Mechano-Physical Properties and Microstructure of Carbon Nanotube Reinforced Cement Paste after Thermal Load.

nanomaterials Article

Mechano-Physical Properties and Microstructure of Carbon Nanotube Reinforced Cement Paste after Thermal Load Maciej Szelag ˛

ID

Faculty of Civil Engineering and Architecture, Lublin University of Technology, 40 Nadbystrzycka Str., 20-618 Lublin, Poland; [email protected]; Tel.: +48-81-538-4428 Received: 3 August 2017; Accepted: 6 September 2017; Published: 11 September 2017

Abstract: The article presents the results obtained in the course of a study on the use of carbon nanotubes (CNTs) for the modification of a cement matrix. Carbon nanotubes were introduced into a cement paste in the form of an aqueous dispersion in the presence of a surfactant (SDS—sodium dodecyl sulfate), which was sonicated. The selected physical and mechanical parameters were examined, and the correlations between these parameters were determined. An analysis of the local microstructure of the modified cement pastes has been carried out using scanning electron microscope (SEM) and X-ray microanalysis (EDS). In addition, the effect of carbon nanotubes on the change in characteristics of the cementitious material exposed to the sudden, short-term thermal load, was determined. The obtained material was characterized by a much lower density than a traditional cement matrix because the phenomenon of foaming occurred. The material was also characterized by reduced durability, higher shrinkage, and higher resistance to the effect of elevated temperature. Further research on the carbon nanotube reinforced cement paste, with SDS, may contribute to the development of a modified cement binder for the production of a lightweight or an aerated concrete. Keywords: carbon nanotubes; cement paste; elevated temperature; sodium dodecyl sulfate (SDS); water dispersion

1. Introduction Cement composites are one of the most widely used construction materials in the world. The reasons for this are, primarily, technological ease of production, availability of raw materials necessary for production, low cost of production, high durability over many decades of operation, the possibility of making any shape, recyclability, and much more [1]. Increasing demands on the durability of cementitious materials, the emergence of new and much more complex constructions, and the aggressive environment, require researchers to seek new methods and ways to improve the performance of concrete and the cement matrix itself. The necessity of examining the characteristics of the cement paste is highly justified, because it is the basic ingredient of mortars and concretes. The cement matrix defines, primarily, the technical and functional characteristics of the cementitious composite [1–3]. In order to improve the properties of the cement matrix, a number of additives and admixtures are used, including e.g., pozzolanic additives (microsilica, fly ash, metakaolinite, zeolites, etc.) [1,4]. A separate group of modifications use scattered reinforcement in the form of various types of fibers, e.g., steel, polypropylene, basalt, glass, aramid, natural, etc. [5–8]. In recent years, research into the use of carbon nanotubes (CNTs) as a nano-reinforcement of cement matrix has been conducted in many countries [9–13].

Nanomaterials 2017, 7, 267; doi:10.3390/nano7090267

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Nanomaterials 2017, 7, 267

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Carbon nanotubes, due to their particular mechanical, thermal, and electrical characteristics, have revolutionized the electronics industry and have become the basis of nanotechnology as a discipline of technology and science [14]. CNTs are rolled up graphene plates. Such a structure can have a fully enclosed surface, because nanotubes are ended at one or both ends by the semicircular canisters. CNTs’ characteristic feature are their large ratio of length to diameter. They can be single-, double-, and multi-wall [15]. The methods of CNT production are very complex, which makes it now an expensive material. The common feature of the CNT manufacturing process is the slow condensation of a hot steam of carbon atoms. The obtained nanotubes take two forms: the first is deposit made of tangled, amorphous and randomly mixed nanotubes, the second form is a row of parallel nanotubes on a catalyst supported substrate [16,17]. The CNT’s morphology is unique. This material is characterized by a very large surface area, new and variable electronic properties, a very high Young’s modulus (about 2 TPa), very strong bonds between carbon atoms that give unmatched durability, high tensile strength (about 50 GPa) and bending strength, and very good conductivity of heat and electricity. Their diameter usually oscillates in the range from 1 to 100 nm, and length, from 10 nm to 10−2 m [16,18,19]. For many years, research has been ongoing on the use of CNTs in medicine and electronics. An exceptionally new approach is the use of CNTs in concrete technology as a nano-reinforcement [20], in which a small addition can improve the mechanical properties of a cementitious composite. In some studies [21,22], it has been shown that the use of multi-wall carbon nanotubes positively affects the nano- and micro-mechanical properties of a cement paste. In addition, the cracking resistance of the cement matrix, and the local stiffness of the calcium-silicate-hydrate (CSH) phase has been increased. It has also been shown that the porosity of cement matrix has been reduced. The studies [23] confirmed a decrease of the local porosity after the application of multi-wall carbon nanotubes. An increase in the compressive strength of cementitious composites was also observed. With the use of the scanning electron microscope (SEM) it was noted that CNTs create bridges between nanoand micro-cracks in the binder. This resulted in increased tensile strength and limiting further the cracks’ propagation—the same conclusions were made by the American scientists [24]. Based on the above-mentioned studies, CNTs are dosed in the amount of 0.04% to 0.5% by weight of cement. The biggest problem in the use of CNTs as an additive to cementitious composites is their uneven dispersion and low adhesion to cement paste. Applying the CNTs alone to the paste does not produce any effect, as they tend to agglomerate, due to their large surface area. Most often, CNTs are used as an aqueous suspension in the presence of a surfactant that has been previously sonicated, in order to obtain a high CNT dispersion. For this purpose, the supercritical-fluid chromatography (SFC) techniques were used [21,22]. As a surfactant among others, H2 SO4 , HNO3 [23,25], and isopropanol [24] were used. A stable dispersion of CNTs is also obtained in the presence of SDS (sodium dodecyl sulfate) [26–31], NaDDBS (sodium dodecyl benzene sulfonate), and DMAc (di-methyl acetamide) [26]. A very high degree of CNT dispersion and stable aqueous solution (stable for a few months) is obtained in the presence of SDS, which was investigated by UV spectroscopy and transmission electron microscopy (TEM) [27]. Some studies on the use of the CNT dispersion with SDS [32–34], NaDDBS [35], and DMAc for cementitious composites, were conducted, but this aspect is not yet well researched. The article presents research on the use of CNTs as a nano-reinforcement of cement matrix. Selected physical (apparent density, linear shrinkage) and mechanical parameters (compressive strength and bending tensile strength) of the CNT modified pastes were examined, and the correlations between these parameters were calculated. CNTs with SDS (as a surfactant) were used as an aqueous dispersion, which was sonicated. An analysis of the microstructure of cement matrix using scanning electron microscope (SEM) and X-ray microanalysis (EDS) was also performed. In addition, the CNT modified cementitious material was subjected to a sudden, short-term thermal load (thermal shock), and the selected physical and mechanical properties of the material were redefined. So far, no studies have been published in the literature, in which the effect of CNT addition on the modification of the cement matrix performance under thermal shock was determined.


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2. Materials and Methods 2.1. Manufacturing of the Modified Cement Pastes This study was conducted on four series of cement paste specimens. For each series, samples were made with three different water/cement ratios (w/c) equal to 0.4, 0.5, and 0.6. Two series were a classical, unmodified cement matrix—a combination of water and cement. In the other two series, CNT were added in the amount of 0.1% by weight of cement. The following cement paste series were made for this study:

• • • •

C42—consisting of CEM I 42.5R and water, C42CNT—consisting of CEM I 42.5R, 0.1% CNT, and water, Nanomaterials 2017, 7, 267 C52—consisting of CEM I 52.5R and water, 2. Materials and Methods C52CNT—consisting of CEM I 52.5R, 0.1% CNT, and water.

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2.1. Manufacturing of the Modified Cement Two Portland cements—CEM I 42.5RPastes and CEM I 52.5R (Cemex, Chełm, Poland) were used for this study.This CEM I 42.5R is guaranteed reachof the minimum strength For of 42.5 28 days; study was conducted on fourtoseries cement paste specimens. eachMPa series,after samples CEMwere I 52.5R—52.5 MPa different after 28water/cement days. Cements a very similar chemical made with three ratioshave (w/c) equal to 0.4, 0.5, and 0.6. Twocomposition, series were a but unmodified cement surface matrix—a combination and In theoxide othercomposition two series, of differclassical, significantly in a specific area, as shownofinwater Table 1. cement. The similar CNT were added in the amount of 0.1% by weight of cement. The following cement paste series were[36]). the cements directly translates to a similar mineral composition (calculated by Bogue’s formula made for this study: Table Chemical and mineral components, the Blaine specific surface area of the Portland cements used. • 1. C42—consisting of CEM I 42.5R andand water, • C42CNT—consisting of CEM I 42.5R, 0.1% CNT, and water, • C52—consisting of CEM I 52.5R and water, Chemical Analysis (%) Cement’s Class SiO Fe O3 MgO Cl Na2 O K2 O 2 2 O3 0.1%CaO • C52CNT—consisting of2CEM I Al 52.5R, CNT, and water. SO3 CEM I 42.5R 20.18 3.39 4.38 64.79 1.17 2.91 0.083 0.26 0.49 Two Portland cements—CEM I 42.5R and CEM I 52.5R (Cemex, Chełm, Poland) were used for CEM I 52.5R 20.19 3.30 4.33 64.76 1.17 3.16 0.078 0.26 0.48

this study. CEM I 42.5R is guaranteed to reach the minimum strength of 42.5 MPa after 28 days; CEM Mineral Composition (%) 2 /g) I 52.5R—52.5 MPa after 28 days. Cements have a very similar chemical but differ Blaine Specific composition, Surface Area (cm Cement’s Class in aC3S C2S C3Aas shown C4AF significantly specific surface area, in Table 1. The similar oxide composition of the CEM I 42.5R 5.88 4010 formula [36]). cements directly 63.41 translates 8.92 to a similar mineral10.31 composition (calculated by Bogue’s CEM IAs 52.5R 62.97 9.28 5.90 10.03 4596 a nano-reinforcement, multi-wall carbon nanotubes were used—Nanocyl™ NC7000 (Nanocyl, Sembreville, Belgium). They were dosed in the amount of 0.1% by weight of cement. Their average diameter is 9.5 nm, average length—1.5 μm,nanotubes specific surface m2/g, tensile As a nano-reinforcement, multi-wall carbon werearea—250–300 used—Nanocyl™ NC7000 strength—about 60 GPa, Young’s modulus—1 TPa, carbon purity—90%, and metal oxide—10%. (Nanocyl, Sembreville, Belgium). They were dosed in the amount of 0.1% by weight of cement. CNTs used in this study are shown in Figure 1, which was taken by means of a SEM. Their average diameter is 9.5 nm, average length—1.5 µm, specific surface area—250–300 m2 /g, tensile As a surfactant, SDS (sodium dodecyl sulfate—C12H25OSO2ONa—MERCK Milipore, Billerica, strength—about 60 GPa, Young’s modulus—1 TPa, carbon purity—90%, and metal oxide—10%. CNTs MA, USA) was used, in order to prepare the CNT dispersion. SDS used is a white powder, with a used specific in this study shown Figure 1, which was taken means a SEM. 3, bulk densityare of 1.1 g/cmin density of 0.49–0.56 g/cm3by , and pH ofof6–9.

(b)

(a)

Figure 1. SEM images of carbon nanotubes (CNTs) used: (a) magnification 50,000×; (b) magnification 200,000×.

Figure 1. SEM images of carbon nanotubes (CNTs) used: (a) magnification 50,000×; (b) magnification 200,000×. Table 1. Chemical and mineral components, and the Blaine specific surface area of the Portland cements used. Cement’s Class

SiO2

Fe2O3

Al2O3

Chemical Analysis (%) CaO MgO SO3

Cl

Na2O

K2O


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Nanomaterials 2017, 7, 267 CEM I 52.5R SDS (sodium 62.97 9.28 5.90 10.03 4596 As a surfactant, dodecyl sulfate—C 12 H25 OSO2 ONa—MERCK

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Milipore, Billerica, CEM I 52.5Rin order 62.97 9.28 the 5.90 10.03 MA, USA) was used, to prepare CNT dispersion. SDS used is4596 a white powder, with 2.2. Preparation of Mixture and Specimens a specific density of 1.1 g/cm3 , bulk density of 0.49–0.56 g/cm3 , and pH of 6–9.

