Materials Science and Engineering C 33 (2013) 427–433

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The impact of zirconium oxide radiopacifier on the early hydration behaviour of white Portland cement Nichola J. Coleman a,⁎, Qiu Li b a b

School of Science, University of Greenwich, Chatham Maritime, Kent, ME4 4TB, UK School of Civil Engineering, University of Leeds, Leeds, LS2 9JT, UK

a r t i c l e

i n f o

Article history: Received 19 May 2012 Received in revised form 29 July 2012 Accepted 17 September 2012 Available online 23 September 2012 Keywords: Cement hydration Mineral trioxide aggregate Nuclear magnetic resonance spectroscopy Portland cement Radiopacifier Zirconium oxide

a b s t r a c t Zirconium oxide has been identified as a candidate radiopacifying agent for use in Portland cement-based biomaterials. During this study, the impact of 20 wt.% zirconium oxide on the hydration and setting reactions of white Portland cement (WPC) was monitored by powder X-ray diffraction (XRD), 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), transmission electron microscopy (TEM) and Vicat apparatus. The presence of 20 wt.% zirconium oxide particles in the size-range of 0.2 to 5 μm was found to reduce the initial and final setting times of WPC from 172 to 147 min and 213 to 191 min, respectively. Zirconium oxide did not formally participate in the chemical reactions of the hydrating cement; however, the surface of the zirconium oxide particles presented heterogeneous nucleation sites for the precipitation and growth of the early C-S-H gel products which accelerated the initial setting reactions. The presence of zirconium oxide was found to have little impact on the development of the calcium (sulpho)aluminate hydrate phases. © 2012 Elsevier B.V. All rights reserved.

1. Introduction Mineral trioxide aggregate (MTA) is a proprietary dental restorative comprising a mixture of Portland cement (PC) and bismuth oxide, Bi2O3, which is used as a root-filling material [1–4]. MTA has been approved by the US Federal Drug Administration for use in surgical endodontic procedures and is produced commercially as ‘Gray’ and Tooth-colored ProRoot MTA (Tulsa Dental Products, OK, USA). In both products, the principal constituent is PC blended with 20 wt.% bismuth oxide to confer radiopacity [4]. MTA is presented as a powder which is mixed manually with supplied sterile water. The principal components of PC are the impure phases; ‘alite’ (tricalcium silicate, Ca3SiO5), ‘belite’ (dicalcium silicate, Ca2SiO4), ‘aluminate’ (tricalcium aluminate, Ca3Al2O6) and ‘ferrite’ (tetracalcium aluminoferrite, Ca2(Al/Fe)O5) which contain up to ~15% of substituted ions [5,6]. Additionally, up to 5 wt.% of gypsum, CaSO4.2H2O, is also ground into the cement to regulate the setting of the aluminate phase. On mixing with water, complex hydration reactions result in the formation of an adhesive gel network and the consequent setting of the mixture [7]. The primary constituent of hydrated PC is a nonstoichiometric calcium silicate hydrate (C-S-H) phase, which is of approximate formula Ca3Si2O7.3H2O and is formed along with portlandite (calcium hydroxide, Ca(OH)2) during the hydration of alite and belite [5,7]. C-S-H ⁎ Corresponding author. Tel.: +44 7980 017088; fax: +44 208 331 9805. E-mail address: [email protected] (N.J. Coleman). 0928-4931/$ – see front matter © 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.msec.2012.09.009

