Time dependent driving forces and the kinetics of tricalcium silicate hydration

Simulations of tricalcium silicate (C₃S) hydration using a kinetic cellular automaton program, HydratiCA, indicate that the net rate depends both on C₃S dissolution and on hydration product growth. Neither process can be considered the sole rate-controlling step because the solution remains significantly undersaturated with respect to C₃S yet significantly supersaturated with respect to calcium silicate hydrate (C–S–H). The reaction rate peak is attributed to increasing coverage of C₃S by C–S–H, which reduces both the dissolution rate and the supersaturation of C–S–H. This supersaturation dependence is included in a generalized boundary nucleation and growth model to describe the kinetics without requiring significant impingement of products on separate cement grains. The latter point explains the observation that paste hydration rates are insensitive to water/cement ratio. The simulations indicate that the product layer on C₃S remains permeable; no transition to diffusion control is indicated, even long after the rate peak.     

Early hydration of tricalcium silicate I. Kinetics of the hydration process and the stoichiometry of the hydration products

The initial hydration of C₃S in paste form at room temperature was studied. The process is initiated by a short lasting rapid hydration in which about 1 – 2% of C₃S is hydrated and a hydrate with low C/S and high H/S ratio is formed. After a subsequent induction period of 4 hours a renewed rapid hydration is observed in which a hydrate of constant stoichiometric composition, independent on the time of hydration is formed. This hydrate has a higher C/S and lower H/S ratio than the one formed initially. The liquid phase stays supersaturated with respect to calcium hydroxide for several hours after the induction period is terminated.     

The influence of an ion-exchange resin on the kinetics of hydration of tricalcium silicate

The addition of a finely-ground ion-exchange resin makes it possible to modify the hydration kinetics of C₃S pastes. Analyses of the liquid phase in pastes and more dilute suspensions show that the resin exchanges calcium ions for sodium ions very rapidly during the early stage of hydration and therefore the concentration of silica in solution increases. The resin impacts the hydration of C₃S by other mechanisms which depends on the resin quantity added. For a high resin quantity, the induction period is very short, but the longer-term hydration is enhanced compared to a reference sample without resin. We hypothesize that the surface of the resin can provide sites for the nucleation and growth of C-S-H hydrates and/or portlandite far away from the surface of the C₃S grains. This consequently increases the quantity of hydrates that can precipitate before a continuous hydrate layer forms over the surfaces of C₃S particles.     

Initial hydration of tricalcium silicate as studied by secondary neutrals mass spectrometry: II. Results and discussion

The hydration of tricalcium silicate (C₃S) was studied by secondary neutrals mass spectrometry (SNMS), a method that enables determination of the Ca/Si ratio of the formed calcium silicate hydrate (C-S-H) phase with an extremely low information depth. It was found that the magnitude of this parameter within the hydrate layer formed at the surface of the nonhydrated C₃S is not constant and increases with increasing distance from the liquid-solid interface. It was also found that, at a constant distance from the surface, the Ca/Si ratio declines with hydration time. The kinetics of the hydration process is characterized by a very fast initial reaction, followed by a dormant period and a subsequent period of renewed hydration. The rate of hydration becomes distinctly accelerated by elevated temperature and retarded by the presence of sucrose, while NaCl affects the initial hydration kinetics only to a small degree.     

Hydration of tricalcium silicate

Results of following the quantities of free Ca(OH)₂ and of tricalcium silicate (C₃S) during the hydration of C₃S, and also the influence of the presence of free CaO on this reaction are in accordance with the hypothesis of Stein & Stevels with regard to the hydration of C₃S. at the first contact between C₃S and water, a surface hydrate, invisible by electron microscope methods, is considered to be formed and to retard the reaction strongly. This hydrate is thought to change into one which retards the hydration reaction less and changes later into a third hydrate, tobermorite gel.     

The hydration of Portland cement,C3S and C2S in the presence of a calcium complexing admixture (EDTA)

The effect of EDTA, a calcium chelating agent, on the early hydration of Portland cement, C₃Sand β-C₂S has been studied by solution analysis and electron microscopy. EDTA is a retarded of cement hydration. Under normal conditions of hydration, the silica levels in solution are very low (<0.05 M) but in the presence of EDTA an initial flush of silica appears in the bulk aqueous phase. On continued hydration, following the saturation of EDTA with calcium, the appearance of ‘free’ calcium causes precipitation of C-S-H gel from the bulk solution and changes in microstructure of the colloidal gel around clinker particles in C₃S and β-C₂S pastes are observed. The action of EDTA as a retarding admixture is explained in terms of the membrane model of cement hydration.     