2.2. Preparation of Mixture and Specimens The paste was made by mechanically mixing cement with water. In the recipes that contained 2.2. Preparation Mixture and Specimens Theofpaste was made byinitially mechanically mixing cement with water. In the CNT (C42CNT and C52CNT), their aqueous dispersion was made, andrecipes then it that was contained mixed CNT (C42CNT and dispersion C52CNT), was initially their dispersion was made, andratio then(CNT/SDS). it was mixed with cement. The CNT made withaqueous a surfactant (SDS), in a 1:5 weight The paste was made by mechanically mixing cement with water. In the recipes that contained with Thesolution CNT dispersion was made withan a surfactant (SDS), in a homogenizer 1:5 weight ratio (CNT/SDS). Thecement. aqueous was sonicated using UP400S ultrasonic (Hielscher CNT (C42CNT and C52CNT), initially their aqueous dispersion wasultrasonic made, and then it was (Hielscher mixed with The Gmbh, aqueous solution was sonicated using an UP400S homogenizer Ultrasonics Teltow, Germany). The H22 sonotrode (Hielscher Ultrasonics Gmbh, Teltow, cement. The CNT dispersion was made with a surfactant (SDS), in a 1:5 weight ratio (CNT/SDS). Ultrasonics Teltow, of Germany). H22 sonotrode (Hielscher Ultrasonics Gmbh, Teltow, Germany) withGmbh, a tip diameter 22 mm, aThe maximum amplitude of 100 μm, and a sound pressure The aqueous solution was The sonicated UP400S ultrasonic homogenizer 2a Germany) with tip diameter ofhomogenizer 22 mm,using a maximum amplitude of 100 μm, andequal a sound pressure density of 85 W/cm , was used. hasan a constant operating frequency to 24(Hielscher kHz. Ultrasonics Gmbh, Teltow, Germany). The sonotrode (Hielscher Gmbh, Teltow, density of 85 W/cm was used. Thewas homogenizer has a constant operating frequency equal kHz. During sonication, the2,homogenizer set H22 to continuous mode, with theUltrasonics output power settoat24the During the was set continuous mode, at the maximum level, inhomogenizer conjunction thetoH22 sonotrode, gave 300the W output ofand power. Theset CNT Germany) withsonication, a tipwhich diameter of 22 mm,with a maximum amplitude ofwith 100 µm, apower sound pressure 2 , was maximum which conjunction with H22 sonotrode, gave W ofwith power. CNT dispersion waslevel, sonicated inin a The glass jar for 30 min,the which was inserted into a300 bucket coldThe water density of 85 W/cm used. homogenizer has a constant operating frequency equal to 24 kHz. dispersion was sonicated in a glass jar for 30 min, which was inserted into a bucket with cold water to expedite the heat transfer generated during ultrasonic mixing. The laboratory stand at which During sonication, the homogenizer was set to continuous mode, with the output power set at the to expedite the heat transfer generated during ultrasonic mixing. The laboratory stand at which sonication was performed is shown in Figure Conversely, Figure 3 shows CNT aqueous maximum level, which in conjunction with the 2.H22 sonotrode, gave 300 Wthe of power. The CNT sonication wasand performed is shown in Figure 2. Conversely, Figure 3 shows the CNT aqueous dispersion before after sonication. dispersion was sonicated in a glass jar for 30 min, which was inserted into a bucket with cold water to dispersion before andmade after into sonication. All samples were bars for proper paste and mortar testing, with dimensions of expedite the All heat transfer generated during ultrasonic mixing. The laboratory stand at which sonication were madewas into bars proper paste and and mortar testing, with dimensions 40 × 40 × 160samples mm3. The mixture laid intofor molds in two layers, sequentially compacted using of was performed shown in according Figure 2. was Conversely, Figure 3two shows theand CNT aqueous dispersion before 40 × 40 × is 160 mm3. The mixture laid into molds layers, sequentially a standardized vibrator, to PN-EN 196-1 [37].inThen all samples were stored compacted for 28 days using in and after sonication. a standardized vibrator, according to PN-EN 196-1 [37]. Then all samples were stored for 28 days in air-dry conditions with an average relative humidity of 50%. air-dry conditions with an average relative humidity of 50%.

Figure 2. Laboratory stand for the production of the CNT suspension. Figure 2. Laboratory stand for the production of the CNT suspension.

Figure 2. Laboratory stand for the production of the CNT suspension.

(b)

(a) (a)

(b)

Figure 3. The CNT water suspension: (a) before sonication—visible clusters of CNT in the form of “lumps”; afterCNT sonication—uniform of the sonication—visible suspension, which clusters indirectly correspond to a of Figure(b) 3. The water suspension:color (a) before of CNT in the form Figure 3. The CNT water suspension: (a) before sonication—visible clusters of CNT in the form uniform CNT(b) dispersion. “lumps”; after sonication—uniform color of the suspension, which indirectly correspond to a of “lumps”; (b) after sonication—uniform color of the suspension, which indirectly correspond to uniform CNT dispersion.

a uniform CNT dispersion.


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All samples were made into bars for proper paste and mortar testing, with dimensions of 40 × 40 × 160 mm3 . The mixture was laid into molds in two layers, and sequentially compacted using a standardized vibrator, according to PN-EN 196-1 [37]. Then all samples were stored for 28 days in air-dry conditions with an average relative humidity of 50%. 2.3. Thermal Load After 28 days of maturation, the samples were subjected to a short-term thermal load. The furnace was heated to 250 ◦ C, then the samples were put into it for 4 h. For this purpose, the SLW-400 ECO ´ aski, furnace (POL-EKO-APARATURA sp.j., Wodzisław Sl ˛ Poland) with forced air circulation was used; the furnace operates in the temperature range from +5 to +250 ◦ C. After the samples were heated, they were removed from the furnace, and the cooling was effected by a natural temperature drop at room temperature (about 20 ◦ C). Placing the samples in the preheated furnace can be termed as the thermal shock. The parameters of the temperature load were selected on the basis of tests carried out at different temperatures and at different load duration, within the framework of previous authors’ studies [2,38–41]. In the studies, the surface structure of the thermal cracks of the modified cement pastes was analyzed, using computer image analysis. 2.4. Methods 2.4.1. Determination of Physical Properties Determination of an apparent density (D) of the hardened cement paste was carried out in accordance with EN 12390-7 [42]. Due to the regular shape (rectangular), the specimens have been measured by a caliper in any of the three directions, and then weighed on a laboratory scale. The apparent density has been calculated on the basis of these data. The physical feature was measured for samples after 28 days of maturing (D(R) ), as well as on samples after the temperature load, and cooled down to the room temperature (D(T) ). Results presented in this study are an arithmetic means of 6 samples. The linear shrinkage (S) determination was performed on a Graf–Kaufman device (ToRoPol Sp. z o.o., Warsaw, Poland). This method requires the installation of special metal pins in the specimens’ faces. They were fixed at the time of placing the cement paste in molds specially designed for this. Shrinkage was determined after 3 (S3 ), 7 (S7 ), 14 (S14 ), 21 (S21 ), and 28 (S28(R) ) days of maturation, and on samples subjected to the temperature load, and cooled to the room temperature (S28(T) ). The first measurement was made immediately after the molds were removed (24 h after the paste was placed). Results for each series, and the w/c ratio, are the arithmetic means of the 6 measurements. 2.4.2. Determination of Mechanical Properties The bending tensile strength test (fct ,f ) using three-point bending according to EN 12390-5 [43], was performed. The parameter was calculated using the modulus of rupture (MOR) formula. The strength was examined for samples after 28 days of maturing (fct ,f (R) ), as well as on samples after the temperature load, and cooled down to the room temperature (fct ,f (T) ). Results presented in this study are the arithmetic means of 6 samples. Samples were loaded perpendicular to the direction of molding. The specimens’ halves were used for the compressive strength test. The bending tensile strength test was conducted using the MTS 810 press with the 10 kN head. The compressive strength test (fc ) according to EN 12390-3 [44] was performed. As in the case of the bending tensile strength, the compressive strength test was carried out on the samples after 28 days of maturation (fc (R) ), and on the heat-treated samples (fc (T) ). Results presented are the arithmetic means of the 12 measurements. The test was carried out using the CONTROLS Advantest 9 press (CONTROLS, Milan, Italy), with an arm with a maximum pressure up to 250 kN, and the compression strength device to test 40 × 40 × 40 mm3 specimens. Samples were compressed in a perpendicular direction to the molding.


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2.4.3. Nanomaterials Microstructure Analysis 2017, 7, 267

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2.4.3. Microstructure Analysis

Analysis of a local microstructure of the modified cement pastes was performed on the basis of 2.4.3.Analysis Microstructure Analysis of a local microstructure of the modified cement pastes was performed on the basis of images obtained from the scanning electron microscope (SEM)—Quanta Feg 250 (FEI, Hillsboro, OR, images obtained scanning electron (SEM)—Quanta Feg 250 (FEI, Hillsboro, OR, Analysis of afrom localthe microstructure of themicroscope modified cement pastes was performed on the basis of USA).USA). The analysis waswas carried outout after 28 28 days ofofmaturation on samples(250 (250°C◦ C for analysis carried after days maturation onthe the heat-treated heat-treated samples imagesThe obtained from the scanning electron microscope (SEM)—Quanta Feg 250 (FEI, Hillsboro, OR, 4 h), after the samples to roomtotemperature. Samples were were sputtered by carbon before analysis. for 4 cooling h), after cooling thecarried samples room Samples sputtered by carbon before USA). The analysis was out after 28 temperature. days of maturation on the heat-treated samples (250 °C Images were taken in the high vacuum mode with magnifications from 100 × to 200,000 × . analysis. Images were taken in the high vacuum mode with magnifications from 100× 200,000×. for 4 h), after cooling the samples to room temperature. Samples were sputtered by to carbon before On same the same samples, (Energy Dispersive Spectroscopy, FEI, Hillsboro, OR, USA) On the samples, thethe EDS Dispersive X-ray Spectroscopy, FEI, Hillsboro, OR, USA) analysis. Images were taken inEDS the(Energy high vacuum mode X-ray with magnifications from 100× to 200,000×. analysis was performed order identifyDispersive the chemical chemical composition of of material. The The results are are analysis was performed in in order totoidentify the composition material. On the same samples, the EDS (Energy X-ray Spectroscopy, FEI, Hillsboro, OR,results USA) presented in terms of weight content ofoxide oxidethe equivalents. Series C52CNT with w/c w/c = are 0.4 = 0.4 presented in was terms of weight equivalents. SeriesC52 C52and C52CNT with analysis performed in content order to of identify chemical composition ofand material. The results were analyzed. were presented analyzed.in terms of weight content of oxide equivalents. Series C52 and C52CNT with w/c = 0.4 were analyzed.

3. Results 3. Results

3. Results

3.1. Physical Properties of the Modified Cement Pastes 3.1. Physical Properties of the Modified Cement Pastes 3.1. Physical Properties of the of Modified Cementseries Pastes is presented in Figure 4. The standard deviation The apparent density all analyzed

The apparent density of all analyzed series is presented in Figure 4. The standard deviation reached reached small values; for the reference samples,inand 0.001–0.010 the heat-treated Thevery apparent density0.002–0.012 of all analyzed series is presented Figure 4. The for standard deviation very small values; 0.002–0.012 for the reference samples, and 0.001–0.010 for the heat-treated samples. samples. reached very small values; 0.002–0.012 for the reference samples, and 0.001–0.010 for the heat-treated samples.

(a)

(b)

(a) Figure 4. Apparent density of modified cement pastes (error bars mean (b) standard deviations of Figure 4. Apparent density of modified cement pastes (error bars mean standard deviations of apparent apparent density; due to the very small values, the error bars are invisible) for (a) reference samples; Figure density of modified cement pastes (error bars mean standard deviations of density; due4.toApparent the very small values, the error bars are invisible) for (a) reference samples; (b) samples (b) samples after thermal load. apparent density; due to the very small values, the error bars are invisible) for (a) reference samples; after thermal load. (b) samples after thermal load.

