nanomaterials Article

Effect of Nano-SiO2 on the Hydration and Microstructure of Portland Cement Liguo Wang 1 , Dapeng Zheng 2 , Shupeng Zhang 1 , Hongzhi Cui 2, * and Dongxu Li 1,2, * 1 2

*

Jiangsu National Synergetic Innovation Center for Advanced Materials (SICAM), Nanjing Tech University, Nanjing 211800, China; [email protected] (L.W.); [email protected] (S.Z.) Guangdong Provincial Key Laboratory of Durability for Marine Civil Engineering, College of Civil Engineering, Shenzhen University, Shenzhen 518060, China; [email protected] Correspondence: [email protected] (H.C.); [email protected] (D.L.); Tel.: +86-755-2691-6424 (H.C.)

Academic Editor: Thomas Nann Received: 7 November 2016; Accepted: 6 December 2016; Published: 15 December 2016

Abstract: This paper systematically studied the modification of cement-based materials by nano-SiO2 particles with an average diameter of about 20 nm. In order to obtain the effect of nano-SiO2 particles on the mechanical properties, hydration, and pore structure of cement-based materials, adding 1%, 3%, and 5% content of nano-SiO2 in cement paste, respectively. The results showed that the reaction of nano-SiO2 particles with Ca(OH)2 (crystal powder) started within 1 h, and formed C–S–H gel. The reaction speed was faster after aging for three days. The mechanical properties of cement-based materials were improved with the addition of 3% nano-SiO2 , and the early strength enhancement of test pieces was obvious. Three-day compressive strength increased 33.2%, and 28-day compressive strength increased 18.5%. The exothermic peak of hydration heat of cement increased significantly after the addition of nano-SiO2 . Appearance time of the exothermic peak was advanced and the total heat release increased. Thermogravimetric-differential scanning calorimetry (TG-DSC) analysis showed that nano-SiO2 promoted the formation of C–S–H gel. The results of mercury intrusion porosimetry (MIP) showed that the total porosity of cement paste with 3% nano-SiO2 was reduced by 5.51% and 5.4% at three days and 28 days, respectively, compared with the pure cement paste. At the same time, the pore structure of cement paste was optimized, and much-detrimental pores and detrimental pores decreased, while less harmful pores and innocuous pores increased. Keywords: nano-SiO2 ; cement-based materials; physical and mechanical properties; porosity

1. Introduction Cement-based materials are widely used as building materials around the world. With the development of cement-based materials, its performance is becoming more and more important in modern construction. There are many defects in ordinary cement materials, which will have an adverse impact on the mechanical properties and durability. In recent years, many researchers have conducted many studies on cement modification through adding mineral admixtures in cement, and have achieved significant progress [1,2]. With the development of nanotechnology, its application in cement-based materials has become a research hotspot. Nanomaterials are functional materials with many excellent properties, such as size effects, quantum effects, surface effects, and interfacial effects [3]. These properties can enhance the physical and chemical properties of cement, and open up new areas for cement research. Some researchers used nanomaterial as an additive added into cement-based materials, and have obtained some remarkable results. Recently, many nanomaterials, such as nano-TiO2 [4–8], nano-CaCO3 [9,10], nano-Al2 O3 [11–13], and carbon nano-tubes [14] have been added in cement-based materials to improve various properties of cement. Compared with other nano-materials, nano-SiO2

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has more advantages, since it has a higher pozzolanic activity. Nano-SiO2 has a retardation effect on the cement paste structure, and fills the voids between the cement particles [15]. It was reported that nano-SiO2 can promote the hydration of cement and generated more C–S–H gel [16,17]. Researchers have done much work on the influence of nano-SiO2 on the hydration and mechanical properties of cement. The Table 1 shows the improvement of properties of cementitious composites with nano-SiO2 . In addition, many researchers studied the influence of nano-SiO2 on the durability of concrete, and the results showed that the suitable content of nano-SiO2 can improve the durability of concrete significantly [18–21]. Table 1. Improvement of properties of cementitious composites with Nano-SiO2 . Nano-SiO2

Properties (Improvement)

Particle Size

Concentration

Compressive Strength

Porosity

14 nm 15 nm 15 nm 15 + 5 nm 30 nm 50 nm 80 nm 30–100 nm 120 nm

0.5% 1.5% 7.5% 10% 2.5% 6% 1.0% 5% 4%

25% 23.88% 12.96% 26% 16% 29.88% 13.71% 22.85% 35.86%

−10.2% −1.0%

Ages

References

28 days 90 days 90 days 28 days 28 days 28 days 90 days 28 days 28 days

Stefanidou [22] Naji [15] Hesam [19] Li [23] Zhu [16] Najat [24] Naji [15] Kim [25] Yu [17]

However, little research has been done on the microstructure and macroscopic properties in the same research. In this paper, the influence of nano-SiO2 particles on the hydration, microstructure and mechanical properties of cement was studied systematically by means of hydration heat, X-ray diffraction (XRD), thermogravimetric-differential scanning calorimetry (TG-DSC), scanning electron microscopic (SEM) observation, and mercury intrusion porosimetry (MIP). The impact of nano-SiO2 particles on the hydration process and pore structure of cement at early ages was studied emphatically, which will have a certain effect on the influence of nano-SiO2 on the hydration mechanism of cement and provide a basis for deep research for future generations. 2. Materials and Methods 2.1. Materials and Mix Proportions 2.1.1. Materials The cement used for this experiment was PII 52.5 N ordinary Portland cement supplied by Jiangnan onoda cement Co., Ltd. (Nanjing, China). The physical properties and chemical compositions of Portland cement were showed in Tables 2 and 3. The standard sand used in this study was obtained from China ISO standard sand Co., Ltd. (Xiamen, China). Nano-SiO2 was obtained from Nanjing TANSAIL Advanced Materials Co. Ltd. (Nanjing, China). Characterization of the used nano-SiO2 were shown in Table 4. The water used in this study was all local tap water. Table 2. Physical properties of Portland cement.

Type

Density (g/cm3 )

Surface Area (m2 /kg)

Normal Consistency (%)

PII 52.5

3.12

372

0.30

Setting Time/Min

Compressive Strength/MPa

Flexural Strength/MPa

Initial

Final

3 days

28 days

3 days

28 days

180

260

5.10

8.15

30.75

54.04

Table 3. Chemical compositions of Portland cement/wt %. Type

CaO

SiO2

Al2 O3

Fe2 O3

SO3

MgO

K2 O

Ignition Loss

PII 52.5

64.95

18.31

4.21

2.95

4.22

0.64

0.788

3.21

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Table 4. Characterization of the used nano-silica Mean Particle Surface Purity % pH 2/g) Mean Particle Surface Size (nm) Purity % pH Area (m 2

Type

Appearance

Type

Appearance

TSP-H10 TSP-H10

White powder

Size (nm)

White powder

a.

Area (m /g)

20 20

>99.5 >99.5

4–74–7

Density 3) Density (g/cm 3

Surface Surface Property

0.10 0.10

hydrophilic hydrophilic

Property

(g/cm )

300300

a Data obtained from the supplier.

a Data obtained from the supplier.