is a poorly crystalline, layered, nanoporous gel phase, which constitutes around 70 wt.% of the hydrated cement matrix. It comprises double layers of calcium oxide polyhedra linked on both sides to silicate chains, as shown in Fig. 1. During hydration, anhydrous unpolymerised isolated (Q0) silicate tetrahedra in alite and belite are hydroxylated (Q0(H)) and condense together to form dimers (Q1) [5,7]. Further condensation leads to the formation of short silicate chains comprising mid-chain (Q 2) species and chain-end (Q 1) groups. Aluminate tetrahedra also take up some of the bridging positions within the C-S-H structure. Mid-chain silicate tetrahedra which are bonded to one aluminate tetrahedron are denoted as Q2(1Al) to distinguish them from mid-chain Q2 groups that are linked to two other silicate tetrahedra. The initial reactions of the aluminate and ferrite phases with gypsum and water yield ettringite (‘AFt’, 6CaO.Al2O3.3SO3.32H2O) and its Fe-substituted analogue, which are dispersed throughout the matrix as needle-like crystals [5,7]. When the supply of gypsum is exhausted, the aluminate phase reacts with portlandite and water to form tetracalcium aluminate hydrate, 4CaO.Al2O3.13H2O. Under ambient curing conditions the ettringite phase subsequently decomposes to form ‘monosulphate’ (‘AFm’, 4CaO.Al2O3.SO3.13H2O) and water which are thermodynamically more stable [5,7]. The different categories of Portland cement are based upon variations in composition and physical properties; e.g. ‘white’ Portland cement (WPC), from which Tooth-colored ProRoot MTA is formulated, is produced by minimising the iron- and manganese-bearing components which impart the characteristic grey colour to ‘ordinary’ Portland cement (OPC) [8].

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Bridging Q2

Mid-chain Q2

Chain-end Q1

Ca-O polyhedra Fig. 1. A sketch of the structure of the calcium silicate layer of the C-S-H gel phase (water molecules and hydroxyl groups are not shown).

Bismuth oxide is essentially inert with respect to the hydration chemistry of Portland cements, in that bismuth is not chemically incorporated into the reaction products; however, its presence in the mix retards the hydration processes, increases the overall porosity and decreases the mechanical strength and durability of the resulting cement matrix [4,9]. Bismuth oxide is also reported to exhibit toxicity towards human dental pulp cells [10]. Accordingly, alternative radiopacifying agents with superior physicochemical and biocompatible properties are currently under consideration for use in the next generation of hydraulic calcium silicate cements for hard tissue repair [11–15]. Zirconium oxide, ZrO2, is among these candidate radiopacifiers as it is bioinert, biocompatible and is widely used in the repair of dental and skeletal tissues [15]. 20 wt.% additions of micron-sized ZrO2 particles to WPC are reported to confer radiopacity values in the range of 3.41–6.58 mm Al, which exceed the minimum ISO recommendation of 3 mm Al for root canal sealers (ISO 6876/2002) [12–16]. A recent study using scanning electron microscopy (SEM) to examine 30-day old pastes containing 30 wt.% ZrO2 particles indicates that this additive behaves as an inert filler which does not participate in the cement hydration reactions [17]. Other than the information contained in these few reports, very little is currently known about the influence of zirconium oxide on the hydration chemistry of Portland cements. The principal objective of this study was to investigate the impact of 20 wt.% zirconium oxide on the early hydration chemistry of WPC. ZrO2-blended and unblended cement paste samples were hydrated for 6, 24 and 168 h prior to analysis by 29Si and 27Al magic angle spinning nuclear magnetic resonance spectroscopy (MAS NMR), powder X-ray diffraction (XRD) and transmission electron microscopy (TEM). The impact of ZrO2 on the initial and final setting times of the cement pastes was also determined by penetration using a Vicat apparatus.

2. Materials and methods 2.1. Materials, preparation and characterisation The WPC used in this study was supplied by Lafarge and is commercially available as ‘Snowcrete’. Its chemical composition and principal constituent phases were provided by the manufacturer and are listed in Table 1. Zirconium oxide particles, in the 0.2–5 μm diameter range, ex. Sigma Aldrich, were used as received. Cement paste specimens were prepared in duplicate by manually mixing 10 g of WPC with 3.75 cm3 of distilled water (i.e. at a water:cement ratio of 0.375 by

Table 1 Composition of white Portland cement. Major oxide components

Minor oxide components

Major crystalline phases

Formula

Mass (%)

Formula

Mass (%)

Formula

Mass (%)