Figures 5–7 show the linear shrinkage after 3, 7, 14, 21, and 28 days of maturation of cement Figures 5–7 show the linear shrinkage after 3, 7, 14, 21, and 28 days of maturation of cement paste, paste,Figures respectively, for w/c 0.4, 0.5, and 0.6. 5–7 show the=linear shrinkage after 3, 7, 14, 21, and 28 days of maturation of cement

respectively, for w/c = 0.4, 0.5, and 0.6. paste, respectively, for w/c = 0.4, 0.5, and 0.6.

Figure 5. Increase in linear shrinkage of modified cement paste with w/c = 0.4, as a function of maturation time. in linear shrinkage of modified cement paste with w/c = 0.4, as a function of Figure 5. Increase Figure 5. Increase in linear shrinkage of modified cement paste with w/c = 0.4, as a function of maturation time.

maturation time.


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Figure 6. Increase in linear shrinkage of modified cement paste with w/c = 0.5, as a function of

Figure 6. Increase in linear shrinkage of modified cement paste with w/c = 0.5, as a function of maturation time. in linear shrinkage of modified cement paste with w/c = 0.5, as a function of Figure 6. Increase maturation time. maturation time.

Figure 7. Increase in linear shrinkage of modified cement paste with w/c = 0.6, as a function of maturation time. in linear shrinkage of modified cement paste with w/c = 0.6, as a function of Figure 7. Increase Figure 7. Increase in linear shrinkage of modified cement paste with w/c = 0.6, as a function of maturation time.

maturation Table 2time. shows the linear shrinkage values after thermal load, the increase in shrinkage, and decrease in 2apparent density thermalvalues shock.after thermal load, the increase in shrinkage, and Table shows the linearafter shrinkage decrease in apparent density after thermal shock.after thermal load, the increase in shrinkage, and Table 2 shows the linear shrinkage values Table 2. The linear shrinkage values of samples tested after thermal load, the percentage increase in

decrease in apparent thermal shrinkage, anddensity apparentafter density decreaseshock. due to the thermal shock.

Table 2. The linear shrinkage values of samples tested after thermal load, the percentage increase in shrinkage, and apparent to the thermal shock. C52 Series C42density decrease due C42CNT C52CNT

The linear shrinkage values of0.4 samples tested0.6after thermal load, 0.6 the percentage increase in Table 2.w/c 0.4 0.5 0.6 0.5 0.4 0.5 0.4 0.5 0.6 Series C42 C42CNT C52CNT S28(T) (mm/m) 5.50 6.35 7.58 8.60 due10.15 10.92 4.53 5.90 7.48 8.67 10.10 11.06 shrinkage, and apparent density decrease to the thermal shock.C52 w/c ΔS=S28(T) − S28(W) S28(T) (mm/m) (mm/m) ΔS=S 28(T) − S28(W) Series Increase of S28 (%) (mm/m) Decrease of D (%) w/c Increase of S28 (%) Decrease of D (%) (mm/m)

0.4 3.25 5.50

0.5 3.83 6.35

0.6 4.35 7.58

0.4 5.04 8.60

0.5 6.38 10.15

144.4 3.25

C42 152.1 3.83 9.9 0.5 152.1 9.9 6.35

134.8 4.35 9.8 0.6 134.8 9.8 7.58

141.5 5.04 10.3 0.4 141.5 10.3 8.60

C42CNT 169.1 168.8 6.38 6.86 10.0 9.2 0.5 0.6 169.1 168.8 10.0 10.15 9.2 10.92

10.6 0.4 144.4

0.6 6.86 10.92

0.4 2.84 4.53

0.5 3.46 5.90

0.6 4.35 7.48

0.4 5.50 8.67

0.5 6.35 10.10

168.5 2.84 11.5 0.4 168.5 11.5 4.53

C52 141.9 3.46 11.8 0.5 141.9 11.8 5.90

C52CNT 139.3 173.7 169.4 151.7 4.35 5.50 6.35 6.67 11.6 12.4 11.5 11.3 0.6 0.4 0.5 139.3 173.7 169.4 151.7 0.6 11.67.48 12.4 8.67 11.5 10.10 11.311.06

10.6 S28(T) 5.50 3.2. Mechanical Properties of the Material ∆S=S 28(T) − S28(W) 3.25 3.83 4.35 5.04 6.38 6.86 2.84 3.46 4.35 5.50 3.2.(mm/m) Mechanical Properties of the Materialstrength of all analyzed series of cement paste. Figure 8 shows the compressive Increase of S28 (%) 144.4 152.1 134.8 141.5 169.1 168.8 168.5 141.9 139.3 173.7 Decrease of D (%) 10.6the compressive 9.9 9.8 strength 10.3 9.2 11.5 of11.8 11.6 Figure 8 shows of10.0 all analyzed series cement paste.12.4

3.2. Mechanical Properties of the Material Figure 8 shows the compressive strength of all analyzed series of cement paste.

0.6 6.67 11.06

6.35

6.67

169.4 11.5

151.7 11.3


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(a) (b) (a) (b) Figure 8. Compressive strength of modified cement pastes (error bars mean standard deviations of Figure 8. Compressive strength of modified cement pastes (error bars mean standard deviations of compressive strength) for (a) reference samples; (b) samples after thermal load. Figure 8. Compressive strength of modified cement pastes (error bars mean standard deviations of compressive strength) for (a) reference samples; (b) samples after thermal load. compressive strength) for (a) reference samples; (b) samples after thermal load.

Figure 9 shows the flexural strength of all tested cement paste series.

Figure 9 shows thethe flexural strength cementpaste pasteseries. series. Figure 9 shows flexural strengthofofall alltested tested cement

(b) (b)

(a) (a)

Figure 9. Flexural strength of modified cement pastes (error bars mean standard deviations of flexural strength) for (a) reference (b) cement samplespastes after thermal load. Figure 9. Flexural strengthsamples; of modified (error bars mean standard deviations of flexural Figure 9. Flexural strength of modified cement pastes (error bars mean standard deviations of flexural strength) for (a) reference samples; (b) samples after thermal load.

strength) (a) reference samples; (b) samples after thermal load. Tablefor 3 shows the decline in the bending tensile and compressive strength after the thermal load. Table 3 shows the decline in the bending tensile and compressive strength after the thermal load.

TableTable 3 shows the decline in the bending tensile and compressive strength thermal load. 3. Percentage decrease in the compressive and bending tensile strength due after to thethe thermal shock. 3. Percentage decrease in the compressive and bending tensile strength due to the thermal Table

Tableshock. 3. Percentage decrease in the compressive and bending tensile strength due to the thermal shock. Series w/c Series Series w/c Decrease of

w/c

Decrease of

Decrease of

fc (%) ffct,f (%) c (%) (%) fcfct,f(%)

0.4 36.1 0.4 48.8 0.4 36.1 48.8 36.1

C42 0.5 C42 C42 46.9 0.5 40.4 0.5 46.9 40.4 46.9

0.6 53.5 0.6 58.2 0.6 53.5 58.2 53.5

C42CNT 0.4 C42CNT 0.5 0.6 C42CNT 39.0 42.5 45.1 0.4 0.5 0.6 35.8 40.2 44.9 0.4 42.5 0.5 45.1 0.6 39.0 35.8 39.0 40.2 42.5 44.9 45.1

C52 0.4 0.5 C52 C52 29.1 38.8 0.4 0.5 73.4 69.2 0.4 29.1 38.80.5 73.4 29.1 69.2 38.8

C52CNT 0.4 C52CNT 0.5 0.6 C52CNT 25.4 28.5 30.3 0.4 0.5 0.6 50.90.4 28.5 52.2 0.5 30.3 51.7 0.6 25.4 69.946.0 50.925.4 52.228.5 51.7 30.3 0.6 46.0 0.6 69.90.6 46.0

fct,f (%) 3.3. SEM and EDS Analysis Material 48.8of the40.4 58.2 35.8 40.2 44.9 73.4 69.2 69.9 50.9 52.2 51.7 3.3. SEM and EDS Analysis of the Material Figures 10 and 11 show sample SEM images for series C52 and C52CNT, and EDS spectra for 3.3. SEM and EDS of the thesample Material theseFigures areas. Table 4 shows oxide composition offor theseries two samples with w/c = 0.4. More images 10Analysis and 11 show SEM images C52 and C52CNT, and EDSSEM spectra for are shown discussed the Discussion section. these areas.and Table 4 showsin thedetail oxideincomposition of the two samples with w/c = 0.4. More SEM images Figures 10 and 11 show sample SEM images for series C52 and C52CNT, and EDS spectra for are shown and discussed in detail in the Discussion section.

these areas. Table 4 shows the oxide composition of the two samples with w/c = 0.4. More SEM images are shown and discussed in detail in the Discussion section.


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Nanomaterials 2017, 7, 267 Nanomaterials 2017, 7, 267

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Figure 10. EDS spectra of series C52 with w/c = 0.4 together with the SEM image of the analyzed area. Figure 0.4 together togetherwith withthe theSEM SEMimage imageof ofthe theanalyzed analyzedarea. area. Figure 10. 10. EDS EDS spectra spectra of ofseries seriesC52 C52with withw/c w/c == 0.4

Figure 11. EDS spectra of series C52CNT with w/c = 0.4 together with the SEM image of the analyzed Figure 11. EDS spectra of series C52CNT with w/c = 0.4 together with the SEM image of the analyzed area. Figure 11. EDS spectra of series C52CNT with w/c = 0.4 together with the SEM image of area. the analyzed area. Table 4. Oxide composition of cement paste after the elevated temperature interaction, and Table 4. Oxide composition of of cement paste after the elevated temperature interaction, and percentage difference in a content individual respect to series C52. Table 4. Oxide composition of cement paste aftercomponents the elevatedwith temperature interaction, and percentage percentage difference in a content of individual components with respect to series C52. difference in a content of individual components with respect to series C52. Oxide Composition (% by Weight) Series Oxide (% 3by Weight) Na 2O MgO Al2O5 Composition SiO2 SO K2O CaO Fe2O3 Series Oxide Composition (% by Weight) Na 2 O MgO Al 2 O 5 SiO 2 SO K 2O CaO Fe 2O 3 Series C52 0.79 1.06 6.58 22.87 3.783 0.52 61.30 3.12 Na2 O MgO Al2 O5 SiO2 SO3 K2 O CaO Fe2 O3 C52 0.79 1.06 6.58 22.87 3.78 0.52 61.30 3.12 C52CNT 0.58 0.92 5.06 18.93 5.20 0.94 65.02 3.34 C52 0.79 0.58 1.06 0.92 6.58 5.06 22.87 3.785.20 0.52 61.30 3.12 C52CNT 18.93 0.94 65.02 3.34 DifferenceC52CNT between series (%) 0.58 −26.0 −12.8 −23.1 −17.2 +37.4 +80.9 +6.1 +6.8 0.92 5.06 18.93 5.20 0.94 65.02 3.34 Difference between series −23.1 −17.2 −17.2 +37.4 +37.4 +80.9 +80.9 +6.1 +6.1 +6.8 Difference between series (%) (%)−26.0−26.0 −12.8−12.8 −23.1 +6.8

4. Discussion 4. 4. Discussion Discussion 4.1. Physical Properties of the Modified Cement Pastes 4.1. Physical Physical Properties Properties of the Modified Cement Pastes 4.1.1. Apparent Density Apparent Density Density 4.1.1. Apparent Apparent density is one of the basic physical parameters of building materials. One of the Apparent density of the basic physical parameters building materials. One of4,the variables densityis isone one of the basic physical of building materials. One ofbeen the variables determining the density of the cement pasteparameters is theofw/c ratio. Based on Figure it has determining the density of the cement paste is the w/c ratio. Based on Figure 4, it has been noted that variables determining the density of the cement paste is the w/c ratio. Based on Figure 4, it has been noted that as the w/c increases, the apparent density decreases. Thus, samples with w/c = 0.5 and 0.4 noted that as theDw/c increases, the apparent density decreases. Thus, samples with w/c = 0.5 and 0.4 reached greater (R) values by an average of 10% and 20% respectively, than samples with w/c = 0.6. reached D(R) values by an average of 10% and 20% respectively, than samples with w/c = 0.6. Identicalgreater differences were observed between the heat-treated samples. Cement has a density of about Identical differences were observed between the heat-treated samples. Cement has a density of about