0

10

20

30

40

50

60

70

80

90

2()

(a)

(b)

Figure1. 1. (a) (a) Scanning Scanning electron electron microscopic microscopic (SEM) (SEM) patterns patterns of of nano-silica; nano-silica; (b) (b) X-ray X-ray diffraction diffraction (XRD) (XRD) Figure patternsof ofnano-silica. nano-silica. patterns

The composition and morphology of nano-silica can be analyzed by scanning electron The composition and morphology of nano-silica can be analyzed by scanning electron microscopic microscopic (SEM) observation and X-ray diffraction (XRD). Nano-silica showed in Figure 1 and the (SEM) observation and X-ray diffraction (XRD). Nano-silica showed in Figure 1 and the mean particle mean particle size of nano-silica was about 20 nm. The samples were scanned from 5° to 85° 2θ at a size of nano-silica was about 20 nm. The samples were scanned from 5◦ to 85◦ 2θ at a scanning speed scanning speed of 10°/min (Figure 1). The crystallization degree of nano-SiO2 is very poor which of 10◦ /min (Figure 1). The crystallization degree of nano-SiO2 is very poor which revealed the revealed the nano-silica possesses very strong reactive activity. nano-silica possesses very strong reactive activity. 2.1.2. Sample Sample Preparation Preparationfor forNano-SiO Nano-SiO2 and and Ca(OH) Ca(OH)2 Reaction Reaction Test Test 2.1.2. 2 2 Inthe thesystem systemofofnano-SiO nano-SiO2 2-Ca(OH) -Ca(OH)22-H -H22O, O, Ca(OH) Ca(OH)22,, nano-SiO nano-SiO22,, and mass ratio ratio were were shown shown In and water water mass in Table 5. Nano-SiO 2 and Ca(OH) 2 powder adequately mixed two times in a mechanical agitator (4000 in Table 5. Nano-SiO2 and Ca(OH)2 powder adequately mixed two times in a mechanical agitator r/min), and 15 min each time. The preparation of nano-SiO 2 and Ca(OH) 2 reaction samples followed (4000 r/min), and 15 min each time. The preparation of nano-SiO2 and Ca(OH)2 reaction samples GB/T 1346-2001. followed GB/T 1346-2001. Table Table5.5.Mix Mixproportions proportionsof ofpaste pastemade madefrom fromCa(OH) Ca(OH)2 2, ,nano-SiO nano-SiO22,, and and water. water.

Number Number A

A

Nano-SiO(g) 2 (g) Nano-SiO 2 100

100

Ca(OH) Ca(OH)22(g) (g) 54 54

WaterWater (g) (g) 230 230

2.1.3. Preparation of Cement Mortar 2.1.3. Preparation of Cement Mortar The mix proportions were shown in Table 6. In the system 1 wt %, 3 wt %, and 5 wt % nano-SiO2 The mix proportions were shown in Table 6. In the system 1 wt %, 3 wt %, and 5 wt % nano-SiO2 (by cement mass) was added in cement. In order to keep with the same fluidity of cement paste, 0.13 (by cement mass) was added in cement. In order to keep with the same fluidity of cement paste, wt %, 0.26 wt % superplasticizer was adding into cement mortar. In order to solve the problem of 0.13 wt %, 0.26 wt % superplasticizer was adding into cement mortar. In order to solve the problem aggregation of nano-SiO2, the pre-process method of ultrasonic dispersion was adopted. Specific mixing of aggregation of nano-SiO2 , the pre-process method of ultrasonic dispersion was adopted. Specific process is as follows: all nano-SiO2 and water were mixed first, and using an ultrasonic machine with mixing process is as follows: all nano-SiO2 and water were mixed first, and using an ultrasonic 50 W power for 5 min. Then superplasticizer was mixed in a mortar mixer with sand and cement for 2 machine with 50 W power for 5 min. Then superplasticizer was mixed in a mortar mixer with sand and min at low low speed the sonicated mixture wasmixture added and min at cement for 2speed. min atAfter low speed. Aftermixing, low speed mixing, the sonicated wasmixed addedfor and1 mixed low1 speed min atand high speed. Cement mortar wasmortar cast in was the mold of 4 cm × 4 for min atand low2speed 2 min at high speed. Cement cast inwith the dimensions mold with dimensions cm × 16 cm immediately after mixing. The specimens were striped after 24 h and cured in water at in 20 of 4 cm × 4 cm × 16 cm immediately after mixing. The specimens were striped after 24 h and cured ◦ ± 1 °C for specified ages (3 days, 7 days, and 28 days). Three samples of each mortar type were water at 20 ± 1 C for specified ages (3 days, 7 days, and 28 days). Three samples of each mortar type subjected to flexural andand compressive strength tests each of cement cement were subjected to flexural compressive strength tests eachtime. time.The Thebasic basic properties properties of samples tested were based on GB/T 17671-1999. samples tested were based on GB/T 17671-1999.

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Table 6. Mix proportions of the samples. Number

Cement (g)

SiO2 (%) (by Cement Mass)

Sand (g)

Water (g)

Superplasticizer (%)

1 2 3 4

450 450 450 450

0 1 3 5

1350 1350 1350 1350

225 225 225 225

0 0 0.13% 0.26%

2.2. Testing Procedures In this experiment, different contents of nano-SiO2 were added with molding paste. The resulting cement paste was immediately poured into a 10 mm × 10 mm × 10 mm mold. Then the samples were cured at 20 ± 1 ◦ C. After 24 h, the specimens were removed from the mold and cured at the same condition samples are packed in small bottles with anhydrous ethanol for microscopic tests. Microstructural properties of hydrated cubes were evaluated. XRD was used to analyze the hydration products phase of cement pastes. A few samples dried at 50 ◦ C for 12 h were crushed and grounded to powder. XRD was performed using a D max/RB diffractometer (Rigaku, Tokyo, Japan) with a copper target, 40 kV, 100 mV. The scan range was 5◦ –80◦ , 0.02◦ /step, 0.4 s/step. The hydration exothermic rate of each paste was measured by a TAM air calorimeter (TA Instruments Co., New Castle, DE, USA) to assess the effect of nano-SiO2 on the hydration of cement paste. The w/c is 0.5, and the hydration exothermic rate within 72 h was tested. A scanning electron microscope (SEM, JMS-5900, JEOL, Tokyo, Japan) was used to analyze the morphology of cement paste. Small fractured samples at every hydration age were soaked in anhydrous ethanol to stop hydration and dried at 50 ◦ C for 12 h. Then the sample was coated with 20 nm of gold to make it conductive. Differential thermal analysis (TG-DSC, NETZSCH, ATA409, NETZSCH, Selb, Germany) was used to test the absorption capacity of nano-SiO2 . In this test, the heating rate is 20 ◦ C/min. The pore sizes of cement samples were tested by mercury intrusion porosimetry (MIP, poremaster-60, Quantachrome, Houston, TX, USA). A few samples dried at 50 ◦ C for 12 h were crushed into 2–5 mm small pieces. The pressure of mercury was fixed at 30,000 psi. 3. Results and Discussion 3.1. Activity of Nano-SiO2 In order to define the microstructure of the reaction product produced by nano-SiO2 and Ca(OH)2 , the corresponding SEM images of the products were also investigated. SEM images of microstructure of the products at different ages were shown in Figure 2. An obvious change can be seen with the increasing ages. Many needle-like and bar-like crystals emerged at the surface of early ages. As the curing age increasing, the resulting flocculated structure tends to become denser. In order to obtain more information of the products, the chemical element of products determined by Energy Dispersive Spectrometer (EDS) and the results were showed in Table 7. It can be seen that the shape and the element percentage of the products was close to C–S–H gel, so we can infer that the production of the reaction was C–S–H gel.