CaO SiO2 Al2O3 SO3

69.2 25.0 1.76 2.00

MgO P2O5 Fe2O3 SrO

0.49 0.43 0.33 0.14

Ca3SiO5 Ca2SiO4 Ca3Al2O6 Ca2(Al/Fe)O5

65 22 4.1 1.0

mass). The samples were packed into polypropylene tubes, hermetically sealed and cured at 37.5 °C (i.e. ‘body temperature’). Specimens WPC-6, WPC-24 and WPC-168 were cured for 6, 24 and 168 h, respectively, before hydration was quenched by solvent exchange with propan-2-ol. Samples blended with zirconium oxide, viz. WPC-Zr-6, WPC-Zr-24 and WPC-Zr-168, were prepared similarly with partial replacement of the WPC by 20 wt.% ZrO2 at a water:solids ratio of 0.3. Initial and final setting times of the ZrO2-blended and unblended WPC pastes were determined in triplicate in accordance with ASTM C191-08 using a manual Vicat apparatus [18]. Powder XRD was performed on all specimens using a Philips D8 diffractometer with Cu Kα = 1.5406 Å, a step-size of 0.019° from 5° to 45° and a measuring time of 141.8 s per step. TEM images of WPC-24 and WPC-Zr-24 were obtained by dispersing the ground samples in methanol prior to deposition onto a carbon film grid. Bright field images were obtained using a JEOL JEM200CX microscope and Gata Orius SC200 digital camera. MAS NMR spectra were collected for all samples on a JEOL JNM-ECX 300 MHz spectrometer. Single pulse 27Al MAS NMR spectra were obtained with a pulse delay of 0.5 s, an acquisition time of 0.01024 s and 7000 scans. 1H–29Si cross polarisation (CP) MAS NMR spectra were obtained with a contact time of 10−3 s, a pulse delay of 5 s, and an acquisition time of 0.0256 s. 68,000, 34,000 and 17,000 scans were collected for the 1H–29Si CP MAS NMR spectra of the 6-, 24- and 168-hour specimens, respectively. Single pulse 29Si MAS NMR spectra were obtained with a pulse delay of 5 s, and an acquisition time of 0.02048 s. 119,000, 68,000 and 34,000 scans were collected for the 29Si MAS NMR spectra of the 6-, 24- and 168-hour specimens, respectively. 29Si and 27Al chemical shifts were referenced to tetramethylsilane (TMS) and the aluminium hexaquo-ion [Al(H2O)6], respectively. The free induction decay (FID) profiles were processed by Delta software (provided by JEOL) to obtain spectra which were then analysed using Igor Pro software.

2.2. Method for the analysis of early hydration products by NMR

29

Si MAS

The following method, after Love et al. [19], was used to analyse the Si MAS NMR spectra of the hydrated cement pastes in order to remove the contribution of unreacted alite which typically obscures the resonances arising from the early Q0(H) and Q1 hydration products. In the case of each hydrated cement paste sample, the intensity of the 29Si MAS NMR spectrum of the anhydrous WPC was adjusted such that the Q0 signal of alite was of equal intensity to that of the sample. This WPC background spectrum was then subtracted from that of the sample paste prior to deconvolution using iterative fitting of the 29Si resonances to Voigt lineshapes. The chemical shifts of the silicate hydration products of each paste were identified from its 1H–29Si CP MAS NMR spectrum and this information was used as a first approximation in the deconvolution of the ‘subtracted’ 29Si MAS NMR spectrum. It should be noted that the majority of the sharp Q0 signal arising from belite remains in the subtracted spectrum as the hydration kinetics of this phase are very slow relative to those of alite. The application of this method to the 29

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analysis of the 29Si MAS NMR spectrum of sample WPC-168 is illustrated in Fig. 2.

429

Table 2 Initial and final setting times for ZrO2-blended and unblended white Portland cement pastes. Standard deviations of the mean values are given in parentheses.