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three times higher than water. Due to the reduction in the amount of cement in the material’s volume (increase in w/c), the amount of water is increased, so the cementitious material has a lower resultant as the w/c increases, apparent density decreases. samples w/c = 0.5 and density. The apparentthe density is also dependent on theThus, volume of thewith capillary pores that0.4 arereached formed greater D values by an average of 10% and 20% respectively, than samples with w/c = 0.6. Identical during the (R) cement hydration process. According to Power’s law, under identical curing conditions, differences observed the samples. Cement hashigher a density of about pores three the capillarywere pores’ volumebetween is shaped byheat-treated the w/c ratio—the higher w/c, the the capillary times higher than water. Due to the reduction in the amount of cement in the material’s volume volume [45]. Water, when evaporating, leaves more empty pores if a higher w/c ratio is used, which (increase amount of water is increased, so the cementitious material has a lower resultant results inina w/c), lowerthe apparent density of cementitious material. density. apparent is CEM also dependent onC52CNT) the volume of the capillary pores TheThe cement pastesdensity made of I 52.5R (C52, were characterized by 3%that andare 1%formed higher during the cement hydration process. According to Power’s law, under identical curing conditions, D(R) and D(T) values than the series C42 and C42CNT. The impact of the cement’s class is negligible the capillary pores’ volume is matrix. shaped However, by the w/cthe ratio—the w/c, the(CEM higherI the capillary pores on the density of the cement cement higher of higher class 52.5R) has a larger volume [45]. Water, when evaporating, more pores if a higher ratio is used, surface area, and therefore, its grains areleaves smaller thanempty the CEM I 42.5R grains.w/c As the size of the which grains results in a according lower apparent cementitious decreases, to the density Power’soflaw mentionedmaterial. above, the distance between them is also smaller, Theresults cement made of CEM I 52.5R (C52, In C52CNT) characterized by 3% higher which inpastes smaller capillary pore volume. such a were situation, the volume of and the 1% hydration D and D values than the series C42 and C42CNT. The impact of the cement’s class is negligible products is larger, resulting in a slight increase in apparent density. (R) (T) on the density the aqueous cement matrix. However, was the cement of higher class (CEM 52.5R) has a larger The use ofofthe CNT dispersion the main factor causing the Ichange of apparent surface and therefore, itsC52CNT grains are smaller than the CEM 42.5Rapparent grains. As the sizeby ofan theaverage grains density.area, Series C42CNT and were characterized by a Ilower density, decreases, according to the Power’s law mentioned above, the distance between them is also smaller, of 31%, than series C42 and C52. After the thermal load, the above difference was the same. whichThe results smaller capillary pore In such a formation situation, of theair volume of the the hydration use ofinthe CNT dispersion with volume. SDS resulted in the pores during mixing products larger, which resulting in aenclosed slight increase apparentvolume. density. A similar result was obtained by of cementis paste, were in the in material’s The use of [46]. the aqueous CNT dispersion was is the main factor causing the change of apparent Sobolkina et al. SDS, as an anionic surfactant, commonly used as a detergent component, and density. C42CNTspecific and C52CNT wereagents—gaseous characterized by and a lower apparent density,As byaan average exhibits Series characteristics to blowing foaming substances. surfactant, of 31%, than series and of C52. thermal load, theand above difference was the same.in contact SDS adsorbs on theC42 surface theAfter CNTsthe during sonication, reduces the surface tension of the CNT dispersion SDSofresulted in the formation of air poressurface during the mixing withThe theuse aqueous phase. Possiblewith ways SDS adsorption on the CNT’s have been of cement paste, which were enclosed in the material’s volume. A 12. similar result was obtained by investigated by Richard et al. [47], as schematically shown in Figure The mechanism of nanotube Sobolkina et al. [46]. SDS, as an anionic surfactant, is commonly used as a detergent component, and isolation from a bundle (Figure 13 (i)), with the combined assistance of sonication and surfactant exhibits characteristics specific to blowing foaming substances. As a surfactant, adsorption, was proposed by Strano et al.agents—gaseous [48]. The role of and ultrasonic treatment is likely to provide SDS on the surface ofto the CNTs during sonication, andOnce reduces theor surface tension in contact highadsorbs local shear, particularly the nanotube bundle end (ii); spaces gaps at the bundle ends with the aqueous phase. Possible ways of SDS adsorption on the CNT’s surface have been investigated are formed, they are propagated by surfactant adsorption (iii); ultimately separating the individual by Richardfrom et al.the [47], as schematically shown in Figure 12. The mechanism of nanotube isolation nanotubes bundle (iv). from As a bundle (Figure with described the combined assistance surfactant adsorption, wasa a result of the13i), process above, there isofa sonication formation and of the CNT suspension with proposed by of Strano et al. [48]. role of treatment is likely to provide highThe local shear, high degree dispersion in theThe volume ofultrasonic the dispersion medium—water in this case. solution particularly the nanotube bundle end (ii);soOnce spaces or is gaps at the bundle endscement are formed, has a highertoviscosity compared to water, when cement added, the resulting paste they may are propagated by surfactant adsorption (iii); ultimately separating the individual nanotubes from undergo foaming in the course of mixing. Thus, the cementitious material is characterized by a much the bundle (iv). lower density.

Figure Differentpossible possibleorganizations organizationsofofthetheSDS SDS molecules surface a CNT: Figure 12. 12. Different molecules on on thethe surface of aofCNT: (a) (a) perpendicular adsorption; half-cylindersoriented orientedparallel paralleltotothe the tube tube axis; (c) half-cylinders perpendicular adsorption; (b)(b) half-cylinders half-cylinders oriented perpendicular to the tube axis [47]. Reproduced with permission of [47]. Copyright AAAS, 2003.


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oriented2017, perpendicular to the tube axis [47]. Reproduced with permission of [47]. Copyright AAAS, Nanomaterials 7, 267 11 of 22 2003.

Figure Figure 13. 13. Mechanism Mechanism of of nanotube nanotube isolation isolation from from bundle bundle obtained obtained by by sonication sonication and and surfactant surfactant stabilization: (i) nanotubes bundle; (ii) providing high local shear by sonication; (iii) surfactant stabilization: (i) nanotubes bundle; (ii) providing high local shear by sonication; (iii) surfactant adsorption; individual nanotube nanotube from from the the bundle bundle [48]. [48]. Reproduced adsorption; (iv) (iv) separation separation of of the the individual Reproduced with with permission permission of of [48]. [48]. Copyright Copyright American American Scientific Scientific Publishers, Publishers, 2003. 2003.

The decrease in apparent density after thermal shock (Table 2) was similar for all samples. It has As a result of the process described above, there is a formation of the CNT suspension with been noted that a slight difference exists in the criterion of cement used. The apparent density a high degree of dispersion in the volume of the dispersion medium—water in this case. The solution decrease for C42 and C42CNT samples was in the range of 9.2–10.6%, while for C52 and C52CNT— has a higher viscosity compared to water, so when cement is added, the resulting cement paste may 11.3–12.4%. Drop in apparent density after heating is due to the evaporation of free water from the undergo foaming in the course of mixing. Thus, the cementitious material is characterized by a much material’s volume. No CNT additive was found to affect this characteristic. lower density. The decrease in apparent density after thermal shock (Table 2) was similar for all samples. It has 4.1.2. Linear Shrinkage been noted that a slight difference exists in the criterion of cement used. The apparent density decrease As and the cement increases (and decreases the amount in the material’s volume for C42 C42CNTcontent samples was in the range of 9.2–10.6%, whileofforwater) C52 and C52CNT—11.3–12.4%. (decrease in w/c), cement tends toisexperience linear shrinkage. This phenomenon has Drop in apparent densitymatrix after heating due to thesmaller evaporation of free water from the material’s been confirmed in this study, and this is shown in Figures 5–7. This relationship is particularly volume. No CNT additive was found to affect this characteristic. evident with the increase in the sample’s maturation time. Shrinkage after 28 days, for samples with 4.1.2. Linear w/c = 0.5, wasShrinkage lower by an average of 16%, and 28% for samples with w/c = 0.4, compared to w/c = 0.6 specimens. Due to the maturation of(and samples in thethe air-dry conditions, average relative As the cement content increases decreases amount of water)with in theanmaterial’s volume humidity of 50%, the main factor shaping shrinkage (especially after 14–28 days of maturation) is the (decrease in w/c), cement matrix tends to experience smaller linear shrinkage. This phenomenon has drying shrinkage associated with the evaporation of free water contained in samples. been confirmed in this study, and this is shown in Figures 5–7. This relationship is particularly evident has been observed that the growth rate ofShrinkage shrinkageafter decreases during the maturation with Itthe increase in the sample’s maturation time. 28 days, for samples with w/c time. = 0.5, Within the first 7 days, unmodified cement pastes (C42, C52) achieved 36% of the shrinkage measured was lower by an average of 16%, and 28% for samples with w/c = 0.4, compared to w/c = 0.6 specimens. after 28 the days. On the other hand, the samples—56%. Increased of the CNT pastes of allows Due to maturation of samples inCNT the air-dry conditions, with an porosity average relative humidity 50%, greater matrix deformation due to the removal of free water from the material. This state translates the main factor shaping shrinkage (especially after 14–28 days of maturation) is the drying shrinkage into an intense increase in shrinkage, especially in the initial stage of maturation. In addition, associated with the evaporation of free water contained in samples. shrinkage from autogenic processes is the greatest in the early phase of the cement It has been observed that the growth rate of shrinkage decreases during the binding. maturation time. There was no significant effect of the cement’s class on the results obtained after 28 daysmeasured (average Within the first 7 days, unmodified cement pastes (C42, C52) achieved 36% of the shrinkage difference was only 4%). In this respect, the chemical composition of the cement is most affected. after 28 days. On the other hand, the CNT samples—56%. Increased porosity of the CNT pastes allows However, both cements used in this study are chemically very similar (Table 1). greater matrix deformation due to the removal of free water from the material. This state translates into Adding CNTs to paste, in the form of initial the aqueous with had ashrinkage negative an intense increase in cement shrinkage, especially in the stage ofdispersion maturation. InSDS, addition, effect on shrinkage. After 3 days of maturation, the series C42CNT and C52CNT achieved higher from autogenic processes is the greatest in the early phase of the cement binding. values of the physical parameter ofclass 140%, the series C42 The There was no significant effectbyofan theaverage cement’s oncompared the resultsto obtained after 28 and daysC52. (average relative increase in shrinkage diminished over time, and after 28 days of maturation, the CNT difference was only 4%). In this respect, the chemical composition of the cement is most affected. samples a higher shrinkage of 49% the classical cement(Table paste.1). It is worth noting that However,reached both cements used in this study arethan chemically very similar despite the rapid increase in shrinkage in the first weeks of dispersion maturation,with the SDS, absolute in Adding CNTs to cement paste, in the form oftwo the aqueous hadincrease a negative shrinkage, between 14 and 28 days for the series C42CNT and C52CNT, was significantly lower than effect on shrinkage. After 3 days of maturation, the series C42CNT and C52CNT achieved higher values the unmodified cement paste. of the physical parameter by an average of 140%, compared to the series C42 and C52. The relative increase in shrinkage diminished over time, and after 28 days of maturation, the CNT samples reached