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(a) 1 day

(b) 3 days

(c) 7 days

(d) 28 days

Figure 2. of of thethe products at different ages: (a) 1day; (b) 3 (b) days; (c) 7 Figure 2. SEM SEMimages imagesofofmicrostructure microstructure products at different ages: (a) 1day; 3 days; days; (d) 28 days. 1, 2, 3, 4 are the position numbers in Table 7. (c) 7 days; (d) 28 days. 1, 2, 3, 4 are the position numbers in Table 7. Table 7. Chemical element compositions of the products tested by energy dispersive spectrometer Table 7. Chemical element compositions of the products tested by energy dispersive spectrometer (EDS). (EDS). Position Number Position Number 1 2 3 4

1 2 3 4

C

O C 30.26 30.26 49.49 26.0926.09 50.87 26.54 26.54 52.67 19.03 56.26 19.03

Atom/% Atom/% O Si Si Ca Ca 49.49 5.63 5.63 14.6214.62 8.62 50.87 8.6213.6413.64 7.41 52.67 7.4113.3813.38 9.43 15.28 56.26 9.43 15.28

Ca/Si Ca/Si 2.60 2.60 1.55 1.55 1.81 1.81 1.62 1.62

XRD analyses were wereused usedtotoinvestigate investigatethe thecomposition composition hydration products. Figure 3 shows XRD analyses of of hydration products. Figure 3 shows the the XRD analysis of nano-SiO and Ca(OH) , and we could get the reaction degree of nano-SiO 2 2 XRD analysis of nano-SiO2 and Ca(OH)2, and we could get the reaction degree of nano-SiO2 with2 with Ca(OH) from The nano-SiO poor of degree of crystallization, so the diffraction peak of2 2 has Ca(OH) 2 from2 it. Theit.nano-SiO 2 has poor degree crystallization, so the diffraction peak of Ca(OH) Ca(OH) focused and the intensity the of Ca(OH) [4]. from It canFigure be seen 2 should should be focusedbe and the intensity reflected thereflected contained ofcontained Ca(OH)2 [4]. It can be2seen 3, from Figure 3, the diffraction peak intensity of Ca(OH) was significantly becoming lower at the ages 2 the diffraction peak intensity of Ca(OH)2 was significantly becoming lower at the ages of 1 h, 6 h, 12 h, of h, 6 three h, 12 days, h, oneseven day, days, three and days, days, and 28As days, respectively. it relative shown diffraction in Table 8, one1 day, 28seven days, respectively. it shown in Table.8,As the ◦ the diffraction peak intensities 18 were 100%, 92.12%, 90.47%, peakrelative intensities at 18° were 100%, 92.12%,at90.47%, 87.73%, 50.84%, 28.69%, and87.73%, 26.89%,50.84%, and the 28.69%, relative ◦ were 100%, 92.27%, 91.48%, 89.36%, and 26.89%, and the relative diffraction peak intensities at 33 diffraction peak intensities at 33° were 100%, 92.27%, 91.48%, 89.36%, 50.04%, 29.16%, and 28.78%, ◦, 50.04%, 29.16%, and 28.78%, Researchers diffuse diffraction 29.1of respectively. Researchers foundrespectively. diffuse diffraction peaks at found 29.1°, 31.8°, 49.8°, and 54.9°peaks in the at study ◦ , 49.8◦ , and 54.9◦ in the study of C–S–H synthesis. This was a relatively low Ca/Si C–S–H 31.8 C–S–H synthesis. This was a relatively low Ca/Si C–S–H gel after analysis. We got the main hydration gel after analysis. We2got the main products of nano-SiO2of reacted with peak Ca(OH) the 2 , and products of nano-SiO reacted with hydration Ca(OH)2, and the characteristics diffraction were similar characteristics of diffraction peak were similar with the researchers’ results [26]. The C–S–H gel peak with the researchers’ results [26]. The C–S–H gel peak existed at 12 h and one day, and became higher existed 12 hand and28one day,This andshowed becamethat higher seven of days and 282 with days.Ca(OH) This showed that the at sevenatdays days. the at reaction nano-SiO 2 in cement can reaction of nano-SiO with Ca(OH) in cement can occur and form C–S–H gel. 2 gel. 2 occur and form C–S–H

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Nanomaterials 2016, 2016, 6, 6, 241 241 Nanomaterials

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--Ca(OH)2





 



 

--C-S-H --Ca(OH)2 28days--CaCO3 --C-S-H 28days--CaCO3 7days







7days 3days 3days 1day 1day 12h 12h 6h 6h 1h 0

20

40

60

80

60

80

。

0

2θ（ ）

20

40

100

1h

100

。

2θ（ ）

Figure 3. XRD patterns of the reaction of nano-SiO2 with Ca(OH)2. Figure 3. 3. XRD XRD patterns patterns of of the the reaction reaction of with Ca(OH) Ca(OH)22.. Figure of nano-SiO nano-SiO22 with Table 8. Peak intensity of Ca(OH)2. Age 1h Age Age 6h 1 h1hh 12 6 12 h6hhh 24 hh 24 312days 3 days h 724 7days days 3 days 28 days 28 days 7 days 28 days

CH (Peak Height) 18 CH (Peak CH2551 (Peak Height) 18 Height) 2350 2551 2551 2308 2350 2350 2238 2308 2308 2238 1297 1297 2238 732 732 1297 686 686 732 686

Table 8. Ca(OH) Increment Relative CH Height) Increment Table 8.Diffraction Peak intensity of(Peak 2.. (%) Peak Intensities 33 (%) Increment Diffraction Height) Increment Relative Diffraction CH (Peak CH3711 (Peak 0% (%) Relative 100% 0% Increment (%) Increment Peak Intensities Height) (%) Peak Intensities 33 33 (%) −7.88% 92.12% 3424 −7.73% 3711 0% 0%0% 100%100% 3711 0% −9.53% 90.47% 3395 −8.52% −7.88% 92.12% 3424 −7.73% −7.88% 92.12% 3424 −7.73% −12.27% 87.73% 3316 −10.64% −9.53% 90.47% 3395 −8.52% −9.53% 90.47% 3395 −8.52% −12.27% 87.73% 3316 −10.64% −49.16% 50.84% 1887 −49.96% − 49.16% 50.84% 1887 − 49.96% −12.27% 87.73% 3316 −10.64% −71.31% 28.69% 1082 −70.84% −71.31% 28.69% 1082 −70.84% −49.16% 50.84% 1887 −73.11% 26.89% 1068 −−49.96% 71.22% −73.11% 26.89% 1068 −71.22% −71.31% 28.69% 1082 −70.84% −73.11% 26.89% 1068 −71.22%

Relative Diffraction Peak Intensities Relative Diffraction Relative Diffraction 100% Peak Intensities Peak Intensities 92.27% 100% 100% 91.48% 92.27% 92.27% 89.36% 91.48% 91.48% 89.36% 50.04% 50.04% 89.36% 29.16% 29.16% 50.04% 28.78% 28.78% 29.16% 28.78%

1day 100 3days 7days 95 1day 28days 100 3days 28days 7days 90 7days 95 28days

0.6 0.4

TG% TG%

0.4 0.2

28days

0.2 0.0

7days

85 90

3days 85 80

0.0 -0.2

28days

-0.2 -0.4

-0.4 -0.6 0

200

400

600

800

Temperature/℃

-0.6 0

200

400

600

800

3days 7days 1day 28days 3days 1day 7days 1day 3days 1000 1day

75 80

DSC(mv/mg) DSC(mv/mg)