3. Results and discussion

Sample

Initial set (min)

Final set (min)

3.1. Setting times

WPC WPC-Zr

172 (6.0) 147 (3.0)

213 (1.0) 191 (4.6)

The initial and final setting times for WPC and WPC-Zr are listed in Table 2. The presence of 20 wt.% zirconium oxide was found to reduce both the initial and final setting times of WPC by 25 min and 22 min, respectively. The ability of a radiopacifying agent to reduce setting time is a distinct advantage, as a common complaint regarding MTA is that its setting time (which is reported to vary between 50 min and 2 h 45 min) is inconveniently long [20]. 3.2. Powder XRD Powder XRD data confirm that the principal constituent phases of the anhydrous WPC used in this study are alite, belite and tricalcium aluminate (Fig. 3). The formation of ettringite (AFt) and portlandite is noted in the diffraction patterns of both the ZrO2-blended and unblended pastes after 6 h of hydration (Fig. 3). In both cement pastes, the reflections of portlandite develop at the expense of those of alite as hydration proceeds. The reflections of ZrO2 persist in the diffraction patterns of the blended pastes throughout the 7-day hydration period and there is no evidence for the formation of any other zirconium-bearing phases within this timeframe (Fig. 3). 3.3.

27

Al MAS NMR

27

Al MAS NMR spectroscopy is used to discriminate between different aluminium co-ordination environments in solids such as glasses, ceramics, clays, zeolites and cements [21]. Aluminium in tetrahedral

co-ordination gives rise to resonances in the approximate chemical shift range of 100 to 50 ppm; five-co-ordinate aluminium resonates between ca. 40 and 30 ppm; and octahedrally co-ordinated aluminium has a chemical shift range of ca. 20 to −10 ppm [21–23]. These chemical shifts are expressed relative to the resonance of the aluminium hexaquoion [Al(H2O)6]. The 27Al MAS NMR spectrum of anhydrous WPC (Fig. 4) comprises a broad resonance at ~ 80 ppm which arises from tetrahedrally co-ordinated aluminium species which are substituted into the alite and belite phases [23]. Resonances from aluminium in the aluminate and ferrite phases do not appear in the spectrum due to extensive line-broadening. In both the presence and absence of zirconium oxide, the intensity of the signal at ~ 80 ppm is greatly reduced within 6 h of hydration and is replaced by two sharp peaks at ~ 14 ppm and ~ 10 ppm which are assigned to octahedrally co-ordinated aluminium in ettringite and tetracalcium aluminate hydrate (C4AH13), respectively (Fig. 4) [24]. As hydration proceeds, a further broad resonance develops at ~ 65 ppm in the spectra of both the ZrO2-blended and unblended pastes which is attributed to the substitution of aluminate tetrahedra in the bridging positions of the C-S-H gel phase [25]. A shoulder at ~ 4 ppm in the spectra of WPC-168 and WPC-Zr-168 (Fig. 4) is tentatively assigned to the presence of a third poorly crystalline calcium aluminate hydrate phase or to the formation of aluminium hydroxide gel [25,26]. There is no evidence to indicate that the presence of zirconium oxide has any impact on the development

Residue

WPC

WPC-6

Intensity (arb)

Original WPC spectrum WPC background spectrum

Intensity (arb)

WPC-24

WPC-168

∗

∗

WPC-Zr-24

Subtracted and fitted spectra Deconvoluted spectra

WPC-Zr-168 5

Fig. 2. An example of the method used for the analysis of early hydration products by 29 Si MAS NMR.

∗

∗∗

WPC-Zr-6

Chemical shift

∗

10

15

20

25

30

35

40

45

2 Theta (°) Fig. 3. Powder XRD data for anhydrous WPC and hydrated paste samples of ZrO2-blended and unblended WPC. Key: ● alite; ○ belite; ⋄ aluminate; ♦ AFt; ■ portlandite and * ZrO2.

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WPC

AFt

C4 AH 13

∗

∗

WPC-168

WPC-6

WPC-24 Intensity (arb)

Intensity (arb)

WPC-24

WPC-168

WPC-6

WPC-Zr-168

WPC-Zr-6

WPC-Zr-24

WPC-Zr-24

WPC-Zr-6 WPC-Zr-168 120

80

40

0

-40

-80

-40

-120

-60

-80

-100

-120

Chemical shift (ppm)

Chemical shift (ppm) Fig. 4. 27Al MAS NMR spectra of anhydrous WPC and hydrated paste samples of ZrO2-blended and unblended WPC. Spinning side-bands are denoted by asterisks.