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a higher shrinkage of 49% than the classical cement paste. It is worth noting that despite the rapid Nanomaterials 2017, 7, 267 12 between of 22 increase in shrinkage in the first two weeks of maturation, the absolute increase in shrinkage, 14 and 28 days for the series C42CNT and C52CNT, was significantly lower than the unmodified By analyzing the increase in shrinkage (ΔS) after thermal shock (Table 2), it was observed that cement thispaste. value increases as the w/c ratio increases. However, there was no significant influence of the By analyzing increase in shrinkage (∆S) after thermal 2), it was observed that this cement’s class the on this parameter. The shrinkage size of cementshock paste (Table after thermal shock is the result valueof increases as the ratioofincreases. However, was no influence of the two factors. At w/c the stage heating, cement pastethere expands, duesignificant to the thermal expansion. Atcement’s the class same on this parameter. Thewater shrinkage size offrom cement paste after thermal shock is the result of two time, there is strong evaporation the material’s structure, which results in shrinkage thatAt increases with increasecement in waterpaste content in the sample’s volume (increase in w/c). From thesame factors. the stage ofan heating, expands, due to the thermal expansion. At the large and interconnected capillaries, water can easily evaporate. time, there is strong water evaporation from the material’s structure, which results in shrinkage that increasein inwater shrinkage was observed for samples with CNTs (average 67% higher thanlarge increases Significant with an increase content in the sample’s volume (increase in w/c). From the the C42 and C52 samples), but the percentage increase was similar to the unmodified specimens, and and interconnected capillaries, water can easily evaporate. ranged from 135% to 174%, in comparison with the values obtained before the thermal load. This is Significant increase in shrinkage was observed for samples with CNTs (average 67% higher than due to the above discussed relationship with increased porosity of the cementitious material. the C42 and C52 relation samples), but the percentage similar the unmodified specimens, A close between shrinkage and increase apparent was density of thetoexamined pastes was observed and ranged from14). 135% to 174%, comparison with the values obtained beforeand thethe thermal load. This is (Figure Empirical datainwere considered in the CNT application criterion, proposed linear due toregression the above discussed relationship with increased porosity of the cementitious material. models, to a large degree, reflect these data. The coefficient of determination of the linear regression equation (R2) forshrinkage the series C42 C52, hasdensity a very high value, equal topastes 0.94, both A close relation between andand apparent of the examined wasbefore observed and after the exposure to theconsidered elevated temperature. the CNT criterion, modified pastes (C42CNT andlinear (Figure 14). Empirical data were in the CNTFor application and the proposed 2 value is equal to 0.79 and 0.90, respectively, for the reference and heat-treated C52CNT), the Rto regression models, a large degree, reflect these data. The coefficient of determination of the linear samples. regression equation (R2 ) for the series C42 and C52, has a very high value, equal to 0.94, both before and As the apparent density of cement paste is a function of its porosity, the above relationship after the exposure to the elevated temperature. For the CNT modified pastes (C42CNT and C52CNT), confirms previous considerations that one of the major factors influencing the shrinkage of the the R2 value is equal to 0.79 and 0.90, respectively, for the reference and heat-treated samples. cement matrix is its porosity.

(a)

(b)

Figure 14. Relationship between linear shrinkage and apparent density of tested materials for (a) Figure 14. Relationship between linear shrinkage and apparent density of tested materials for reference samples; (b) samples after thermal load. (a) reference samples; (b) samples after thermal load.

4.2. Mechanical Properties of the Examined Material

As the apparent density of cement paste is a function of its porosity, the above relationship 4.2.1.previous Compressive Strength that one of the major factors influencing the shrinkage of the cement confirms considerations matrix is its Theporosity. characteristic for cement composites is that the smaller w/c ratio, the higher values of mechanical parameters. This has been confirmed in this study, for both the reference and post-

4.2. Mechanical Properties the Examined Material samples with w/c = 0.5 and 0.4 achieved higher fc(R) temperature samples of (Figure 8). The reference values by an average of 26% and 75%, respectively, than samples with w/c = 0.6. Considering this

4.2.1. dependence Compressive Strength after thermal shock, these differences are even greater—38% and 116%, respectively.

analyzing thefor results of compressive strength in the usedw/c criterion, was higher found that The By characteristic cement composites is that thecement smaller ratio,it the values the higher values (both before and after the influence of temperature) were obtained by samples made of mechanical parameters. This has been confirmed in this study, for both the reference and of the higher class cement, i.e., CEM I 52.5R (series C52 and C52CNT). The effect of the cement’s class post-temperature samples (Figure 8). The reference samples with w/c = 0.5 and 0.4 achieved higher is more visible for the CNT material, as the C52CNT samples have reached higher fc(R) and fc(T) values fc (R) values by an average of 26% 75%, respectively, than C42CNT. samples with w/c = 0.6. Considering by an average of 43% and 77%,and respectively, than the series On the other hand, the above this dependence after thermal shock, these differences are even greater—38% and 116%, respectively.


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By analyzing the results of compressive strength in the cement used criterion, it was found that the higher values (both before and after the influence of temperature) were obtained by samples made of the higher class cement, i.e., CEM I 52.5R (series C52 and C52CNT). The effect of the cement’s class is more visible for the CNT material, as the C52CNT samples have reached higher fc (R) and fc (T) values by an average of 43% and 77%, respectively, than the series C42CNT. On the other hand, the above relationship for the classical cement paste (C52 compared to C42) is much lower—14% and 29%, respectively. The experience of other researchers indicates that the introduction of additional porosity into the material’s volume is beneficial, in terms of some physical properties, e.g., density, heat transfer coefficient, capillary rise. However, the presence of extra pores in the material, which cause discontinuity of the structure, usually results in a deterioration in mechanical properties [49–51]. It is similar in the case of the cement pastes tested, in which series C42CNT reached 56% and 55% of compressive strength of the series C42, for fc (R) and fc (T) , respectively. For the series in which CEM I 52.5R was used (C52CNT compared to C52), these differences are equal to 45% and 38%, respectively. The negative strength effect is reduced with the use of the higher class cement. This is a valuable guideline for practical use of the CNT cement matrix with SDS. By analyzing the decrease in compressive strength (Table 3) after thermal load, it was noted that with the increase in the w/c ratio, the degree of material degradation also increased. These results confirm the generally well-known fact that a cement matrix with a higher compressive strength is more resistant to elevated temperatures (also thermal shock). It is noteworthy that smaller decreases in compressive strength were observed for the CNT samples. This phenomenon is especially visible for the series made of CEM I 52.5R; the fc decrease for the series C52 was in the range of 29–46%, and for the series C52CNT, 25–30%. This indicates the positive effect of the CNT on the mechanical strength of cement matrix under the influence of elevated temperature, despite the increased porosity of the material. CNTs (well dispersed in a cement matrix) may form bridges between hydrates, which result in an increase in local stiffness of the CSH phase [21,22]. Also, other phenomenon occur, such as crack bridging [52–54], delay in micro crack propagation [52], and chemical binding of hydrated phases [23,54,55]. Thus, all the above mentioned mechanisms may result in an increase in the coherence of the cementitious material subjected to thermal shock. The use of the above described method of the temperature loading caused the formation of thermal cracks due to the volume deformation of sample—in the heating phase, the thermal expansion of the material; in the cooling phase, the shrinkage. The steam pressure created in the pores and capillaries of cement paste causes crack propagation, and transformation of the technological cracks that are present in the structure of the material before applying service loads, into macro-cracks visible on the sample’s surface. In this way, the cement paste structure defects were extracted without excessive deterioration of the mechanical properties of the tested material. A relationship between compressive strength and apparent density can be observed (Figure 15). This dependence was described by linear regression equations, separately for the classical cement pastes (C42, C52) and the cement matrix containing CNT (C42CNT, C52CNT). The determination coefficient (R2 ) of the equation describing the relationship for the classical cement pastes was equal to 0.98 and 0.91, respectively, for the reference and heat-treated samples. It means that the applied model of regression covers more than 90% of achieved data. For the cement pastes with CNT, the R2 was equal to 0.78 and 0.52. In the case of the reference samples, the good fit of the regression line to the empirical data is observed. The R2 value for samples after thermal load is not very high, but it should be kept in mind that the equation describes the dependence on materials made of two different cements.


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(a)

(b)

Figure 15. Relationship between compressive strength and apparent density of tested cement pastes Figure 15. Relationship between compressive strength and apparent density of tested cement pastes for (a) (a) reference reference samples; samples; (b) (b) samples samples after after thermal thermal load. load. for

4.2.2. Flexural Strength 4.2.2. Flexural Strength As with compressive strength, one of the major factors which shapes bending tensile strength, As with compressive strength, one of the major factors which shapes bending tensile strength, is is the w/c ratio. Based on the results shown in Figure 9, the cement pastes with w/c = 0.5 obtained the w/c ratio. Based on the results shown in Figure 9, the cement pastes with w/c = 0.5 obtained higher higher fct,f(R) and fct,f(T) values by an average of 26% and 46%, respectively, than samples with w/c = 0.6. fct ,f (R) and fct ,f (T) values by an average of 26% and 46%, respectively, than samples with w/c = 0.6. The same relation between samples, both for w/c = 0.4 and 0.6; 65% and 82%. Clearly, the difference The same relation between samples, both for w/c = 0.4 and 0.6; 65% and 82%. Clearly, the difference between the results obtained is higher for the heat-treated samples, in terms of w/c. Cement paste between the results obtained is higher for the heat-treated samples, in terms of w/c. Cement paste with w/c = 0.6 contains the most water, which results in the highest vapor pressure during heating. with w/c = 0.6 contains the most water, which results in the highest vapor pressure during heating. This phenomenon, combined with the thermal deformation of the material, results in a greater This phenomenon, combined with the thermal deformation of the material, results in a greater decrease decrease in the thermal shock resistance with the higher w/c ratio. in the thermal shock resistance with the higher w/c ratio. The differences in fct,f(R) between samples with and without CNTs are less than in the case of fc(R). The differences in fct ,f (R) between samples with and without CNTs are less than in the case of fc (R) . The bending tensile strength of the reference samples of series C52CNT and C42CNT is reduced by The bending tensile strength of the reference samples of series C52CNT and C42CNT is reduced by approximately 30%, in comparison to the series C52 and C42. It should be remembered that the CNT approximately 30%, in comparison to the series C52 and C42. It should be remembered that the CNT cement pastes have much higher porosity than conventional cement matrix, which is the main cause cement pastes have much higher porosity than conventional cement matrix, which is the main cause of reduced strength. of reduced strength. On the other hand, after exposure to the elevated temperature, particularly in the case of samples On the other hand, after exposure to the elevated temperature, particularly in the case of samples containing CEM I 52.5R, the C52CNT samples had higher fct,f(T) values, by an average of 18%, than containing CEM I 52.5R, the C52CNT samples had higher fct ,f (T) values, by an average of 18%, than series C52. The beneficial effect of using CNTs is larger, the lower the w/c ratio is. It can be stated, as series C52. The beneficial effect of using CNTs is larger, the lower the w/c ratio is. It can be stated, as in the case of fc, that the CNT forms mechanical connections with the cement hydration products in in the case of fc , that the CNT forms mechanical connections with the cement hydration products in the material’s nano-structure. This translates into increased cohesion of cement paste in the aspect of the material’s nano-structure. This translates into increased cohesion of cement paste in the aspect of its stretching. It is so important that when using the CNT application method, the foam effect was its stretching. It is so important that when using the CNT application method, the foam effect was achieved, which translates into a significant reduction in weight of the cementitious material. achieved, which translates into a significant reduction in weight of the cementitious material. There is a very strong relationship between bending tensile strength and apparent density of the There is a very strong relationship between bending tensile strength and apparent density modified cement pastes (Figure 16), but only for the reference samples. The coefficient of of the modified cement pastes (Figure 16), but only for the reference samples. The coefficient of determination of the linear regression equation reaches a very high value, equal to 0.93 and 0.94, determination of the linear regression equation reaches a very high value, equal to 0.93 and 0.94, respectively, for pastes with and without CNT. On the other hand, after the influence of the elevated respectively, for pastes with and without CNT. On the other hand, after the influence of the2 elevated temperature, the regression model no longer reflects empirical data in such a good way (R = 20.72 for temperature, the regression model no longer reflects empirical data in such a good way (R = 0.72 the CNT matrix, R2 = 0.25 for the classical cement paste). In addition, on the fct,f and D graph, the main for the CNT matrix, R2 = 0.25 for the classical cement paste). In addition, on the fct ,f and D graph, advantage of the CNT cementitious material is visible—the reduction in weight without loss of the main advantage of the CNT cementitious material is visible—the reduction in weight without loss bending tensile strength (mainly after the influence of the elevated temperature). of bending tensile strength (mainly after the influence of the elevated temperature).