The TG-DSC TG-DSCcurves curvesfor forthe the reaction of nano-SiO 2 with Ca(OH)2 at one day, three days, seven The reaction of nano-SiO 2 with Ca(OH)2 at one day, three days, seven days, days, and 28 days, respectively, were shown in Figure 4. We could obtain the reaction degree of nanoand 28 respectively, were in Figure 4. We 2could reaction degree of nano-SiO 2 Thedays, TG-DSC curves for theshown reaction of nano-SiO with obtain Ca(OH)the 2 at one day, three days, seven SiO2 with Ca(OH) 2the by the TG-DSC curves. It can be found that the endothermic peak of Ca(OH) 2 was was with Ca(OH) by TG-DSC curves. It can be found that the endothermic peak of Ca(OH) 2 2 days, and 28 days, respectively, were shown in Figure 4. We could obtain the reaction degree of nanomost obvious after one day. The endothermic peak of Ca(OH) 2 waswas gradually weakened as time went obvious after one day. The endothermic peak of Ca(OH) gradually weakened as 2 SiO2 with Ca(OH)2 by the TG-DSC curves. It can be found that the endothermic peak of Ca(OH)2 time was by. Additionally, we can the mass ofloss Ca(OH) 2 based on the on TGthe curves. Ca(OH)Ca(OH) 2 content went by. Additionally, weobtain can the loss mass of Ca(OH) TG curves. 2 based most obvious after one day. The obtain endothermic peak of Ca(OH) 2 was gradually weakened as time went2 in the sample the ratio of the massthe lossmass of sample 2(Ca(OH) dehydration peak) to the mass content in the by sample byobtain the ratio of(Ca(OH) sample peak) to loss the 2 dehydration by. Additionally, we can the of mass loss of loss Ca(OH) 2 based on the TG curves. Ca(OH) 2 content Ca(OH) 2 (analytical reagent) (mass ratio) was estimated. The results are shown in Figure 5. The mass loss Ca(OH) (analytical reagent) (mass ratio) was estimated. The results are shown in Figure 2 in the sample by the ratio of the mass loss of sample (Ca(OH)2 dehydration peak) to the mass loss5. reaction amounts of Ca(OH) 2 were 89.57%, 94.91%, 95.28%, and 95.54% at oneat day, The reaction amounts of Ca(OH) 89.57%, 94.91%, 95.28%, and 95.54% onethree day, days, three seven days, 2 were Ca(OH) 2 (analytical reagent) (mass ratio) was estimated. The results are shown in Figure 5. The days, and 28and days, The reaction between nano-SiO 2 and Ca(OH) 2 was the most intense seven days, 28 respectively. days, respectively. The reaction between nano-SiO and Ca(OH) was the most 2 days, seven reaction amounts of Ca(OH)2 were 89.57%, 94.91%, 95.28%, and 95.54% 2at one day, three in one day andday theand reaction was basically completed in threeindays. intense in one the reaction was basically completed three days. days, and 28 days, respectively. The reaction between nano-SiO2 and Ca(OH)2 was the most intense in one day and the reaction was basically completed in three days. 0.6

70 75 70

1000

Temperature/℃ Figure 4. 4. Thermogravimetric-differential Thermogravimetric-differentialscanning scanningcalorimetry calorimetry (TG-DSC) curves of the products of Figure (TG-DSC) curves of the products of the the reaction of nano-SiO 2 with Ca(OH) 2 . reaction4.ofThermogravimetric-differential nano-SiO2 with Ca(OH)2 . Figure scanning calorimetry (TG-DSC) curves of the products of the reaction of nano-SiO2 with Ca(OH)2.

7 of 15 7 of 15 7 of 15

Ca(OH) % 2% Ca(OH) 2

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Ca(OH)2

95 96

Ca(OH)2

94 95 93 94 92 93 91 92 90 91 89 90

0

4

8

12

16

20

24

28

32

24

28

32

Age/days

89 0

4

8

12

16

20

Age/days Figure 5. The mass loss of Ca(OH)2 at different ages. Figure 5. The mass loss of Ca(OH)2 at different ages.

Figure 5. The lossMortar of Ca(OH)2 at different ages. 3.2. Compressive and Flexural Strength of mass Cement

Mechanicaland properties all the specimens were measured at different ages. The effect of adding 3.2. Compressive and Flexuralof Strength of Cement Cement Mortar Mortar 3.2. Compressive Flexural Strength of 0%, 1%, 3%, and 5% nano-SiO2 on the flexural and compressive strength of cement paste were shown Mechanical properties properties of all all the the specimens specimens were were measured measured at at different different ages. ages. The The effect effect of of adding Mechanical adding in Figure 6. Compared withof the control samples, the flexural strength of cement paste increased 6.9%, 0%, 1%, 3%, and 5% nano-SiO22 on on the the flexural flexural and and compressive compressive strength of cement paste were shown 6.7%, and −1.8%, at three days, seven days, and 28 days, respectively, when the content of nano-SiO2 thethe control samples, the the flexural strength of cement pastepaste increased 6.9%, in Figure Figure 6.6.Compared Comparedwith with control samples, flexural strength of cement increased was 1%, and compressive strength increased by 16.8%, 4.7%, and −3.2%, respectively. When the 6.9%, 6.7%, and − three days, seven and respectively, 28 days, respectively, of2 6.7%, and −1.8%, at1.8%, three at days, seven days, anddays, 28 days, when the when contentthe of content nano-SiO content of was nano-SiO 2 was 3%, flexural strength increased by 30.4%, 22.2%, and 6.7% and the nano-SiO 1%, and compressive strength increased by 16.8%, 4.7%, and − 3.2%, respectively. 2 was 1%, and compressive strength increased by 16.8%, 4.7%, and −3.2%, respectively. When the compressive strength increased2 was by 33.2%, 29.1%, and 18.5%, respectively. When the dosage of nanoWhen of nano-SiO 3%, flexural increased by 30.4%, and 6.7% contenttheofcontent nano-SiO 2 was 3%, flexural strengthstrength increased by 30.4%, 22.2%,22.2%, and 6.7% and and the SiO 2 reaching 5%, flexural strength increased by 31.4%, 27.4%, and 9.8%, and the compressive the compressive strength increased by 33.2%, 29.1%, and 18.5%, respectively. When the dosage of compressive strength increased by 33.2%, 29.1%, and 18.5%, respectively. When the dosage of nanostrength increased 44.9%, 29.7%, and 10.6% at three days, 28and days, respectively. It nano-SiO reachingby 5%, flexural strength increased by days, 31.4%,seven 27.4%, andand 9.8%, the compressive 2 SiO2 reaching 5%, flexural strength increased by 31.4%, 27.4%, and 9.8%, and the compressive strength 44.9%, 29.7%,the andcontent 10.6% of at nano-SiO three days, seven days, 28strength days, respectively. could be increased concludedby that the higher 2, the better theand early of cement. strength increased by 44.9%, 29.7%, and 10.6% at three days, seven days, and 28 days, respectively. It It could be itconcluded the higher the content of nano-SiO betterthe theamount early strength of cement. However, will lead that to a slow development of the late strength of nano-SiO 2 was 2 , thewhen could be concluded that the higher the content of nano-SiO 2, the better the early strength of cement. However, it will lead to a slow development of the late strength when the amount of nano-SiO was 2 too high. Nano-SiO2 could provide obvious increases of the strengths both at early and late dates However, it will lead to a slow development of the late of strength when the amount of nano-SiO 2 was too high. Nano-SiO could provide obvious increases the strengths both at early and late dates 2 when the dosage of nano-SiO2 was 3%. There were reasons for this phenomenon: on the one hand, too high. could provide obvious of thefor strengths both at early late hand, dates when the Nano-SiO dosage of 2nano-SiO 3%. Thereincreases were reasons this phenomenon: onand the one 2 was nano-SiO2 had smaller particles, and can fill between the cement particles, so that the density of the when the2dosage of nano-SiO 2 was were reasons for this phenomenon: on the one of hand, nano-SiO had smaller particles, and3%. canThere fill between the cement particles, so that the density the paste increased. Nano-SiO2 can also be a nucleation point to promote cement hydration and paste increased. Nano-SiO can also be a nucleation point to promote cement hydration and strengthen nano-SiO 2 had smaller particles, and can fill between the cement particles, so that the density of the 2 strengthen the of connection of cement hydration products [23,27]. On the other hand, nano-SiO 2 can the connection cement hydration products [23,27]. On the other hand, nano-SiO can also react with paste increased. Nano-SiO2 can also be a nucleation point to promote cement hydration and 2 alsohydration react withproduct the hydration product Ca(OH) 2 to produce more C–S–H gel [15]. We believe that too the Ca(OH) more C–S–H gel [15]. We thathand, too high or too2 low 2 to produce strengthen the connection of cement hydration products [23,27]. Onbelieve the other nano-SiO can high or too low content of nano-SiO 2 are to not conducive to the upgrading of cement strength. Zhu [16] content of nano-SiO are not conducive the upgrading of cement strength. Zhu [16] also 2 also react with the hydration product Ca(OH)2 to produce more C–S–H gel [15]. We believeobtained that too alsosame obtained theThis same results. may duethe toquantity the fact of that the quantity of SiO 2 nano-particles the results. may be dueThis to the factbethat SiO present in the mix 2 nano-particles high or too low content of nano-SiO2 are not conducive to the upgrading of cement strength. Zhu [16] is higherinthan required to amount combinerequired with the to liberated lime during the process ofduring hydration. present the the mixamount is higher than the combine with the liberated lime the also obtained the same results. This may be due to the fact that the quantity of SiO2 nano-particles This leads excess silica andsilica causing a deficiency strength as it replaces part of the process of to hydration. Thisleaching leads to out excess leaching out andincausing a deficiency in strength as present in thematerial mix is higher thannot thecontribute amount required to combine with the liberated during the cementitious but does todoes its strength [15]. From Figure 6a,b,[15]. itlime can be known it replaces part of the cementitious material but not contribute to its strength From Figure process of hydration. This excessenhancement silica leachinginout and causing a deficiency as that showed theleads most to obvious three-day strength, followed in bystrength seven-day 6a,b,nano-SiO it can be2known that nano-SiO 2 showed the most obvious enhancement in three-day strength, it replaces part of the cementitious material but does not contribute to its strength [15]. From Figure strength, the effect on the strength of effect 28 dayonwas weakest. followed and by seven-day strength, and the thethe strength of 28 day was the weakest. 6a,b, it can be known that nano-SiO2 showed the most obvious enhancement in three-day strength, followed by strength of 28 day was the weakest. 9.0 seven-day 3days strength, and the effect on the 65 7days 28days 3days 7days 28days