A Reaction Zone Hypothesis for the Effects of Particle Size and Water‐to‐Cement Ratio on the Early Hydration Kinetics of C3S

The hydration kinetics of tricalcium silicate (C₃S) has been the subject of much study, yet the experimentally observed effects of the water‐to‐cement (w/c) ratio and particle size distribution have been difficult to explain with models. Here, we propose a simple hypothesis that provides an explanation of the lack of any significant effect of w/c on the kinetics and for the strong effect of the particle size distribution on the amount of early hydration associated with the main hydration peak. The hypothesis is that during the early hydration period the calcium–silicate–hydrate product forms only in a reaction zone close to the surface of the C₃S particles. To test the hypothesis, a new microstructure‐based kinetics (MBK) model has been developed. The MBK model treats the C₃S particle size distribution in a statistical way to save computation time and treats the early hydration as essentially a boundary nucleation and growth process. The MBK model is used to fit kinetic data from two published studies for C₃S with different size distributions, one for alite (impure C₃S) pastes and one for stirred C₃S suspensions. The model is able to fit all the data sets with parameters that show no significant trend with particle size, providing support for the reaction zone hypothesis.     

A soft X-ray microscope investigation into the effects of calcium chloride on tricalcium silicate hydration

Calcium chloride (CaCl₂) is one of the most recognized and effective accelerators of hydration, setting, and early strength development in portland cement and tricalcium silicate (C₃S) pastes. The mechanisms responsible for this acceleration, as well as the microstructural consequences, are poorly understood. Soft X-ray transmission microscopy has recently been applied to the study of cementitious materials and allows the observation of hydration in situ over time. This technique was applied to the examination of tricalcium silicates hydrating in a solution containing CaCl₂. It appears that CaCl₂ accelerates the formation of “inner product” calcium silicate hydrate (C-S-H) with a low-density microstructure.     

Combined effect of lignosulfonate and carbonate on pure portland clinker compounds hydration. IV. Hydration of tricalcium aluminate-sodium oxide solid solution

The combined effect of lignosulfonate and sodium carbonate on the hydration of C₃A and C₃ANa₂O solid solution was examined by DTG and TG curves, by XRD analysis and by zeta potential measurements. It is confirmed that the simultaneous addition of lignosulfonate and carbonate completely blocks the C₃A hydration with an induction period whose length is proportional to the percentage of admixtures. On the other hand, no induction period was observed in the hydration of C₃ANa₂O solid solution in the presence of both lignosulfonate and carbonate. The effect of the admixtures on the zeta potential is substantially the same for C₃A and C₃ANa₂O solid solution. The liquefying effect of NC and lgs combined addition seems to be more pronounced on C₃A than on C₃ANa₂O solid solution.     

Action of triethanolamine on the hydration of tricalcium aluminate

Hydration characteristics of tricalcium aluminate and tricalcium aluminate + gypsum were studied following addition of 0.5, 1.0, 5.0 or 10.0% triethanolamine (TEA) at a solution/C₃A ratio of 1.0 after hydration periods of 1 to 60 min. TEA accelerated the hydration of C₃A to the hexagonal aluminate hydrate and its conversion to the cubic aluminate hydrate. The rate of hydration increased with increased amounts of TEA, which also accelerated the formation of ettringite in the C₃A-gypsum-H₂O system.     

Hydration of cubic tricalcium aluminate in the presence of calcined clays

Calcined clay blended cements play a major role in cement industry's strategy to reduce CO₂ emissions. During their hydration, an accelerated aluminate reaction is often observed to affect the sulfate balance. The objective of this study was to provide insights into the influence of different calcined clays on the hydration of cubic tricalcium aluminate (C₃A). A cementitious model system consisting of cubic C₃A, quartz powder, calcium sulfate and a model pore solution was investigated. The influence of three different calcined clays and one nanolimestone was examined by a successive replacement of the quartz powder and variation of the calcium sulfate. Heat flow and hydrate phase development were followed by isothermal calorimetry and quantitative in-situ X-ray diffraction. Accelerated ettringite formation and sulfate depletion were observed for all calcined clays, while the nanolimestone exhibited the opposite effect. It was found that adsorption of SO₄ ions and/or Ca-SO₄-complexes at the surface of calcined clay particles is the main factor inhibiting retardation of the C₃A hydration in absence of a silicate reaction. In the Al-rich systems a retardation through sulfate adjustment seems to be impeded by additional Al ions, which react with Ca adsorbed onto and leached from the C₃A surface.     