Fig. 5. 1H–29Si CP MAS NMR spectra of hydrated paste samples of ZrO2-blended and unblended WPC.

of the calcium (sulpho)aluminate hydrate phases in white Portland cement.

isolated hydrated Q 0(H) species are present at ~−75 ppm in the CP spectra of both WPC-6 and WPC-Zr-6 (Fig. 5). Both spectra also contain significant intensity in the Q 1 and Q2 regions which develop into the respective partially resolved peaks at −80 ppm and −86 ppm as hydration proceeds.

3.4. 1H– 29Si CP MAS NMR The technique of 1H–29Si CP MAS NMR is used to distinguish between anhydrous and hydrated silicate species in glasses, ceramics and cements [21]. Only hydrated silicate species appear in 1H–29Si CP MAS NMR spectra, as the maintenance of the resonance signal relies on the relaxation of 29 Si nuclei via neighbouring protons (i.e. Si–OH silanol groups). It should be noted that the signal intensities recorded in CP spectra are functions of relaxation and contact parameters, number of protons and their 1H–29Si inter-nuclear distances, hence, they are not proportional to the relative abundance of the different Qn species [21]. The chemical shifts of given resonances in both 1H–29Si crosspolarisation and single pulse 29Si MAS NMR spectra are concurrent and reflect the degree of polymerisation of the silicate species from which they arise [27]. Isolated Q0 silicate species resonate in the approximate range of −65 to −75 ppm; dimeric and chain-end Q1 groups give rise to signals between ca. −78 and −82.5 ppm; and resonances in the range of −84 to −87.5 ppm derive from mid-chain Q2 species [19,25,27]. Additionally, the replacement of a neighbouring silicate for aluminate tetrahedron increases the chemical shift by ~5 ppm, such that mid-chain Q 2(1Al) species in an hydrated cement would appear ‘downfield’ of mid-chain Q2 species at ca. −81 to −83 ppm [21,27]. 1 H– 29Si CP MAS NMR spectra of the hydrated cement paste samples prepared during this study are shown in Fig. 5. These CP spectra were collected to locate the approximate chemical shifts of the early silicate hydration products. This technique enables the detection and location of early Q0(H) and Q1 species which are not readily observed in the corresponding 29Si MAS NMR spectrum due to the superposition of the Q0 signals from unreacted alite and belite. After 6 h of hydration,

3.5.

29

Si MAS NMR

The 29Si MAS NMR spectrum of anhydrous white Portland cement used in this study is shown in Fig. 2 (labelled ‘WPC background spectrum’). This spectrum comprises a sharp peak at −72 ppm which is assigned to Q0 species in belite and a broad signal spanning the region −68 ppm to −78 ppm arising from various crystallographically distinct Q 0 species in alite [26,28]. The original 29Si MAS NMR spectra of the ZrO2-blended and unblended hydrated pastes are shown in Fig. 6. In both cases, the signal arising from alite diminishes with the formation of signals in the Q 1 and Q 2 regions of the spectrum as hydration proceeds. A qualitative inspection of these 29Si MAS NMR data indicates that the rate of formation of Q1 and Q2 hydration products in the cement paste containing ZrO2 is initially faster than that of its unblended counterpart. 3.6. Deconvolution and analysis of ‘subtracted’

29

Si MAS NMR spectra

The broad signal arising from unhydrated alite obscures the region of the 29Si MAS NMR spectrum in which the early Q 0(H) and Q 1 hydrated silicate products appear. In order to evaluate the early hydration products more accurately, the residual alite signal in the spectra of the hydrated pastes was removed by subtraction of a suitably adjusted WPC background spectrum prior to deconvolution (as outlined in Section 2.2). The subtracted, deconvoluted and fitted 29Si MAS NMR spectra, together with the residue of the subtracted and fitted spectra for the unblended WPC pastes are shown in Figs. 1 and 7: those of the

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431

Intensity (arb)

WPC-6

WPC-168

Intensity (arb)

WPC-24

WPC-6

WPC-Zr-168 Intensity (arb)

WPC-24

WPC-Zr-24

WPC-Zr-6 -40

-60

-80

-100

-120

Chemical shift (ppm) Fig. 6. WPC.