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Figure 6. Flexural and compressive strength of cement contained Nano-SiO (a) (b) 2. (a) Flexural strength; Figure 6. Flexural and compressive strength of cement contained Nano-SiO2 . (a) Flexural strength; and (b) compressive strength. and (b) 6. compressive strength. Figure Flexural and compressive strength of cement contained Nano-SiO2. (a) Flexural strength; and (b) compressive strength.

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3.3. 3.3. Effect Effect of of Nano-SiO Nano-SiO22 on on the the Hydration Hydration Heat Heat of of Cement Cement The with 0%, 0%, 1%, 1%, 3%, 3%, and and 5% 5% amounts amounts of of nano-SiO nano-SiO22 The hydration hydration exothermic exothermic rate rate of of cement cement paste paste with within within 72 72 h h were were shown shown in in Figure Figure 7. 7. There There was was an an obvious obvious difference difference on on heat heat release release compared compared with with pure cement paste. pure cement paste. As the hydration hydration of of Portland Portland cement cement is is an an exothermic exothermic process. process. It As we we know, know, the It was was found found that that the the hydration hydration process process of of cement cement with with nano-SiO nano-SiO22 was was similar similar to to Portland Portland cement cement [17,28]. [17,28]. We We found found that that with the nano-SiO nano-SiO22 content of hydration hydration heat heat was was accelerated. accelerated. The with the content increasing, increasing, the the rate rate of The higher higher the the adding dosage was, the higher hydration rate was, when the ratio was limited in an experiment range, which indicated that nano-SiO22 could promote the hydration process of Portland cement. The second exothermic peak of the sample contained 3% and 5% nano-SiO22 appeared at 8 h after water wetting, which which were were about about 0.013 0.013W/g, W/g, 0.014 W/g. W/g. The Thesecond secondexothermic exothermic peak peak of of the the standard standard sample appeared at about 10.5 h after water wetting, and the hydration rate was about 0.011 W/g. W/g. Compared with the standard sample, the second exothermic peak appeared about 2.5 h earlier and Compared exothermic peak appeared about the heat release rate increased increased about about 0.002 0.002 W/g W/g and 0.003 W/g. W/g. The Theheat heat release release rate rate of of sample with may bebe related with thethe high volcanic activity of 1% Nano-SiO22 also alsoincreased increasedabout about0.001 0.001W/g. W/g.This This may related with high volcanic activity nano-SiO 2 . Nano-SiO 2 has a smaller particle size, a greater quantity of atoms distributing on the of nano-SiO2 . Nano-SiO2 has a smaller particle size, a greater quantity of atoms distributing on surface, activity. Nano-SiO Nano-SiO22 reacted with Ca(OH)22 generated surface, which resulted in higher chemical activity. generated by the hydration of cement, so that Ca(OH)22 was was consumed consumed and and the the chemical chemical equilibrium equilibrium was was broken, broken, 2+ 2+ which promoted the Ca Ca arrive arrive supersaturated supersaturated in advance. Thus, nano-SiO22 shorted shorted the induction period and heat release rate advanced. Thus, the and the the hydration hydrationprocess processwas wasaccelerated acceleratedand andthethe heat release rate advanced. Thus, induction period, acceleration period, andand deceleration phase appeared to advance. the induction period, acceleration period, deceleration phase appeared to advance. It can the heat release of cement with nano-SiO 2 was higher than than the pure can be beseen seenfrom fromFigure Figure7,7, the heat release of cement with nano-SiO higher the 2 was cement sample. With With the increasing of nano-SiO 2 content, the hydration process was accelerated and pure cement sample. the increasing of nano-SiO content, the hydration process was accelerated 2 the release increased significantly. This This was consistent with with the previous strength trend.trend. The andheat the heat release increased significantly. was consistent the previous strength reason may may be that amounts of nano-SiO 2 particles depleted Ca(OH) 2, leading to more The reason be the thatlarge the large amounts of nano-SiO depleted Ca(OH) to 2 particles 2 , leading crystallized Ca(OH) 2. The non-hydrated mineral hydration process was accelerated and the more crystallized Ca(OH) . The non-hydrated mineral hydration process was accelerated and 2 accumulation hydration heat of the cement increased. 12000 0.016

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3.4. 3.4. Effect Effect of of Nano-SiO Nano-SiO22 on on Products Products of of Cement Cement Hydration Hydration To qualitatively evaluate evaluate the the effect effect of of nano-SiO nano-SiO2 on the mineralogy of cement paste, XRD tests To qualitatively 2 on the mineralogy of cement paste, XRD tests were performed and the results were shown in Figure 8. As shown in in Figure Figure 6, 6, mixing mixing 3% 3% nano-SiO nano-SiO2 were performed and the results were shown in Figure 8. As shown 2 in cement made the greatest improvement in mechanical properties of the cement paste, thus, in cement made the greatest improvement in mechanical properties of the cement paste, thus, onlyonly the the sample 3% nano-SiO 2 was tested in this part. sample withwith 3% nano-SiO 2 was tested in this part. It was shown in Figure 8 that the hydration products have not changed after adding 3% nanoSiO2 in cement, while the magnitude of the peaks of hydration products changed a lot. The peak