New insights into tricalcium silicate hydration in paste

The knowledge of the aqueous phase composition during the hydration of tricalcium silicate (C₃S) is a key issue for the understanding of cement hydration. A new in situ method of computing calcium ion concentration from the measurement of the electrical conductivity on paste was coupled to isothermal calorimetry and BET measurements to get new insights on the early hydration of C₃S. Ion concentrations of the aqueous phase are mainly dependent on the degree of hydration and the water to C₃S ratio. In the case of C₃S paste, the calcium and silicon concentrations determined at low degrees of hydration can be related to the equilibrium curve of C-S-H having C/S = 1.27 and named C_1.27SH_y. It is expected that C_1.27SH_y thermodynamically controls the aqueous phase composition at this early stage. Indeed, the formation of C_1.27SH_y is quasi-immediate when C₃S is in contact with water inducing a very rapid increase of the specific surface area that remains constant during the induction period. At higher degrees of hydration, the aqueous phase composition departs from the C_1.27SH_y equilibrium curve. C_1.27SH_y appears to be a metastable C-S-H that could be related to an intermediate phase previously reported. The quasi-immediate precipitation of C_1.27SH_y on C₃S surface explains why calcium and silicon concentrations remain low during early hydration even though C₃S is strongly undersaturated. This also agrees with the control of the end of the induction period by the nucleation and growth of more stable C-S-H.     

Implications for C3S kinetics from combined C3S/CA hydration

This study was performed to understand the influence of a critical amount of CA on the kinetics of C₃S hydration. For this purpose, monoclinic C₃S was blended with 15 wt.% CA and investigated by heat flow calorimetry and in situ XRD at 23°C at a water to cement ratio of 0.5. The binary mixture shows 3 distinct heat flow maxima where the underlying C₃S dissolution is proceeding stepwise. The C₃S dissolution rates during the 3 steps are varying strongly, depending on the hydrate phase precipitated during the respective reaction step. Comparison of these dissolution rates with a pure C₃S reference sample allows the conclusion that the dissolution rate of pure C₃S after the heat flow maximum might be governed by either the remaining available reactive surface of C₃S or a diffusion‐controlled process, which would both be influenced by the respective hydrate phase precipitating on the C₃S surface.     

The Effect of Magnesium and Zinc Ions on the Hydration Kinetics of C3S

Impure tricalcium silicate (C₃S) in portland cement may contain various foreign ions. These ions can stabilize different polymorphs of C₃S at room temperature and may affect its reactivity. In this paper, the effects of magnesium and zinc on the polymorph type, hydration kinetics, and the hydrate morphology of C₃S were investigated. The pure C₃S has the T1 structure while magnesium and zinc stabilize polymorphs M3 and T2/T3, respectively. The two elements have distinct effects on the hydration kinetics. Zinc increases the maximum heat released. Magnesium increases the hydration peak width. The C–S–H morphology is modified, leading to longer needles in the presence of zinc and thicker needles in the presence of magnesium. Zinc is incorporated into C–S–H, while magnesium is only incorporated slightly, if at all, but rather seems to inhibit nucleation. Implementing experimentally measured parameters for C–S–H nucleation and growth in the μic hydration model captured well the observed changes in hydration kinetics. This supports C–S–H nucleation and growth to be rate controlling in the hydration of C₃S.     

Early hydration of cement constituents with organic admixtures

The hydration of C₂S, C₃S, C₃A, C₄AF and type 1 portlant cement in the presence of calcium lignosulfonate and salicylic acid was studied at a high(20/1) water-cement ratio. The effect of these admixtures on the development, microstructure and surface area of the hydration products was investigated.     

Changes in C3S hydration in the presence of cellulose ethers

The influence of cellulose ethers (CE) on C₃S hydration processes was examined in order to improve our knowledge of the retarding effect of cellulose ethers on the cement hydration kinetics. In this frame, the impacts of various cellulose ethers on C₃S dissolution, C-S-H nucleation-growth process and portlandite precipitation were investigated. A weak influence of cellulose ethers on the dissolution kinetics of pure C₃S phase was observed. In contrast, a significant decrease of the initial amount of C-S-H nuclei and a strong modification of the growth rate of C-S-H were noticed. A slowing down of the portlandite precipitation was also demonstrated in the case of both cement and C₃S hydration. CE adsorption behavior clearly highlighted a chemical structure dependence as well as a cement phase dependence. Finally, we supported the conclusion that CE adsorption is doubtless responsible for the various retarding effect observed as a function of CE types.     

