Improved evidence for the existence of an intermediate phase during hydration of tricalcium silicate

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Abstract

Tricalcium silicate (Ca3SiO5) with a very small particle size of approximately 50 nm has been prepared and hydrated for a very short time (5 min) by two different modes in a paste experiment, using a water/solid-ratio of 1.20, and by hydration as a suspension employing a water/solid-ratio of 4000. A phase containing uncondensed silicate monomers close to hydrogen atoms (either hydroxyl groups or water molecules) was formed in both experiments. This phase is distinct from anhydrous tricalcium silicate and from the calcium–silicate–hydrate (C–S–H) phase, commonly identified as the hydration product of tricalcium silicate. In the paste experiment, approximately 79% of silicon atoms were present in the hydrated phase containing silicate monomers as determined from 29Si{1H} CP/MAS NMR. This result is used to show that the hydrated silicate monomers are part of a separate phase and that they cannot be attributed to a hydroxylated surface of tricalcium silicate after contact with water. The phase containing hydrated silicate monomers is metastable with respect to the C–S–H phase since it transforms into the latter in a half saturated calcium hydroxide solution. These data is used to emphasize that the hydration of tricalcium silicate proceeds in two consecutive steps. In the first reaction, an intermediate phase containing hydrated silicate monomers is formed which is subsequently transformed into C–S–H as the final hydration product in the second step. The introduction of an intermediate phase in calculations of the early hydration of tricalcium silicate can explain the presence of the induction period. It is shown that heterogeneous nucleation on appropriate crystal surfaces is able to reduce the length of the induction period and thus to accelerate the reaction of tricalcium silicate with water.

Introduction

Tricalcium silicate (Ca3SiO5) is an inorganic substance that has been found in nature. Modified in composition and crystal structure by ionic substitution and rapid cooling it is called alite, the main constituent of modern Portland cement clinker. It reacts with water at room temperature and forms calcium hydroxide and a semi-crystalline calcium silicate hydrate phase denoted C–S–H (Eq. (1)). The composition and structure of the C–S–H phase are variable and thus not perfectly established. In contact with a saturated Portlandite (Ca(OH)2) solution, it is supposed to have an atomic calcium/silicon-ratio of approximately 1.7, a water content of 20–44%, more or less long chains (dreierketten) of silicate tetrahedra, and structural similarities to a calcium silicate hydrate mineral (tobermorite) [1]. More than twenty crystalline calcium silicate hydrates are known to exist, however, in this paper the term C–S–H is reserved to the final product resulting from tricalcium silicate hydration at ambient temperature.Ca3SiO5 + (1.3 + x) H2O  1.7CaO·SiO2·xH2O + 1.3 Ca(OH)2

Thermodynamic calculations on the reaction of tricalcium silicate with water forming C–S–H and calcium hydroxide have been carried out [[2], [3], Appendix A]. These calculations predict that the solubility of Ca3SiO5 in water is much higher than the solubility of the reaction products. Beside the high solubility of tricalcium silicate, the average dissolution rate of Ca3SiO5 is close to 10 µmol/(s m²) in pure water and decreases to less than 0.1 µmol/(s m²) in a saturated Portlandite solution [4] and even higher rates have been reported in diluted systems [5]. These rates are very fast and supersaturation with respect to C–S–H is expected to be rapidly reached, allowing a homogeneous formation of nuclei of the hydration products directly from the solution. Calculations referring to the conditions commonly used during reaction of Ca3SiO5 with water (water/solid-ratio = 0.50, temperature = 25 °C, specific surface area = 3000 cm²/g, presence of minor amounts of free lime) show that the dissolution of tricalcium silicate proceeds at a very high velocity. Furthermore, they indicate that the formation of the first nuclei of C–S–H can be expected within less than one second after mixing with water. After the formation of these nuclei, there should be a very fast growth of the once formed nuclei in combination with a rapid dissolution of Ca3SiO5. Such a fast reaction is due to the high difference in Gibbs free energy (− 84.4 kJ/mol) between the initial (Ca3SiO5, H2O) and final (C–S–H, Ca(OH)2) state of the reaction in combination with a high dissolution rate of tricalcium silicate. Kinetic calculations assuming the aforementioned conditions reveal that the reaction should be completed within a few hours.