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Nanomaterials 2016, 6, 241 9 of 15 It was shown in Figure 8 that the hydration products have not changed after adding 3% nano-SiO 2

in cement, while the magnitude of the peaks of hydration products changed a lot. The peak intensity intensity of main variations hydrates variations were Table 9. The magnitude of the of intensity of main hydrates were shown in shown Table 9.in The magnitude of the intensity Ca(OH)of2 Ca(OH) 2 at 2θ angle of tested, 18° wasshowing tested, showing an increase 1.42% after one day, the magnitude at 2θ angle of 18◦ was an increase of 1.42%ofafter one day, but thebut magnitude of the of the intensity of Ca(OH) 2 decreased by 32.24%, 33.31%, and 13.07% at three days, seven days, and intensity of Ca(OH)2 decreased by 32.24%, 33.31%, and 13.07% at three days, seven days, and 28 days, 28 days, respectively. This illustrates more Ca(OH) 2 was consumed by nano-SiO2. respectively. This illustrates that morethat Ca(OH) 2 was consumed by nano-SiO2 . The magnitude of the intensity of the not hydrates such as tricalcium tricalcium silicate silicate (C (C3S) and dicalcium The magnitude of the intensity of the not hydrates such as 3 S) and dicalcium silicate (C 2S) was obviously changed. The main peak intensity of C3S and C2S at a 2θ angle of 33° was silicate (C2 S) was obviously changed. The main peak intensity of C3 S and C2 S at a 2θ angle of 33◦ also also tested. It showed that that the peak intensity of C3of S and 2S decreased by 13.08%, 31.40%, 9.08%, and was tested. It showed the peak intensity C3 S C and C2 S decreased by 13.08%, 31.40%, 9.08%, 3.24% at one day, three days, seven days, and 28 days when nano-SiO 2 was was added. It was found that and 3.24% at one day, three days, seven days, and 28 days when nano-SiO added. It was found 2 the Ca(OH) 2 diffraction peak was decreased at the same time, and the C 3 S and C 2 S diffraction peaks that the Ca(OH)2 diffraction peak was decreased at the same time, and the C3 S and C2 S diffraction also decreased significantly, especially at threeatdays. results showedshowed that nano-SiO 2 was reacted peaks also decreased significantly, especially threeThe days. The results that nano-SiO 2 was with Ca(OH) 2 and formed C–S–H gel, prompting the hydration reaction to move forward. All of the reacted with Ca(OH)2 and formed C–S–H gel, prompting the hydration reaction to move forward. above results illustrated that nano-SiO 2 will react with Ca(OH)2 and produce the solid C–S–H gel to All of the above results illustrated that nano-SiO 2 will react with Ca(OH)2 and produce the solid C–S–H promote the hydration of Cof 3S C and accelerated the pace of of cement gel to promote the hydration S and accelerated the pace cementhydration. hydration.This This conclusion conclusion is is 3 consistent with the study of Ye Ye [29]. [29]. 

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Figure 8. XRD patterns of cement pastes with 3% or without of nano-SiO2 at different ages: (a) one Figure 8. XRD patterns of cement pastes with 3% or without of nano-SiO2 at different ages: (a) one day; (b) three days; (c) seven days; and (d) 28 days. day; (b) three days; (c) seven days; and (d) 28 days. Table 9. Peak intensity of Ca(OH)2 and C3S of samples with or without nano-SiO2. Table 9. Peak intensity of Ca(OH)2 and C3 S of samples with or without nano-SiO2 .

Age

Control Control 3% nano-SiO22 3% nano-SiO Control Control 3% nano-SiO2 3% nano-SiO Control 2 3% nano-SiO2 Control

Intensity (Counts) Intensity (Counts) CH C3S + C2S CH C3 S + C2 S 983 1598 983 1598 997 1389 997 1389 2410 679 2410 1633 466679 1633 2597 595466 1732 541 2597 595

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The TG-DSC curves for the cement contained 3% nano-SiO2 were shown in Figure 9, which had The TG-DSC curves forthe thecontrol cementsample. contained 3%be nano-SiO Figure 9, which 2 were an obvious difference with It can found that theshown secondinendothermic peakhad of an obvious difference with the control sample. It can be found that the second endothermic peak of the cement contained 3% nano-SiO2 was obviously stronger than the control sample at different ages. the cement contained 3% nano-SiO2 was obviously stronger than the control sample at different ages. 0.00

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According to the TG curve of cement paste, we can obtain the mass loss of Ca(OH)2 at different According to the TG curve of cement paste, we can obtain the mass loss of Ca(OH) at different ages. It can be seen from Figure 10, the content of Ca(OH)2 was always lower than that of2 the control ages. It can be seen from Figure 10, the content of Ca(OH)2 was always lower than that of the control sample with the hydration time increased. The contents of Ca(OH)2 in cement paste with 3% nanosample with the hydration time increased. The contents of Ca(OH)2 in cement paste with 3% nano-SiO2 SiO2 were 0.27%, 0.82%, 2.24%, and 3.95% lower than those in control sample at one day, three days, were 0.27%, 0.82%, 2.24%, and 3.95% lower than those in control sample at one day, three days, seven days, and 28 days, respectively. By hydration heat analysis (Figure 7) and XRD analysis (Figure seven days, and 28 days, respectively. By hydration heat analysis (Figure 7) and XRD analysis 8), it can be seen that the cement hydration rate was faster at the early age, especially when the nano(Figure 8), it can be seen that the cement hydration rate was faster at the early age, especially when SiO2 was added to the cement. The hydration promotion of C–S–H by nano-silica mainly occurred at the nano-SiO was added to the cement. The hydration promotion of C–S–H by nano-silica mainly three days of 2age. However, the Ca(OH)2 content in the cement paste did not show any significant occurred at three days of age. However, the Ca(OH) content in the cement paste did not show any decrease after adding nano-SiO2 by the TG analysis.2This phenomenon meanly because of that the significant decrease after adding nano-SiO2 by the TG analysis. This phenomenon meanly because of hydration reaction just began at that time and there were much C3S and C2S which were not hydrated. that the hydration reaction just began at that time and there were much C3 S and C2 S which were not The hydration reaction of C3S was quick and at the same time the pozzolanic activity of nano-SiO2 hydrated. The hydration reaction of C3 S was quick and at the same time the pozzolanic activity of was stronger, and the of Ca(OH)2 formed by the hydration reaction of C3S and C2S reacts with nanonano-SiO2 was stronger, and the of Ca(OH)2 formed by the hydration reaction of C3 S and C2 S reacts SiO2 suddenly. Thus, the production and consumption of Ca(OH)2 achieved a dynamic equilibrium, with nano-SiO suddenly. Thus, the production and consumption of Ca(OH)2 achieved a dynamic resulting in the2Ca(OH)2 content tested by TG not decreasing significantly at the three-day age. With equilibrium, resulting in the Ca(OH)2 content tested by TG not decreasing significantly at the three-day the increase of the hydration age, the cement slurry was further hydrated, and the formation of a age. With the increase of the hydration age, the cement slurry was further hydrated, and the formation large number of hydration products, the reaction of nano-SiO2 and Ca(OH)2 is still continuing, so the of a large number of hydration products, the reaction of nano-SiO and Ca(OH)2 is still continuing, content of Ca(OH)2 was significantly lower than the control sample.2 so the content of Ca(OH)2 was significantly lower than the control sample.

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Figure 10. 10. The The mass mass loss loss of of Ca(OH) Ca(OH)2 at at difference difference ages. ages. Figure Figure 10. The mass loss of Ca(OH) 22 at difference ages.