29

Si MAS NMR spectra of hydrated paste samples of ZrO2-blended and unblended

blended WPC-Zr pastes are shown in Fig. 8. In each case, in addition to the early hydration products, the subtracted spectrum also contains a proportion of the unhydrated belite resonance at −72 ppm. The small residues (i.e. the differences between the subtracted and fitted spectra) obtained by this method indicate that this is a suitable approach for the deconvolution and analysis of the resonances arising from early hydration products. Further validation for this method derives from the fact that each of the deconvoluted Q0(H) signals possesses the same shape and line-width, as do the Q1, Q2(1Al) and Q2 signals for both sample-types. The position and number of signals present in the subtracted and deconvoluted 29Si MAS NMR spectra of the pastes blended with ZrO2 do not differ from those of the unblended pastes (shown in Figs. 8 and 7, respectively). This observation supports the argument that zirconium oxide does not directly participate in the hydration reactions of white Portland cement and is, thus, not incorporated into the product phases. The relative abundance of the various silicate species and the degree of hydration of the WPC and WPC-Zr pastes are listed in Table 3 [25]. These data confirm that the presence of 20 wt.% ZrO2 accelerates the initial rate of formation of C-S-H and degree of hydration from 5.7% to 15% within the first 6 h. As hydration proceeds the difference in the hydration rates between the two cement pastes becomes less pronounced and by 1 week the respective degrees of hydration for WPC-168 and WPC-168-Zr are 60% and 65%. 3.7. TEM Representative TEM bright field images of WPC-24 and WPC-Zr24 are shown in Fig. 9. As previously mentioned, the samples were dispersed in methanol prior to mounting and analysis. The advantages of this preparation method are that it is rapid and is appropriate for the analysis of early ‘outer products’ that are formed via dissolution and reprecipitation in the pore spaces between the cement gains. (Conversely, ‘inner products’ are those that form during the later stages of hydration in

Fig. 7. Subtracted, fitted and deconvoluted pastes.

29

Si MAS NMR spectra of unblended WPC

the locations that were formerly occupied by the anhydrous cement grains and are better observed by TEM using thin sections.) Both WPC-24 and WPC-Zr-24 paste samples comprise C-S-H gel and lath-like ettringite crystals of approximately 100 nm in width (as shown in Fig. 9). C-S-H gel was found to be precipitated onto the surfaces of the ZrO2 particles in sample WPC-Zr-24. TEM analysis of this sample also confirmed that the zirconium oxide particles remain intact and that zirconium is not transferred to the ettringite, portlandite or C-S-H gel product phase. 3.8. The role of ZrO2 in the hydration chemistry of WPC The results of powder XRD, 27Al and 29Si MAS NMR and TEM analyses have confirmed that zirconium oxide particles remain intact within the hydrating cement matrix and that zirconium is not formally incorporated into the cement hydration products. This finding is consistent with that of Camilleri et al. [17], who carried out an SEM investigation on 30-day ZrO2-blended WPC pastes and concluded that ZrO2 behaves as an inert filler in the cement matrix. Additionally, deconvolution and analysis of the 29Si MAS NMR spectra of ZrO2-blended and unblended WPC pastes collected during this investigation has revealed that ZrO2 accelerates the initial hydration reactions leading to the formation of the C-S-H gel phase. This observation accounts for the relative reduction in setting times of the WPC-Zr pastes (which were based on a mechanical penetration test). The incorporation of micron- and sub-micron-sized inorganic particles, such as corundum (Al2O3), amorphous silica (SiO2), calcite (CaCO3) and rutile (TiO2), in Portland cement-based mixtures is known to