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(a)

(b)

Figure 16. Relationship between bending tensile strength and apparent density of tested materials for

Figure 16. Relationship between bending tensile strength and apparent density of tested materials for (a) reference samples; (b) samples after thermal load. (a) reference samples; (b) samples after thermal load.

By analyzing the results shown in Figure 9, it can be observed that the class of cement has very

By analyzing the results shown in Figure 9, it can be observed that the class of cement has very little little effect on the fct,f(R) results. The cement pastes made of CEM I 52.5R reached higher fct,f(R) values, effectonly on the fct ,f (R) results. The cement pastes made CEM 52.5R reached fcthigher ,f (R) values, by 5%, than the samples with CEM I 42.5R. Theofeffect ofIthe cement’s classhigher is much in the only by 5%, than samples with CEM with I 42.5R. The effect of the and cement’s class isbecause much higher in the case case of the cementitious materials aggregate (mortars concretes), the resultant of cementitious materials aggregate (mortars andby concretes), because the resultant mechanical strength of with the material is also determined the interfacial transition zone (ITZ).mechanical In the case of of the presence of aggregate in the material, of ITZ depends to a large extent on the strength material is also determined bythe thestrength interfacial transition zone (ITZ). In the case of cement’s class [56]. presence of aggregate in the material, the strength of ITZ depends to a large extent on the cement’s class [56].The effect of the cement’s class is clearly visible after the thermal load. Samples made of the cement of higher class (CEM I 52.5R) achieved lower fct,f(T)the values by anload. average of 30%made from of thethe CEM The effect of the cement’s class is clearly visible after thermal Samples cement I 42.5R specimens. This is due to the material’s fragility. As the compressive strength of cement of higher class (CEM I 52.5R) achieved lower fct ,f (T) values by an average of 30% from the CEM I 42.5R composite increases, the material’s fragility also increases. Fragility, as a physical property of a specimens. This is due to the material’s fragility. As the compressive strength of cement composite material, consists in its cracking under the influence of external forces. Materials that are fragile increases, the material’s also increases. Fragility, as [1] a physical a material, consists absorb relatively littlefragility energy before cracking. The fragility of cementproperty matrix is of a ratio of bending in its tensile cracking under the influence of external forces. Materials that are fragile absorb relatively strength (fct,f) to compressive strength (fc). Then, the fragile material is such, for which the ratio little energy before The fragility [1] of cement matrix is a ratio of bending tensile strength (fct ,f ) to is less thancracking. 0.125. Results in Table 5 confirms that cement pastes with the highest compressive are usually compressive strength (fc ). Then, the fragile material is such, for which the ratiostrength is less than 0.125. the mostinfragile = 0.4). The of cement a much greater compressive effect on the fragility than, Results Table(w/c 5 confirms thatclass cement pasteshas with the highest strength are for usually example, the w/c ratio. For example, the fragility of the reference samples made of CEM I 52.5R is for the most fragile (w/c = 0.4). The class of cement has a much greater effect on the fragility than, higher by an average of 19%, than cement pastes made of CEM I 42.5R. After the temperature load, example, the w/c ratio. For example, the fragility of the reference samples made of CEM I 52.5R is this difference is over 2.5 times bigger, and is equal to 52%. higher by an average of 19%, than cement pastes made of CEM I 42.5R. After the temperature load, this difference is over 2.5 5.times bigger,ofand is equalcement to 52%. Table The fragility the modified pastes expressed in fct,f/fc ratio. Series w/c fct,f(R)/fc(R) Series fct,f(T)/fc(T) w/c

C42 C42CNT C52 C52CNT Table 5. The fragility of the modified cement pastes expressed in fct ,f /fc ratio. 0.4 0.094 0.076 0.4

0.5 0.100 C42 0.112 0.5

0.6 0.092 0.083 0.6

0.4 0.149 0.157 0.4

0.5 0.6 0.4 0.5 0.6 0.4 0.5 0.6 0.159 0.158 0.083 0.088 0.099 0.113 0.117 0.108 C42CNT C52 C52CNT 0.165 0.159 0.031 0.044 0.055 0.074 0.078 0.075 0.5 0.6 0.4 0.5 0.6 0.4 0.5 0.6

fct,f (R) /fThe 0.094 of 0.100 0.149 matrix 0.159has0.158 0.083 0.088 0.099 and 0.113 0.108 CNTs in0.092 the cement reduced fragility both before after0.117 exposure c (R) presence fct,f /fc (T) 0.076 0.112 by 0.083 0.165 0.159 0.031 0.044 to0.055 0.075 to(T)the elevated temperature 45% 0.157 and 77%, respectively. As the length width 0.074 ratio of 0.078 the CNTs

applied is significant (length 1500 times larger than diameter), the CNT anchorage in the hydrates is

The presence CNTsItinis the cement matrixtensile has reduced fragility both before and after exposure locally large inof length. likely that during stress growth in the cementitious material, the to theCNTs elevated 45% results and 77%, respectively. As the length to width of the slidetemperature in the anchor,by which in the change of fragility of the CNT pastesratio towards theCNTs materials with plastic properties [21,22]. The CNTs’ tensile strength is several tens of thousands of applied is significant (length 1500 times larger than diameter), the CNT anchorage in the hydrates times greater than the cement paste, so it is not possible to break CNTs at the stresses present in is locally large in length. It is likely that during tensile stress growth in the cementitious material, cement matrix. the CNTs slide in the anchor, which results in the change of fragility of the CNT pastes towards the materials with plastic properties [21,22]. The CNTs’ tensile strength is several tens of thousands of times greater than the cement paste, so it is not possible to break CNTs at the stresses present in cement matrix.


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When analyzing the decrease in bending tensile strength (Table 3) after temperature load, no strict relationship between this parameter and the w/c ratio was observed. As in the case of fc , the increased resistance of the CNT pastes to the thermal shock was observed because the decrease of fct ,f was in the range of 36–52%, and for the samples of the classical cement paste, 40–73%. Smaller losses in bending tensile strength showed samples made of CEM I 42.5R, which is indirectly related to the fragility phenomenon described above. 4.3. Correlations Between Parameters Table 6 shows a matrix with the Pearson’s correlation coefficients (r) between the investigated parameters of the cement pastes. It is worth noting that almost all parameters are very strongly (r > 0.9) or strongly (r > 0.7) correlated. A change in the parameters tested results in a closely linked change in the remaining characteristics of the cementitious material. This knowledge is useful for estimation purposes. Table 6. Correlation matrix of the parameters examined of the cement pastes. Parameters

fc (R)

fc (T)

fct ,f (R)

fct ,f (T)

D(R)

D(T)

S28(R)

S28(T)

∆S

fc(R) fc(R)

1.00 0.96 0.94 0.29 0.95 0.94 −0.96 −0.95 −0.92

1.00 0.92 0.26 0.84 0.82 −0.89 −0.84 −0.80

1.00 0.54 0.87 0.87 −0.93 −0.91 −0.88

1.00 0.27 0.29 −0.34 −0.33 −0.32

1.00 1.00 −0.96 −0.98 −0.98

1.00 −0.95 −0.98 −0.98

1.00 0.98 0.95

1.00 0.99

1.00

fct,f (R) fct,f (T) D(R) D(T) S28(R) S28(T) ∆S

Only the dependence of the bending tensile strength measured after the temperature load (fct ,f (T) ) with the other parameters is weakly correlated (r < 0.4), and this parameter is not suitable for proper estimation of other properties both before and after the impact of the thermal shock. 4.4. Microstructure of the Material SEM and EDS allowed qualitative and quantitative analysis of a local microstructure of the material. Below, selected images of microstructure of the cement pastes after thermal load are shown and described. Figure 17a shows the microstructure of a classic cement paste (C52). The CSH phase is shown in the form of small isometric grains of hydrated calcium silicates (1) and as a congested gel (2), which corresponds to the type III and IV phases of the CSH, according to the classification proposed by Diamond [57]. In the middle of the photo, there is a clear crack (3) with residual bridges formed from the CSH phase. In the upper part of Figure 17b, a portlandite plate (1) can be observed with hydrated calcium silicates. In the middle of the image (2) there is a CSH phase at borderline of II (the honeycomb CSH phase) and III Diamond’s type. A microcrack (3) passing through the CSH gel is also visible.


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Figure 17. SEM photos of the C52 cement paste microstructure (descriptions of the indications in the Figure 17. SEM photos of the C52 cement paste microstructure (descriptions of the indications in text) for (a) magnification 10,000×; (b) magnification 20,000×. the text) for (a) magnification 10,000×; (b) magnification 20,000×.

The use of CNTs in the form of an aqueous dispersion with SDS, as mentioned earlier, caused The useof ofthe CNTs in the form of anresult aqueous dispersion as mentioned earlier, caused the foaming cement matrix. The of this process iswith the SDS, increased porosity of the material, the foaming of the cement matrix. The result of this process is the increased porosity of the material, with the predominant proportion of pores with diameters greater than 50 μm. Most of these pores withoval theor predominant of pores greater than 50 µm. of these pores are spherical, asproportion shown in Figure 18a.with Suchdiameters pore shape is characteristic forMost the foamed cement are oval or spherical, as shown in Figure 18a. Such pore shape is characteristic for the foamed matrix. cement Thematrix. microstructure of the CNT paste differs from the classical cement paste, not only in terms of The the CNT paste fromof the classical cement not18b only in terms porosity, microstructure but also in theof morphology anddiffers chemistry the material. Thus,paste, Figure shows the of porosity, but also in the morphology and chemistry of the material. Thus, Figure 18b shows condensed CSH gel mainly type IV, with few monosulfates (1) interspersed with calcium hydroxide the condensed with fewwith monosulfates with calcium hydroxide crystals (2); theCSH areagel of mainly higher type CNTIV, coverage surfactant(1) (3)interspersed was also noted. This may indicate crystals (2); the area of higher CNT coverage with surfactant (3) was also noted. This may indicate insufficient dispersion of CNTs in the dispersion medium at the suspension preparation stage. The insufficient dispersion of CNTs in the dispersion medium at the during suspension preparation reason may also be the CNTs’ conglomeration into larger aggregates mechanical mixing.stage. The The reason may also be the CNTs’ conglomeration into larger aggregates during mechanical mixing. micro-cracks, which are present in the microstructure, mostly pass through the condensed CSH gel, The micro-cracks, are present in the microstructure, mostly through the condensed CSH gel, as shown in Figurewhich 18c. Figure 18d shows portlandite plates in thepass void of material that are connected as shown in Figure 18c. Figure 18d shows portlandite plates in the void of material that are connected by CNTs and thin ettringite needles. by CNTs andsame thin ettringite On the samples, needles. the EDS X-ray microanalysis was performed, the results of which are On the same samples, the EDS X-ray microanalysis was thermally performed, the results ofstable which shown in Table 4. Cement hydration products are predominantly and chemically at are shown in Table 4. Cement hydration products are predominantly thermally and chemically the furnace temperature (250 °C). Thus, the resulting chemical composition greatly reflects the stable at the furnace temperature (250 ◦being C). Thus, the resulting chemical composition greatly material’s composition in a state before subjected to the temperature load. According toreflects Zhang the material’s composition in a state before being subjected to the temperature load. According to and Ye [58], from 30 °C to 400 °C, the microstructure of cement paste does not change too much—the ◦ C to 400 ◦ C, the microstructure of cement paste does not change Zhang and Ye [58], from 30 volumes of capillary pore from room temperature to 400 °C is almost the same. Other literature too much—the volumes of capillary porerange fromup room temperature to 400 ◦ Cofisfree almost the same. reports show that within the temperature to 250 °C, the evaporation water and the ◦ C, the evaporation of free Other literature reports show that within the temperature range up to 250 decomposition of some components of cement matrix, such as tobermorite gel [59] and gypsum [60], water and the Also, decomposition of cement matrix, such ascontent tobermorite gel [59] mainly occurs. a reductionofinsome AFmcomponents phases (hydrated calcium aluminates) was observed and gypsum [60], mainly occurs. Also, a reduction in AFm phases (hydrated calcium aluminates) [61]. Ettringite is intrinsically unstable in a cement paste above about 70 °C, but if enough sulfate is ◦ C, but content was observed [61]. Ettringite is intrinsically unstable paste 70 cement present, it can exist in cement pastes at temperatures up in to aatcement least 90 °C above [61,62].about Other if enough sulfate is present, it can exist cement pastesstability at temperatures up to at least 90 ◦analyzed. C [61,62]. hydration products are characterized byin a high chemical in the temperature range Other cement hydration products are characterized by a high chemical stability in the temperature range analyzed.