It can be concluded that thermodynamic and kinetic calculations indicate that the reaction of Ca3SiO5 with water to form C–S–H and calcium hydroxide should start directly after contact of this mineral with water and proceed immediately in one kinetic step as observed for the hydration of tricalcium aluminate. In contrast to these predictions, a time lapse denoted as dormant or induction period is observed before the reaction starts to accelerate (Fig. 1). After releasing minor amounts of heat for a short period of time, often attributed to wetting and dissolution, no significant adsorption or liberation of heat is observed for some hours. Acceleration of the reaction starts after about 2–6 h when the main hydration period begins.

The discrepancy between predicted and observed behavior for the Ca3SiO5 hydration has been discussed for a long time and a few hypotheses have been put forward to explain the experimental results. A review of these ideas considering studies until 1997 was given by Taylor [1] and it was concluded that in the first seconds of hydration, a phase distinct from C–S–H forms on the surface of tricalcium silicate thereby preventing its continued dissolution. The solubility of this first hydrate (termed product B by Taylor based on a designation by Jennings [12]) controls the ionic concentrations in the aqueous phase and generates a slight supersaturation with respect to C–S–H as the final product. The first hydrate is structurally distinct from the C–S–H phase since it contains only monomeric silicate tetrahedra (Q0 units) whereas the SiO4 tetrahedra are condensed to more or less long chains in the C–S–H phase. Because the first hydrate exhibits a solubility higher than that of C–S–H, it is metastable with respect to the C–S–H and finally transforms into this phase. Taylor [1] concludes from this data that the reaction of tricalcium silicate with water proceeds in two steps. In the first step, Ca3SiO5 reacts with water and forms an intermediate phase which is converted in the second step into C–S–H as the final product. Both reactions proceed simultaneously during the whole time of hydration (Fig. 2).

This peculiarity can be explained by the comparatively slow condensation/oligomerization of silicate tetrahedra. Tricalcium silicate includes only isolated SiO4 tetrahedra (Q0 sites) and thus, monomeric silicate ions are formed during the fast dissolution of this phase. As discussed before, the dissolution proceeds at such a high velocity that a supersaturation, allowing a homogeneous nucleation of C–S–H, is reached within less than one second. Despite the supersaturation should allow precipitation, C–S–H cannot form in such a short period of time (less than 1 s). Formation of C–S–H requires the condensation of silicate tetrahedra forming dimers or longer chains. Since these condensed units are complicated to form, the precipitation of C–S–H nuclei cannot take place instantly and tricalcium silicate continues to dissolve very fast. Within a fraction of a second, concentrations of Ca2+ and H3SiO4 ions are reached in the aqueous phase that allow nucleation of a calcium silicate hydrate containing only monomeric silicate species and which can be related to the product B defined by Taylor [1]. This phase is assumed to form directly on the surface of tricalcium silicate [1], its precipitation causing a reduction of the silicon concentration in the aqueous phase within the first seconds or minutes [2], [10], [11]. To summarize, the hydration of tricalcium silicate obeys directly Ostwald's rule of stages. The very fast dissolution of tricalcium silicate is responsible for the formation of a phase containing silicate monomers only. This initial reaction is very rapid, but the process continues much more slowly after the first precipitation of hydrates. The reduction of the rate of reaction indicates that the intermediate phase forms a kind of protective layer around the anhydrous core of the particles [1]. After formation of the intermediate phase (product B), the ionic concentrations in the solution are controlled by the solubility of this phase.

A number of spectroscopic results is available that supports the presence of an intermediate phase (product B) that is responsible for the isolation of anhydrous cores of Ca3SiO5 particles from the solution [6], [7], [8], [9]. Most important is the paper by Rodger et al. employing 29Si magic-angle spinning (MAS) and 29Si{1H} cross-polarization CP/MAS nuclear magnetic resonance (NMR) spectroscopy [6]. In that study it was shown that resonances from the C–S–H, originating from condensed silicate tetrahedra in Q1 (end members in chains) and Q2 (middle groups in chains) SiO4 units, are not observed before the end of the induction period. Instead, minor amounts of hydrated monomeric silicate tetrahedra (Q0) were detected during the induction period.