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Intensity (arb)

WPC-Zr-6

Intensity (arb)

WPC-Zr-24

Table 3 Relative abundance of Qn species and degree of hydration for ZrO2-blended and unblended white Portland cement pastes. Sample

Q0 (%)

Q0(H) (%)

Q1 (%)

Q2(1Al) (%)

Q2 (%)

Degree of hydration (%)

WPC-6 WPC-24 WPC-168 WPC-Zr-6 WPC-Zr-24 WPC-Zr-168

94.3 57.8 40.2 85.0 48.1 35.0

0.34 1.41 4.48 3.60 3.57 1.60

2.61 23.5 29.1 7.24 26.6 32.9

0.73 5.87 9.17 1.34 10.5 11.6

2.03 11.4 17.0 2.85 11.2 18.8

5.7 42 60 15 52 65

effects’ depend upon chemical structure and are generally found to be inversely related to particle size [29,30]. 29 Si MAS NMR analysis has demonstrated that the incorporation of 0.2–5 μm ZrO2 particles in white Portland cement paste increases the degree of hydration by 160% within the first 6 h. TEM analysis has also shown that the early C-S-H gel products are found in direct contact with the surface of the ZrO2 particles. Hence, these findings suggest that the accelerating effect of zirconium oxide is a catalytic phenomenon which derives from the presentation of suitable nucleation sites for the precipitation and growth of the C-S-H gel product phase. Powder XRD, 27 Al MAS NMR and TEM analyses have indicated that the hydration of the aluminate and ferrite phases is little affected by the presence of ZrO2, as the rates of development of ettringite and tetracalcium aluminate hydrate are not dissimilar in the two cement pastes and these

(a)

WPC-Zr-168 Intensity (arb)

C-S-H AFt

(b)

AFt

Fig. 8. Subtracted, fitted and deconvoluted 29Si MAS NMR spectra of ZrO2-blended WPC pastes.

ZrO 2 accelerate the initial hydration reactions and result in a cement matrix with improved microstructure, durability and mechanical strength [29,30]. Enhanced microstructure and mechanical strength arise from the superior density of packing afforded by the occupation of the interstices between the larger (ca. 20–50 μm) cement grains by the fine admixture particles. The acceleration of the early hydration reactions is attributed the presence of heterogeneous nucleation sites on the surfaces of the particles which promote the precipitation and development of the initial cement hydration products. The extents of these ‘filler

C-S-H & portlandite

Fig. 9. TEM images of (a) WPC-24 and (b) WPC-Zr-24.

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phases are not found in intimate contact with the surface of the ZrO2 particles. 4. Conclusions This paper examines the impact of 20 wt.% zirconium oxide particles in the size-range of 0.2 to 5 μm on the early hydration chemistry of white Portland cement. Zirconium oxide, a candidate radiopacifying agent, does not formally participate in the chemical reactions of the hydrating cement; however, the surface of the zirconium oxide particles presents heterogeneous nucleation sites for the precipitation and growth of the early C-S-H gel products which accelerates the initial setting reactions. Conversely, the presence of zirconium oxide was found to have little impact on the development of the calcium (sulpho)aluminate hydrate phases. Solid state 27Al and 29Si MAS NMR spectroscopy is a powerful tool for the analysis of aluminosilicate materials which has been widely applied to study the hydration behaviour of Portland cement systems for construction and environmental engineering. Surprisingly, to date, this technique has been entirely overlooked by researchers working in the field of Portland cement-based biomaterials. In addition to understanding the effect of novel radiopacifiers and other admixtures on the hydration of Portland cements, solid state NMR could also provide valuable information on the impact of the clinical environment (i.e. exposure to physiological fluids, antiseptics and anaesthetics) on these materials. References [1] M. Torabinejad, D. White, Tooth filling material and use. US Patent 1995, No. 5415547. [2] H.W. Roberts, J.M. Toth, D.W. Berzins, D.G. Charlton, Dent. Mater. 24 (2008) 149–164. [3] M. Parirokh, M. Torainejad, J. Endod. 36 (2010) 16–27. [4] B.W. Darvell, R.C.T. Wu, Dent. Mater. 27 (2011) 407–422.