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Figure 18. 18. SEM ofof the indications in Figure SEM photos photos of ofthe theC52CNT C52CNTcement cementpaste pastemicrostructure microstructure(descriptions (descriptions the indications the text) for (a) magnification 1000×; (b) magnification 25,000×; (c) magnification 20,000×; (d) in the text) for (a) magnification 1000×; (b) magnification 25,000×; (c) magnification 20,000×; magnification 40,000×. (d) magnification 40,000×.

Characteristic for hardened cementitious materials is the high content of SiO2 and CaO, as these Characteristic for hardened cementitious materials is the high content of SiO2 and CaO, as these oxides correspond to the CSH phase (in the 28 day cement paste, the content of the CSH phase can oxides correspond to the CSH phase (in the 28 day cement paste, the content of the CSH phase can exceed 70–80%), and cement hydration products, such as portlandite. The EDS and SEM analysis exceed 70–80%), and cement hydration products, such as portlandite. The EDS and SEM analysis confirmed that hydrated calcium silicates present in various morphological forms are the primary confirmed that hydrated calcium silicates present in various morphological forms are the primary constituent of the hardened cement paste. constituent of the hardened cement paste. In the case of the CNT samples, the SiO2 content is noticeably lower (17.2% less than the In the case of the CNT samples, the SiO2 content is noticeably lower (17.2% less than unmodified cement paste). Additionally, the reduction of aluminum and magnesium phases were the unmodified cement paste). Additionally, the reduction of aluminum and magnesium phases observed (23.1% and 12.8%, respectively). A weight content reduction of hydrated phases is caused were observed (23.1% and 12.8%, respectively). A weight content reduction of hydrated phases is by additional porosity, which is the main reason for the decrease in the material strength. caused by additional porosity, which is the main reason for the decrease in the material strength. Modification of the cement matrix due to the presence of SDS was also detected—the SO3 content Modification of the cement matrix due to the presence of SDS was also detected—the SO content is greater by 37.4%, compared to the conventional cement paste. It is also interesting to note3 that the is greater by 37.4%, compared to the conventional cement paste. It is also interesting to note that CNT paste is characterized by the higher content of sulfur than aluminum compounds. This is rarely the CNT paste is characterized by the higher content of sulfur than aluminum compounds. This is observed when Portland cement is used. rarely observed when Portland cement is used. Literature analysis indicates the very high stability of CNTs in water dispersion with addition Literature analysis indicates the very high stability of CNTs in water dispersion with addition of SDS, however, the surfactant used adversely modifies the chemistry of cement matrix. For this of SDS, however, the surfactant used adversely modifies the chemistry of cement matrix. For this purpose, alternative surfactants should be sought, which would result in a good degree of CNT purpose, alternative surfactants should be sought, which would result in a good degree of CNT dispersion in water, with no reactivity (or reduction of adverse effects to the minimum) with the dispersion in water, with no reactivity (or reduction of adverse effects to the minimum) with the cement cement paste components. paste components. Another option to improve performance of the CNT paste may be the application of defoaming agents [23,25,63]. For example, tests were performed using oligomers of polyether [64] and tributyl


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Another option to improve performance of the CNT paste may be the application of defoaming agents [23,25,63]. For example, tests were performed using oligomers of polyether [64] and tributyl phosphate [65] as defoaming agents. In both cases, defoamer improved ultrasonic SDS dispersion by breaking thick bundles into thinner pieces, and decreased the amount of air bubbles. However, further studies on the use of CNT in the presence of SDS, due to the foaming properties of the surfactant, may be the basis for the development of a modified cement binder used in the production of aerated concretes, or in combination with lightweight aggregates, for the production of lightweight concretes. 5. Conclusions CNT is the material that has revolutionized materials engineering, and scientists around the world are exploring the use of CNTs in many areas (e.g., medicine, electronics, materials, etc.). This paper describes the results of investigations concerning the possibility of using CNTs to modify the cement matrix. A number of physical and mechanical parameters were examined, including apparent density, linear shrinkage, bending tensile and compressive strength. The correlations between these parameters were calculated. The microstructure of the modified cement pastes was also analyzed by means of SEM and EDS. Moreover, the behavior of the CNT cementitious material subjected to the short-term thermal stress at 250 ◦ C was investigated, which is the first such test described in the literature. A thorough analysis of the obtained results enables the formulation of the following conclusions: 1.

2.

3.

4. 5. 6. 7.

8. 9.

10.

The use of the aqueous CNT dispersion using the SDS surfactant caused foaming of the cement paste. This is due to the fact that SDS, as an anionic surfactant, has characteristics specific to blowing agents. Due to the foam effect, cement pastes with CNTs were characterized by a lower apparent density, by 31% compared to the conventional cement pastes; the same density difference occurred both before and after the thermal load. Samples containing CNTs were characterized by a large initial growth rate of shrinkage (after 3 days of maturing, the shrinkage was as much as 140% greater than the unmodified samples); at the time of maturing, the increase in shrinkage significantly decreased. After the temperature load, the change in linear shrinkage of samples with and without CNT was similar, and ranged from 135–174% with respect to the reference values. A very high dependence between shrinkage and apparent density of modified cement pastes was observed (correlation coefficients ranged from −0.95 to −0.98). CNT modified cement pastes have an average of 50% less compressive strength compared to the conventional cement matrix, due to the increased porosity. The CNT addition increased the resistance of the cement matrix to elevated temperatures in terms of the compressive strength; in the case of CEM I 52.5R, the reduction in the compressive strength of CNT modified samples ranged between 25–30%, and for unmodified samples, 29–46%. Bending tensile strength of the pastes with CNT (before thermal load) was about 30% lower than the pastes without the additive. After exposure to the elevated temperature, in the samples containing CEM I 52.5R, CNT modified cement pastes achieved about 18% greater bending tensile strength compared to the unmodified samples. It has been found that the CNT additive forms mechanical connections with the cement hydration products in the material’s nano-structure, and improves the cohesion of the cement paste, both in terms of its stretching and compression. This is important because of the CNT application method used, where a foam effect was achieved—a significant reduction in the weight of the cementitious material. Increased thermal resistance of the cement pastes with CNTs in the aspect of the bending tensile strength of cement matrix was observed; the reduction of this parameter for the CNT samples ranged between 36–52%, and for the samples of the classical cement paste, 40–73%.


11.

12.

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SEM and EDS analysis allowed for the identification of structural differences between individual samples of the cementitious material. It has been noted that the presence of CNTs with SDS adversely affects the chemistry of the cement paste (e.g., lower content of SiO2 , greater content of CaO, SO3 ). Further studies on the use of CNTs in conjunction with SDS, for the modification of cement matrix, may contribute to the development of a modified cement binder used in the manufacture of lightweight or aerated concrete.

Acknowledgments: This work was financially supported by the Polish Ministry of Science and Higher Education with the statutory research number S-14/B/2016 and the “Młoda Kadra” project—507-3-14-140. Conflicts of Interest: The author declares no conflict of interest.

References 1. 2.

3. 4.

5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20.

Neville, A.M. Properties of Concrete, 4th ed.; Arkady: Warszawa, Poland, 2000. Szelag, ˛ M. Wpływ Składu Kompozytów Cementowych Na Geometri˛e Ich Sp˛ekan´ Termicznych (The Influence of Cement Composites’ Composition on the Geometry of Their Thermal Cracks); Lublin University of Technology: Lublin, Poland, 2017. Kurdowski, W. Chemia Cementu I Betonu (Chemistry of Cement and Concrete); Wydawnictwo Polski Cement, Wydawnictwo Naukowe PWN: Kraków, Poland, 2010. Zegardlo, B.; Szelag, M.; Ogrodnik, P. Ultra-high strength concrete made with recycled aggregate from sanitary ceramic wastes—The method of production and the interfacial transition zone. Constr. Build. Mater. 2016, 122, 736–742. [CrossRef] Wang, Y.; Wu, H.; Li, V. Concrete reinforcement with recycled fibers. J. Mater. Civ. Eng. 2000, 12, 314–319. [CrossRef] Zollo, R. Fiber-reinforced concrete: An overview after 30 years of development. Cem. Concr. Compos. 1997, 19, 107–122. [CrossRef] Naaman, A.; Wongtanakitcharoen, T.; Hauser, G. Influence of different fibers on plastic shrinkage cracking of concrete. ACI Mater. J. 2005, 102, 49–58. Fic, S.; Brzyski, P.; Szelag, ˛ M. Composite based on foam lime mortar with flax fibers for use in the building industry. Ecol. Chem. Eng. A 2013, 20, 899–907. Cwirzen, A.; Habermehl-Cwirzen, K.; Penttala, V. Surface decoration of carbon nanotubes and mechanical properties of cement/carbon nanotube composites. Adv. Cem. Res. 2008, 20, 65–73. [CrossRef] Wang, B.; Liu, S.; Han, Y.; Leng, P. Preparation and durability of cement-based composites doped with multi-walled carbon nanotubes. Nanosci. Nanotechnol. Lett. 2015, 7, 411–416. [CrossRef] Ibragimov, R.; Izotov, V. Effect of carbon nanotubes on the structure and properties of cement composites. Inorg. Mater. 2015, 51, 834–839. [CrossRef] Luo, J.; Duan, Z.; Zhao, T.; Li, Q.; Jiao, S.; Jiang, Z.; Bu, J. Effect of multi-wall carbon nanotube on fracture mechanical property of cement-based composite. Adv. Superalloys 2011, 146–147, 581. [CrossRef] Manzur, T.; Yazdani, N.; Emon, M. Potential of carbon nanotube reinforced cement composites as concrete repair material. J. Nanomater. 2016, 2016. [CrossRef] Ramsden, J. Nanotechnology: An introduction; William Andrew: Norwich, NY, USA, 2011. Stankiewicz, N.; Lelusz, M. Nanotechnologia w budownictwie—Przeglad ˛ zastosowan´ (Nanotechnology in ´ civil engineering—Application review). Bud. I Inz˙ ynieria Srodowiska 2014, 5, 101–112. Czarnecki, L. Nanotechnologia w budownictwie (Nanotechnology in construction). Prz. Bud. 2011, 1, 40–53. Kellsall, R.W.; Hamley, I.W.; Geoghegan, M. Nanotechnologie (Nanotechnologies); Wydawnictwo Naukowe PWN: Warszawa, Poland, 2009. Huczko, A. Nanorurki W˛eglowe. CZARNE Diamenty XXI Wieku (Carbon Nanotubes. Black Diamonds of the 21st Century); BEL Studio: Warszawa, Poland, 2004. Huczko, A. Fulereny i nanorurki (Fulerens and nanotubes). Academia 2006, 2, 16. Zharikov, V.V.; Yezerskii, V.A.; Kuznetsova, N.V.; Sterkhov, I.I. Tsementny kompozitsionny materialy s ispol’zovaniemotkhodovlitye˘ınogoproizvodstva. Tekhnologiibetonov 2011, 11–12, 19–21.


21. 22.

23. 24.

25. 26. 27. 28. 29. 30.

31. 32. 33.