Other hypothesis than the protective layer theory have been proposed to explain the existence of an induction period. The reduction of the dissolution rate after the initial reaction in the first minutes has been attributed to the presence of calcium hydroxide or absence of crystallographic defect sites but is in contrast with experimental data. Reported dissolution rates [4], [5] are high, even in the aforementioned conditions and will unavoidably lead to ionic concentrations much beyond those observed during hydration of tricalcium silicate [12]. Therefore, the lack of other explanations for the presence of an induction period during Ca3SiO5 hydration leads to the conclusion that product B acts as a protective layer around the anhydrous particle cores. However, there is no direct experimental evidence to prove this conclusion.

Until now, the question of the existence of an intermediate phase, despite its importance for understanding the reaction of Portland cement clinker with water, is not well resolved. For this reason, we focus on this issue in the present paper. The principal aim of this investigation is to complement the results reported by Rodger et al. [6] and Damidot et al. [13] by synthesizing samples containing a high amount of the intermediate phase. Such samples would allow a better characterization of the intermediate phase which was detected only in minor amounts in the previous studies (i.e., 2% in ref. [6]). An important tool in the present characterization is 29Si MAS NMR employing 1H cross-polarization which allows selective observation of silicate species with hydrogen atoms in their near vicinity. Furthermore, the 29Si NMR resonances observed for the intermediate phase are compared with signals reported for crystalline phases in the CaO–SiO2–H2O system in order to identify structural similarities between the intermediate phase and the crystalline phases if such relations exist. To obtain a high amount of the intermediate phase, the starting material had a very small particle size, which is essential since the surface layer is very thin. If coarse particles are used, the fraction of intermediate phase relative to unreacted Ca3SiO5 is so low, that the intermediate phase, if existing, can hardly be detected. With progressively lower particle sizes, the amount of intermediate phase relative to unreacted Ca3SiO5 increases, favoring the detection of this phase.

Section snippets

Materials and sample preparation

Ca3SiO5 was prepared by high-temperature solid-state reaction between calcium carbonate and amorphous silicon dioxide. The starting materials were mixed in the stoichiometric ratio for tricalcium silicate and then fired in a platinum crucible at 1520 °C. The procedure was repeated twice at a temperature of 1550 °C to obtain a free lime content of 0.4 wt.%. The synthesized material was ground in a special milling device (MiniCer, Netzsch) aimed to obtain grain sizes in the range of 30–80 nm.

Powder XRD measurements

The results from the XRD examinations are illustrated in Fig. 4. In the anhydrous tricalcium silicate (nano-Ca3SiO5), only the triclinic form of Ca3SiO5 is identified. However, the peaks are very broad as a result of small crystallite sizes and serious microstrain introduced by grinding [16]. Quantitative phase analysis using the Rietveld method and an anhydrous sample spiked with 20% zincite (ZnO) as internal standard reveals an amorphous content as high as 41%. In contrast to this, the

Discussion

Nanosized tricalcium silicate was produced for this study with particle diameters between 40 and 200 nm. The specific surface measured by nitrogen absorption was approximately 22 m²/g. An estimation of the mean particle diameter from the specific surface results in a value of 88 nm if a spherical shape and a uniform particle size are assumed. The existence of nano-cracks cannot be excluded. It can be assumed that the surface accessible to water has been exactly determined by nitrogen adsorption

Conclusions

The early hydration of laboratory made tricalcium silicate has been addressed in this study. A material with a very high specific surface area has been produced in order to increase the fraction of the early hydration product relative to the particle core of anhydrous Ca3SiO5. As a result of this high surface area, the starting material attracted a small amount of isopropanol (used in the grinding process) and water before initiation of the hydration experiments. The presence of these

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