3.5. Effect Effect of of Nano-SiO Nano-SiO2 on Microstructure of the Cement Paste Paste onMicrostructure Microstructure of of the the Cement Cement 3.5. Effect of Nano-SiO 22 on Paste Figure 11showed showed SEMmicrographs micrographs of pure cement paste and cement with 3% nano-SiO 2 at of of pure cement paste andand cement withwith 3% nano-SiO Figure 11 11 showedSEM SEM micrographs pure cement paste cement 3% nano-SiO 2 at 2 at three threeand days and 28 days to directly explore the role of nano-SiO 2 modifying the properties of cement days days directly explore the role nano-SiO the properties of cement paste. three days28and 28 to days to directly explore theofrole of nano-SiO 2 modifying the properties of cement 2 modifying paste. Figure 11 showed that the sample had a certain development in three days. Figure 11a showed Figure 11 showed that the sample had a certain development in three days. Figure 11a showed the paste. Figure 11 showed that the sample had a certain development in three days. Figure 11a showed the microstructures existence of needle-hydrates and hexagonal flake of Ca(OH) 2, but the structure microstructures existence of needle-hydrates and hexagonal flake flake of Ca(OH) structure of the the microstructures existence of needle-hydrates and hexagonal of Ca(OH) , but the structure 2 , but 2the of the cement pastes was verywith loose with large number of micron pores. The morphology of C–S– cement pastespastes was very largeaanumber of micron pores. pores. The morphology of C–S–H was of the cement wasloose very looseawith large number of micron The morphology of C–S– H was non-compact and fibrous. The sample with 3% nano-SiO 2 was shown in the Figure 11b. As we non-compact and fibrous. The sample with 3% nano-SiO was shown in the Figure 11b. As we know, 2 H was non-compact and fibrous. The sample with 3% nano-SiO 2 was shown in the Figure 11b. As we know, the structural defects can affect the mechanical properties, and comparing with the control the structural defectsdefects can affect mechanical properties, and comparing with thewith control know, the structural canthe affect the mechanical properties, and comparing the sample, control sample, the hydration products of the sample with 3% nano-SiO 2 were different from the control the hydration products of the sample with 3% nano-SiO were different from the control samples. 2 sample, the hydration products of the sample with 3% nano-SiO2 were different from the control samples. The hydration products became more compact after the addition of nano-SiO 2, most of the The hydration productsproducts became more compact after theafter addition of nano-SiO the of barite 2 , most2,of samples. The hydration became more compact the addition of nano-SiO most the barite ettringite crystals and hexagonal flake of Ca(OH) 2 had been covered by C–S–H. At 28 days, in ettringite crystals and and hexagonal flake of of Ca(OH) byC–S–H. C–S–H.AtAt days, 2 had barite ettringite crystals hexagonal flake Ca(OH) 2 hadbeen been covered covered by 2828 days, in Figure 11c,11c, thethe C–S–H gel with a denser and finer structure of was observed compared with figure in Figure C–S–H gel with a denser and finer structure of was observed compared with Figure 11c, the C–S–H gel with a denser and finer structure of was observed compared with figure 11a, but11a, the micron pores ofpores the cement paste remained and it had a lot of built-in directional Ca(OH)2 Figure the pores micron of the paste cement paste remained and it had a lot ofdirectional built-in directional 11a, but the but micron of the cement remained and it had a lot of built-in Ca(OH)2 crystals embedded in the pores, and each area had relatively independent system. As shown in Figure Ca(OH)2embedded crystals embedded in the and each area had relatively independent system. shown crystals in the pores, andpores, each area had relatively independent system. As shownAs in Figure 11d, the structure of cement paste with 3% nano-SiO 2 at 28 days was more compact and the Ca(OH)2 in Figure 11d, the of structure cement with 3% nano-SiO 28 days more and compact and the2 2 at 11d, the structure cementof paste withpaste 3% nano-SiO 2 at 28 days was morewas compact the Ca(OH) crystal cannot becannot found,beand the hydration productsproducts was much more which have become a whole. Ca(OH) crystal found, and the hydration was much more which have become 2 crystal cannot be found, and the hydration products was much more which have become a whole.a Thus, a finer structure formed in the paste, which results in higher strength. whole.a Thus, a finer structure in thewhich paste, results which results in strength. higher strength. Thus, finer structure formedformed in the paste, in higher

(a) (a)

(b) (b) Figure 11. Cont.

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Figure 11.SEM SEM images: (a)control control cement three days (b) cementwith withnano-SiO nano-SiO 2 3% three days Figure 11. SEM images: (a) control cement three days(b) (b)cement cement with nano-SiO 2 3% at at three days Figure 11. images: (a) cement atatat three days 2 3% at three days (c) control cement at 28 days; and (d) cement with nano-SiO 2 3% at 28 days. (c) control cement 28 days; cement with nano-SiO23% 3%at at 28 28 days. days. (c) control cement at 28atdays; andand (d) (d) cement with nano-SiO 2

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3.6. Effect of Nano-SiO on Pore StructureofofCement CementPaste Paste 3.6.Effect Effect Nano-SiO 2 on2 Pore Structure 3.6. ofofNano-SiO 2 on Pore Structure of Cement Paste The structure of cement pastereflects reflectsits itscompaction compaction rate, which impact on on Thepore porepore structure ofcement cement paste rate,which whichhad hadsignificant significant impact The structure of paste reflects its compaction rate, had significant impact on the mechanical property. The more dense the cement paste, the stronger the anti-penetration ability, themechanical mechanicalproperty. property.The Themore moredense densethe thecement cementpaste, paste,the the stronger stronger the the anti-penetration anti-penetration ability, ability, the the stronger the outside corrosion resistance. The pore structure of cement paste can be characterized thestronger stronger theoutside outsidecorrosion corrosionresistance. resistance.The Thepore porestructure structureof ofcement cementpaste pastecan canbe becharacterized characterized the the by the porosity and pore size distribution. According to study of Renhe, et al. [30], the pores in the bythe the porosity porosity and and pore size distribution. et et al.al. [30], thethe pores in the by distribution. According Accordingtotostudy studyofofRenhe, Renhe, [30], pores in cement paste can be divided into innocuous pores (the diameter < 20 nm), less harmful pore (20–50 cement paste can can be divided into into innocuous porespores (the diameter < 20 nm), pore (20–50 the cement paste be divided innocuous (the diameter < 20 less nm),harmful less harmful pore nm), detrimental pores (50–200 nm), and much-detrimental pores (>200 nm). Here, the harmful pores (20–50 nm), detrimental (50–200 nm),much-detrimental and much-detrimental pores nm). Here, the harmful nm),are detrimental pores pores (50–200 nm), and pores (>200(>200 nm). thestability. harmful pores mainly is respect of the cement-based materials’ mechanical properties andHere, volume pores are mainly is respect of the cement-based materials’ mechanical properties and volume stability. are mainly is respect of the cement-based materials’ mechanical properties and volume stability. Figure 12 showed MIP of cement pastes without nano-SiO2 and cement paste with 3% nano-SiO2 Figure MIP ofofcement cement paste 3% Figure 12showed showed MIPseven cement pastes without nano-SiO 2and andfrom cement paste with 3%nano-SiO nano-SiO at one 12 day, three days, days,pastes and 28without days. Asnano-SiO it can be 2seen Figure 10,with the curves of the 2 2 atatone day, three days, seven days, and 28 days. As it can be seen from Figure 10, the curves of one day,sample three with days,3% seven days, anda 28 it can be seen from 10, the ofthe the entire nano-SiO 2 had leftdays. shift atAs different hydration time.Figure This means thatcurves the pores entire sample with 3% nano-SiO had a left shift at different hydration time. This means that the pores 2 2 had entiresize sample with nano-SiO a left shift time. This means that the pores refined and3% pore structure improved with at thedifferent additionhydration of nano-SiO 2. size 2 .2. sizerefined refinedand andpore porestructure structureimproved improvedwith withthe theaddition additionofofnano-SiO nano-SiO

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Figure 12. Pore size distribution of cement pastes with 3% or without nano-SiO2 at different ages: (a) 1 day; (b) 3 days; (c) 7 days; and (d) 28 days.