34. 35. 36. 37. 38. 39.

40. 41.

42. 43. 44.

21 of 22

Konsta-Gdoutos, M.S.; Metaxa, Z.S.; Shah, S.P. Highly dispersed carbon nanotube reinforced cement based materials. Cem. Concr. Res. 2010, 40, 1052–1059. [CrossRef] Konsta-Gdoutos, M.S.; Metaxa, Z.S.; Shah, S.P. Multi-scale mechanical and fracture characteristics and early-age strain capacity of high performance carbon nanotube/cement nanocomposites. Cem. Concr. Compos. 2010, 32, 110–115. [CrossRef] Li, G.Y.; Wang, P.M.; Zhao, X. Mechanical behaviour and microstructure of cement composites incorporating surface-treated multi-walled carbon nanotubes. Carbon 2005, 43, 1239–1245. [CrossRef] Makar, J.; Margeson, J.; Luh, J. Carbon Nanotube/Cement Composites—Early Results and Potential Applications. In Proceedings of the 3rd International Conference on Construction Materials: Performance, Innovations and Structural Implications, Vancouver, BC, Canada, 22–24 August 2005; pp. 1–10. Li, G.Y.; Wang, P.M.; Zhao, X. Pressure-sensitive properties and microstructure of carbon nanotube reinforced cement composites. Cem. Concr. Res. 2007, 29, 377–382. [CrossRef] Vaisman, L.; Wagner, D.H.; Marom, G. The role of surfactants in dispersion of carbon nanotubes. Adv. Coll. Interface Sci. 2006, 128, 37–46. [CrossRef] [PubMed] Yu, J.; Grossiord, N.; Koning, C.E.; Loos, J. Controlling the dispersion of multi-wall carbon nanotubes in aqueous surfactant solution. Carbon 2007, 45, 618–623. [CrossRef] Jiang, L.; Gao, L.; Sun, J. Production of aqueous colloidal dispersions of carbon nanotubes. J. Coll. Interface Sci. 2003, 260, 89–94. [CrossRef] Rausch, J.; Zhuang, R.; Mader, E. Surfactant assisted dispersion of functionalized multi-walled carbon nanotubes in aqueous media. Compos. Part A Appl. Sci. Manuf. 2010, 41, 1038–1046. [CrossRef] Grossiord, N.; Regev, O.; Loos, J.; Meuldijk, J.; Koning, C. Time-dependent study of the exfoliation process of carbon nanotubes in aqueous dispersions by using UV-visible spectroscopy. Anal. Chem. 2005, 77, 5135–5139. [CrossRef] [PubMed] Rastogi, R.; Kaushal, R.; Tripathi, S.; Sharma, A.; Kaur, I.; Bharadwaj, L. Comparative study of carbon nanotube dispersion using surfactants. J. Coll. Interface Sci. 2008, 328, 421–428. [CrossRef] [PubMed] Liu, Q.; Sun, W.; Sun, B.; Sun, X.; Jia, L.; Ma, Z. Preparation of carbon nanotubes solution and its effectson mechanical properties of cement mortar. J. Southeast Univ. (Nat. Sci. Ed.) 2014, 44, 662–667. Adresi, M.; Hassani, A.; Javadian, S.; Tulliani, J. Determining the surfactant consistent with concrete in order to achieve the maximum possible dispersion of multiwalled carbon nanotubes in keeping the plain concrete properties. J. Nanotechnol. 2016, 2016. [CrossRef] Adresi, M.; Hassani, A.; Tulliani, J.; Lacidogna, G.; Antonaci, P. A study of the main factors affecting the performance of self-sensing concrete. Adv. Cem. Res. 2017, 29, 216–226. [CrossRef] Han, B.; Zhang, K.; Yu, X.; Kwon, E.; Ou, J. Fabrication of piezoresistive CNT/CNF cementitious composites with superplasticizer as dispersant. J. Mater. Civ. Eng. 2012, 24, 658–665. [CrossRef] Bogue, R.H. The Chemistry of Portland Cement; Reinhold Publishing Corporation: Stafford, TX, USA, 1947. En 196-1; Methods of Testing Cement—Part 1: Determination of Strength; European Committee for standardization: Brussels, Belgium, 2016. Fic, S.; Szelag, M. Analysis of the development of cluster cracks caused by elevated temperatures in cement paste. Constr. Build. Mater. 2015, 83, 223–229. [CrossRef] Szelag, ˛ M.; Fic, S. The use of image analysis techniques to describe a cluster cracks in the cement paste with the addition of metakaolinite. In Proceedings of the 18th International Conference on Civil, Environmental and Structural Engineering, Kuala Lumpur, Malaysia, 11–12 February 2016. Szelag, ˛ M.; Fic, S. Analiza rozwoju sp˛ekan´ klastrowych w zaczynie cementowym modyfikowanym mikrokrzemionka.˛ Bud. I Archit. 2015, 14, 117–127. Szelag, ˛ M. The influence of microsilica on the cluster cracks’ geometry of cement paste. In Proceedings of the 18th International Conference on Civil Engineering and Building Materials, Rome, Italy, 8–9 December 2016. En 12390-7; Testing Hardened Concrete—Part 7: Density of Hardened Concrete; European Committee for Standardization: Brussels, Belgium, 2009. En 12390-5; Testing Hardened Concrete—Part 5: Flexural Strength of Test Specimens; European Committee for Standardization: Brussels, Belgium, 2009. En 12390-3; Testing Hardened Concrete—Part 3: Compressive Strength of Test Specimens; European Committee for Standardization: Brussels, Belgium, 2009.


45.

46.

47. 48.

49. 50. 51. 52. 53.

54. 55.

56. 57. 58.

59. 60. 61. 62. 63. 64. 65.

22 of 22

Collepardi, M.; Coppola, L.; Zaffaroni, P. Progress in concrete science and technology for maritime works. In Proceedings of the International Conference “Innovative Marine Materials and Technologies”, La Spezia, Italy, 1–3 October 1992; pp. 11–22. Sobolkina, A.; Mechtcherine, V.; Khavrus, V.; Maier, D.; Mende, M.; Ritschel, M.; Leonhardt, A. Dispersion of carbon nanotubes and its influence on the mechanical properties of the cement matrix. Cem. Concr. Compos. 2012, 34, 1104–1113. [CrossRef] Richard, C.; Balavoine, F.; Schultz, P.; Ebbesen, T.; Mioskowski, C. Supramolecular self-assembly of lipid derivatives on carbon nanotubes. Science 2003, 300, 775–778. [CrossRef] [PubMed] Strano, M.; Moore, V.; Miller, M.; Allen, M.; Haroz, E.; Kittrell, C.; Hauge, R.; Smalley, R. The role of surfactant adsorption during ultrasonication in the dispersion of single-walled carbon nanotubes. J. Nanosci. Nanotechnol. 2003, 3, 81–86. [CrossRef] [PubMed] Alexanderson, J. Relations between structure and mechanical-properties of autoclaved aerated concrete. Cem. Concr. Res. 1979, 9, 507–514. [CrossRef] Narayanan, N.; Ramamurthy, K. Structure and properties of aerated concrete: A review. Cem. Concr. Compos. 2000, 22, 321–329. [CrossRef] Kim, H.; Jeon, J.; Lee, H. Workability, and mechanical, acoustic and thermal properties of lightweight aggregate concrete with a high volume of entrained air. Constr. Build. Mater. 2012, 29, 193–200. [CrossRef] Chen, S.; Zou, B.; Collins, F.; Zhao, X.; Majumber, M.; Duan, W. Predicting the influence of ultrasonication energy on the reinforcing efficiency of carbon nanotubes. Carbon 2014, 77, 1–10. [CrossRef] Parveen, S.; Rana, S.; Fangueiro, R.; Paiva, M. Microstructure and mechanical properties of carbon nanotube reinforced cementitious composites developed using a novel dispersion technique. Cem. Concr. Res. 2015, 73, 215–227. [CrossRef] Xu, S.; Liu, J.; Li, Q. Mechanical properties and microstructure of multi-walled carbon nanotube-reinforced cement paste. Constr. Build. Mater. 2015, 76, 16–23. [CrossRef] Cwirzen, A.; Habermehl-Cwirzen, K.; Shandakov, D.; Nasibulina, L.; Nasibulin, A.; Mudimela, P.; Kauppinen, E.; Penttala, V. Properties of high yield synthesised carbon nano fibres/portland cement composite. Adv. Cem. Res. 2009, 21, 141–146. [CrossRef] Scrivener, K.; Crumbie, A.; Laugesen, P. The interfacial transition zone (ITZ) between cement paste and aggregate in concrete. Interface Sci. 2004, 12, 411–421. [CrossRef] Diamond, S. The microstructures of cement paste in concrete. In Proceedings of the 8th International Congress on the Chemistry of Cement, Rio de Janeiro, Brazil, 22–27 September 1986; Volume 2, pp. 122–147. Zhang, Q.; Ye, G.; Fai, L. Microstructure analysis of heated portland cement paste. In Proceedings of the Twelfth East Asia-Pacific Conference on Structural Engineering and Construction (EASEC12), Hong Kong, China, 26–28 January 2011; Volume 14. Naus, D.J. The Effect of Elevated Temperature on Concrete Materials and Structures—A Literature Review (No. Ornl/tm-2005/533); Oak Ridge National Laboratory: Oak Ridge, TN, USA, 2006. Alarcon-Ruiz, L.; Platret, G.; Massieu, E.; Ehrlacher, A. The use of thermal analysis in assessing the effect of temperature on a cement paste. Cem. Concr. Res. 2005, 35, 609–613. [CrossRef] Taylor, H.; Famy, C.; Scrivener, K. Delayed ettringite formation. Cem. Concr. Res. 2001, 31, 683–693. [CrossRef] Zhou, Q.; Glasser, F. Thermal stability and decomposition mechanisms of ettringite at

In vitro evaluation of carbon-nanotube-reinforced bioprintable vascular conduits.

A Thermal Model for Carbon Nanotube Interconnects.

polymer composites.

Effect of carbon nanotube functionalization on mechanical and thermal properties of cross-linked epoxy-carbon nanotube nanocomposites: role of strengthening the interfacial interactions.

Investigation on the Mechanical Properties of a Cement-Based Material Containing Carbon Nanotube under Drying and Freeze-Thaw Conditions.

Mechanical Properties and Microstructure of Class C Fly Ash-Based Geopolymer Paste and Mortar.

Extremely High Thermal Conductivity of Aligned Carbon Nanotube-Polyethylene Composites.

Enhanced thermal transport at covalently functionalized carbon nanotube array interfaces.

Thermophysical and Mechanical Properties of Hardened Cement Paste with Microencapsulated Phase Change Materials for Energy Storage.

Control of porosity and pore size of metal reinforced carbon nanotube membranes.

Data for the physical and mechanical properties of staple fibers cement paste composites.

Thermal rectification in a polymer-functionalized single-wall carbon nanotube.

Carbon nanotube-reinforced hydroxyapatite composite and their interaction with human osteoblast in vitro.

Functionalized carbon nanotube reinforced scaffolds for bone regenerative engineering: fabrication, in vitro and in vivo evaluation.

Carbon Nanotube Membranes: Synthesis, Properties, and Future Filtration Applications.

Nano-Silica Sol-Gel and Carbon Nanotube Coupling Effect on the Performance of Cement-Based Materials.

Investigation of the Mechanical Properties and Microstructure of Graphene Nanoplatelet-Cement Composite.

Characterization of multiwalled carbon nanotube-reinforced hydroxyapatite composites consolidated by spark plasma sintering.

Nanoscale resolution scanning thermal microscopy using carbon nanotube tipped thermal probes.

Free vibration of functionally graded carbon-nanotube-reinforced composite plates with cutout.

Investigating the Suitability of Carbon Nanotube Reinforced Polymer in Transcatheter Valve Applications.

single-walled carbon nanotube composites.

Synergetic chemical coupling controls the uniformity of carbon nanotube microstructure growth.

Carbon nanotube buckypaper reinforced acrylonitrile-butadiene-styrene composites for electronic applications.