Effect of Carbon‐Based Materials on the Early Hydration of Tricalcium Silicate

Two types of carbon‐based materials, i.e., mesoporous carbon and HNO₃‐oxidized carbon nanotubes, with nearly the same specific surface area and abundant in surface oxygen‐containing functional groups were selected in order to examine their effect on the hydration of tricalcium silicate (C₃S), the main portland cement component, in early stages. Different methods, including XPS and TG‐MS analyses, electrokinetic potential measurements, as well as determination of adsorption capacity for calcium ions from aqueous solutions, were used to investigate the physicochemical surface properties of the selected carbon‐based materials. It was found that the carbon‐based materials with high specific surface area and rich in oxygen‐containing functional groups on their surfaces have a catalytic effect on early C₃S hydration. It was observed that the modification of C₃S paste with the selected materials added in high concentrations (1 wt% and higher) led to an increase in the rate and degree of C₃S hydration in the early stages. The mechanism of early C₃S hydration accelerated by carbon‐based materials rich in surface functional groups was clarified by the example of the mesoporous carbon. It was found that the oxygen‐containing functional groups present on the carbon surface have both an influence on the content of calcium ions in the aqueous phase of the C₃S paste and an indirect positive effect in relation to the specific surface of C₃S.     

Effect of particle size distribution of metakaolin on hydration kinetics of tricalcium silicate

The focus of this study is to elucidate the role of particle size distribution (PSD) of metakaolin (MK) on hydration kinetics of tricalcium silicate (C₃S–T₁) pastes. Investigations were carried out utilizing both physical experiments and phase boundary nucleation and growth (pBNG) simulations. [C₃S + MK] pastes, prepared using 8%_mass or 30%_mass MK, were investigated. Three different PSDs of MK were used: fine MK, with particulate sizes <20 µm; intermediate MK, with particulate sizes between 20 and 32 µm; and coarse MK, with particulate sizes >32 µm. Results show that the correlation between specific surface area (SSA) of MK's particulates and the consequent alteration in hydration behavior of C₃S in first 72 hours is nonlinear and nonmonotonic. At low replacement of C₃S (ie, at 8% _mass), fine MK, and, to some extent, coarse MK act as fillers, and facilitate additional nucleation and growth of calcium silicate hydrate (C–S–H). When C₃S replacement increases to 30% _mass, the filler effects of both fine and coarse MK are reversed, leading to suppression of C–S–H nucleation and growth. Such reversal of filler effect is also observed in the case of intermediate MK; but unlike the other PSDs, the intermediate MK shows reversal at both low and high replacement levels. This is due to the ability of intermediate MK to dissolve rapidly—with faster kinetics compared to both coarse and fine MK—which results in faster release of aluminate [Al(OH)₄⁻] ions in the solution. The aluminate ions adsorb onto C₃S and MK particulates and suppress C₃S hydration by blocking C₃S dissolution sites and C–S–H nucleation sites on the substrates’ surfaces and suppressing the post-nucleation growth of C–S–H. Overall, the results suggest that grinding-based enhancement in SSA of MK particulates does not necessarily enhance early-age hydration of C₃S.     

Influence of pozzolanic additives on hydration mechanisms of tricalcium silicate

Pozzolanic mineral additives, such as silica fume (SF) and metakaolin (MK), are used to partially replace cement in concrete. This study employs extensive experimentation and simulations to elucidate and contrast the influence of SF and MK on the early age hydration rates of tricalcium silicate (triclinic C₃S), the major phase in cement. Results show that at low replacement levels (i.e., ≤10%), both SF and MK accelerate C₃S hydration rates via the filler effect, that is, enhanced heterogeneous nucleation of the main hydration product (C–S–H: calcium‐silicate‐hydrate) on the extra surfaces provided by the additive. The filler effect of SF is inferior to that of MK because of agglomeration of the fine particles of SF, which causes significant reduction (i.e., up to 97%) in its surface area. At higher replacement levels (i.e., ≥20%), while SF continues to serve as a filler, the propensity of MK to allow nucleation of C–S–H on its surface is substantially suppressed. This reversal in the filler effect of MK is attributed to the abundance of aluminate [Al(OH)₄^?] ions in the solution—released from the dissolution of MK—which inhibit topographical sites for C–S–H nucleation and impede its subsequent growth. Results also show that in the first 24 hours of hydration, MK is a superior pozzolan compared to SF. However, the pozzolanic activities of both SF and MK are limited and, thus, do not produce significant alterations in the early age hydration kinetics of C₃S. Overall, the outcomes of this study provide novel insights into the mechanistic origins of the filler and pozzolanic effects of SF and MK, and their impact on cementitious reaction rates.     

Early hydration of tricalcium silicate II. The induction period

The initial hydration of C₃S was found to be stimulated by adding to the paste prehydrated C₃S and by lowering the Ca⁺⁺ concentration of the liquid phase with oxalic acid. An addition of crystalline calcium hydroxide did not alter the duration of the induction period. Based on these findings the origin of the induction period in C₃S hydration is discussed.