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In order order to to describe describe the the development development of of the the pore pore structure structure of of cement cement paste paste more more accurately, accurately, In concrete numerical value of porosity of cement pastes at various levels were shown in Figure 13. concrete numerical value of porosity of cement pastes at various levels were shown in Figure 13. The The total porosity of control sample were 33.35%, 26.30%, 19.67%, and 18.66% oneday, day,three threedays, days, total porosity of control sample were 33.35%, 26.30%, 19.67%, and 18.66% atatone seven days, and 28 days, respectively, while the total porosity of sample with 3% nano-SiO were 2 seven days, and 28 days, respectively, while the total porosity of sample with 3% nano-SiO2 were 31.14%, 20.79%, 16.72%, and 13.26% respectively. The total porosity of cement paste decreased by 31.14%, 20.79%, 16.72%, and 13.26% respectively. The total porosity of cement paste decreased by 2.21%, 5.51%, 5.51%, 2.95%, 2.95%, and and 5.4% 5.4% at at one one day, day, three three days, days, seven seven days, days, and and 28 28 days, days, respectively, respectively, after after 2.21%, adding 3% nano-SiO . The improvement effect of total porosity is obvious at three days and 28 days. adding 3% nano-SiO22. The improvement effect of total porosity is obvious at three days and 28 days. The XRD XRD results results showed showed that that the the secondary secondary hydration and Ca(OH) Ca(OH)22 accelerates accelerates the the rate rate The hydration of of nano-SiO nano-SiO22 and of hydration hydration of of C C33S, S,resulting resultingin inmore morecompact compact C–S–H C–S–H gels, gels, increasing increasing the the density density of of the the cement cement paste paste of degree. Porosity improvement is obvious at 28 days; it is mainly due to the hydration of the cement degree. Porosity improvement is obvious at 28 days; it is mainly due to the hydration of the cement paste, the the hydration hydration products products become become more more compact compact and and connected connected as as aa whole. whole. The The total total porosity porosity paste, decreased significantly significantly compared compared to to the the early early age, age, and and the the small small particle particle size size of plays its its decreased of nano-SiO nano-SiO22 plays filling effect, filling between unhydrated particles and voids from further dense cement paste, reducing filling effect, filling between unhydrated particles and voids from further dense cement paste, the total the cementofpaste. reducing theporosity total theof porosity cement paste. The addition of nano-SiO the porosity of cement paste, but but alsoalso affected the pore The addition of nano-SiO2 2not notonly onlyaffected affected the porosity of cement paste, affected the size distribution at different ages. By analyzing the pore size of different ages in Figure 13, after adding pore size distribution at different ages. By analyzing the pore size of different ages in Figure 13, after nano-SiO of 20–50 nm increased from 15.2% to 17.39%, the percentage of detrimental 2 , the porosity adding nano-SiO 2, the porosity of 20–50 nm increased from 15.2% to 17.39%, the percentage of pores decreased from 52.89% 47.53% after day. after This was the filling effectthe of filling nano-SiO 2 promoting detrimental pores decreased to from 52.89% toone 47.53% one day. This was effect of nanodetrimental pores to transform to less harmful pores. The less harmful pores were increased from SiO2 promoting detrimental pores to transform to less harmful pores. The less harmful pores were 23.59% to 48.23%, and the decreased from 29.78% to 4.65% after three to days. Theafter less increased from 23.59% to detrimental 48.23%, andpores the detrimental pores decreased from 29.78% 4.65% harmful pores increased from 43.79% to 71.68%, and the detrimental pores decreased from 24.82% to three days. The less harmful pores increased from 43.79% to 71.68%, and the detrimental pores 4.12% after seven days. This was due to the accelerated rate of hydration, and the hydration products decreased from 24.82% to 4.12% after seven days. This was due to the accelerated rate of hydration, filling the pores, making detrimental pores transform to less harmful pores. The detrimental pores and the hydration products filling the pores, making detrimental pores transform to less harmful decreased from 13.45% to 2.78% and the less harmful pores increased from 20.91% to 25.39% with the pores. The detrimental pores decreased from 13.45% to 2.78% and the less harmful pores increased continuation of hydration after 28 days. from 20.91% to 25.39% with the continuation of hydration after 28 days. From the above analysis, it can be concluded that the total porosity of cement paste was effectively From the above analysis, it can be concluded that the total porosity of cement paste was reduced and the pore size distribution of cement paste has been effectively improved after adding 3% effectively reduced and the pore size distribution of cement paste has been effectively improved after nano-SiO . The pore structure was optimized by adding nano-SiO , which was not only consistent adding 3%2 nano-SiO2. The pore structure was optimized by adding2 nano-SiO2, which was not only with the SEM analysis, but also a key factor to improve the mechanical properties of cement mortar at consistent with the SEM analysis, but also a key factor to improve the mechanical properties of different ages after adding nano-SiO . cement mortar at different ages after2adding nano-SiO2. 35

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Figure Figure 13. 13. The Thepore pore size size distribution distribution and and porosity porosity of of set set cement cement paste paste with with different different days. days.

4. Conclusions 4. Conclusions (1) The reaction between nano-SiO2 and Ca(OH)2 started within 1 h, and the reaction rate was faster (1) The reaction between nano-SiO2 and Ca(OH)2 started within 1 h, and the reaction rate was faster in the three day period, and the C–S–H gel was formed. in the three day period, and the C–S–H gel was formed. (2) When the content of nano-SiO2 was 3%, the compressive strength increased by 33.2%, 29.1%, and 18.5% at three days, seven days, and 28 days, respectively. The compressive strength increased by 44.9%, 29.7%, and 10.6% at 3 days, 7 days, and 28 days, respectively, when the dosage of

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When the content of nano-SiO2 was 3%, the compressive strength increased by 33.2%, 29.1%, and 18.5% at three days, seven days, and 28 days, respectively. The compressive strength increased by 44.9%, 29.7%, and 10.6% at 3 days, 7 days, and 28 days, respectively, when the dosage of nano-SiO2 was 5%. Nano-SiO2 had the most obvious effect on compressive strength at 3 days, followed by 7 days and 28 days. Taking this into account, 3% was the best dosage of nano-SiO2 . Nano-SiO2 promoted the hydration heat of cement paste. The effect was obvious when the dosages of nano-SiO2 were 3% and 5%, and the heat release rate of hydration heat of the second exothermic peak was increased 0.002 W/g, 0.003 W/g respectively. The second exothermic peak appeared approximately 2.5 h earlier. The cumulative heat release of the paste increased with the adding of nano-SiO2 . The content of Ca(OH)2 of cement paste with 3% nano-SiO2 was decreased by 0.27%, 0.82%, 2.24%, and 3.95% at one day, three days, seven days, and 28 days, respectively. The Ca(OH)2 diffraction peak intensity increased by −32.24% and −13.07%, but the tricalcium silicate (C3 S) and dicalcium silicate (C2 S) diffraction peak intensity increased by −31.40% and −3.24% at three days and 28 days, respectively. The addition of nano-SiO2 promoted the formation of C–S–H gel, and the promotion effect mainly occurred in three days. The total porosity of cement paste decreased 2.21%, 5.51%, 2.95%, and 5.4% at one day, three days, seven days, and 28 day, respectively, when the dosage of nano-SiO2 was 3%. Nano-SiO2 optimized the pore structure of cement paste, and much-detrimental pores and detrimental pores decreased while less harmful pores and innocuous pores increased.

Acknowledgments: This research was supported by National High Technology Research and Development Program of China (No.: 2015AA034701) and Shenzhen Foundation Research (No.: JCYJ20160422092836654). Author Contributions: Liguo Wang did the experiments, data analysis and wrote part of this paper. Dapeng Zheng did data analysis and wrote part of this paper. Shupeng Zhang did data analysis. Haibo Wen did the experiments and data analysis. Hongzhi Cui did data analysis. Dongxu Li provided the original ideas, did data analysis. Conflicts of Interest: The authors declare no conflict of interest.

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