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[5] H.F.W. Taylor, Cement Chemistry, first ed. Academic Press, London, 1990. [6] H. Pöllmann, in: J. Bensted, P. Barnes (Eds.), Structure and Performance of Cements, Spon Press, London, 2002, pp. 25–56. [7] E.M. Gartner, J.F. Young, D.A. Damidot, I. Jawed, in: J. Bensted, P. Barnes (Eds.), Structure and Performance of Cements, Spon Press, London, 2002, pp. 57–113. [8] A.K. Chatterjee, in: J. Bensted, P. Barnes (Eds.), Structure and Performance of Cements, Spon Press, London, 2002, pp. 186–236. [9] K.S. Coomaraswamy, P.J. Lumley, M.P. Hofmann, J. Endod. 33 (2007) 295–298. [10] K.S. Min, H.S. Chang, J.M. Bae, S.H. Park, C.U. Hong, E.C. Kim, J. Endod. 33 (2007) 1342–1346. [11] K.S. Coomaraswamy, P.J. Lumley, R.M. Shelton, M.P. Hofmann, Key Eng. Mater. 361–363 (2008) 885–888. [12] E.A. Bortoluzzi, J.M. Guerreiro-Tanomaru, M. Tanomaru-Filho, M.A.H. Duarte, Oral Surg. Oral Med. Oral Pathol. Oral Radiol. Endod. 108 (2009) 628–632. [13] M.A.H. Duarte, G.D. de Oliveira El Kadre, R.R. Vivan, J.M. Guerreiro-Tanomaru, M. Tanomaru-Filho, I.G. de Moraes, J. Endod. 35 (2009) 737–740. [14] J. Camilleri, M.G. Gandolfi, Int. Endod. J. 43 (2010) 21–30. [15] A. Cutajar, B. Mallia, S. Abela, J. Camilleri, Dent. Mater. 27 (2011) 879–891. [16] International Standards Organization, 2002. (ISO 6876–7.8). [17] J. Camilleri, A. Cutajar, B. Mallia, Dent. Mater. 27 (2011) 845–854. [18] , 2008. [19] C.A. Love, I.G. Richardson, A.R. Brough, Cem. Concr. Res. 37 (2007) 109–117. [20] B. Czarnecka, N.J. Coleman, H. Shaw, J.W. Nicholson, Dent. Med. Probl. 45 (2008) 5–11. [21] G. Engelhardt, D. Michel, High-resolution Solid State NMR of Silicates and Zeolites, first ed. John Wiley & Sons, Chichester, 1987.. [22] H. Justnes, I. Meland, O. Bjoergum, J. Krane, T. Skjetne, Adv. Cem. Res. 3 (1990) 105–110. [23] J. Skibsted, H.J. Jakobsen, C. Hall, J. Chem. Soc. Faraday Trans. 90 (1994) 2095–2098. [24] J. Skibsted, E. Henderson, H.J. Jakobsen, Inorg. Chem. 32 (1993) 1013–1027. [25] M.D. Andersen, H.J. Jakobsen, J. Skibsted, Cem. Concr. Res. 34 (2004) 857–868. [26] H. Krøyer, H. Lindgreen, H.J. Jakobsen, J. Skibsted, Adv. Cem. Res. 15 (2003) 103–112. [27] E.T. Lippmaa, M. Mägi, A. Samoson, G. Engelhardt, A.R. Grimmer, J. Am. Chem. Soc. 102 (1980) 4889–4893. [28] J. Skibsted, H.J. Jakobsen, C. Hall, J. Chem. Soc. Faraday Trans. 91 (1995) 4423–4430. [29] W.A. Gutterige, J.A. Dalziel, Cem. Concr. Res. 20 (1990) 778–782. [30] E.H. Kadri, S. Aggoun, G. De Schutter, K. Ezziane, Mater. Struct. 43 (2010) 665–673.