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.     

Induction period of hydration of tricalcium silicate

The length of the induction period of tricalcium silicate hydration has been found to be related to the probability of escape of trapped electrons measured by thermoluminescence on anhydrous sample. Several strong experimental evidences suggest an analogy between the hydration of tricalcium silicate with water vapour and the main reaction of the induction period in paste.On this basis a mechanism is tentatively proposed.     

Early hydration of large single crystals of tricalcium silicate

A reaction product believed to be an initial hydrate layer was observed to have formed on large pure C₃S single crystals after 5 minutes of hydration. This layer then increased in thickness and became covered with micrometer-sized spheres of poorly crystallized Ca(OH)₂ within 30 minutes. Subsequently, the formation of a new hydrate of acicular morphology was observed to occur on the surface of the first-formed hydrate. This transformation was accompanied by the disappearance of the first hydrate layer and the calcium hydroxide spheres. The observation of a transient initial hydrate is in accord with the view that the rate of early C₃S hydration is controlled by the formation and subsequent disappearance of a barrier layer.     

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.     

Mathematical modeling of tricalcium silicate hydration

Based on conceptual models for the stages in the hydration of tricalcium silicate, a mathematical model was developed. The separate resistances in the mathematical model correspond to the phenomenological stages of the conceptual model. Comparison of model output with available hydration data gave a reasonable fit between the model and the data.     

Studies on the hydration of tricalcium silicate pastes III. Influence of admixtures on hydration and strength development

A study has been made of the influence of calcium salts, as set-controlling admixtures, on the hydration of tricalcium silicate. It is found that some admixtures modify the morphology of C-S-H gel and for calcium hydroxide. Changes in capillary porosity distribution can be correlated with changes in the morphology of the outermost C-S-H gel. It is concluded that capillary porosity, as controlled by the degree of hydration is still the dominant factor controlling tensile strength.     

Combined hydration of tricalcium silicate and β-dicalcium silicate

The mutual interaction of tricalcium silicate (C₃S) and β-dicalcium silicate (β-C₂S) in their combined hydration was studied. The rate of β-C₂S hydration was accelerated significantly in the presence of C₃S. The rate of C₃S hydration was retarded, but only in the presence of large amounts of β-C₂S. The stoichiometric composition and the pore structure of the hydrates formed was altered only unsignificantly when both compound hydrated simultaneously.     

Carbonation of the hydration products of tricalcium silicate

C₃S has been hydrated for increasing time and stored for 2.5 years under normal atmosphere, the fresh and aged materials being characterized by X-ray diffraction and infrared spectroscopy. The carbonation occuring during storage gives rise to complete disappearance of CSH gel while portlandite remains in appreciable amount; the siliceous residue is an amorphous silica similar to common silica gels. The carbonates formed are vaterite and aragonite, the latter being relatively more important in samples with a low degree of hydration.     

Low pressure steam hydration of tricalcium silicate

Pastes of C₃S (w/c ratio = 0.5) were steam cured at 25, 40, 60 and 90°C for 1 hour to 30 days. The results obtained have shown that, as the curing temperature rises, the induction period is shortened and the initial rate of hydration of C₃S is increased; at longer curings, on the other hand, such hydration rate is considerably lowered. In order to explain the influence of temperature on the hydration reaction a new hypothesis has been proposed, which takes into account the C/S molar ratio as well as the surface properties of the hydrated silicate.     

Dissolution rates during the early hydration of tricalcium silicate

The hydration of triclinic tricalcium silicate containing foreign oxides was investigated in this study. Two water/solid-ratios of 50 and 0.50 were applied. The kinetics of the reaction was analyzed by a combination of methods including thermal analysis, pore solution analysis, and calorimetry. From these data, the evolution of the rate of reaction during hydration was calculated. The results were compared to the free, unconstrained dissolution of tricalcium silicate in undersaturated solutions. Such dissolution rates were analyzed in a dissolution cell connected to an ICP-OES instrument. A comparison of the free dissolution rate in the absence of precipitation of hydrates to the rate of reaction showed that the dissolution of tricalcium silicate is much faster than the rate of reaction of the global process.     

A kinetic model for the hydration of tricalcium silicate

A kinetic model describing the hydration of C₃S has been developed. The model is predicated on the assumption that the formation of a final hydrate phase initiates in transient hydrate layers which surround the anhydrous grains. This transformation results in the onset of the acceleratory period. The model predicts C-S-H formation to be controlled by interfacial processes during the acceleratory period and by diffusional processes thereafter and that the growth of particles is essentially one-dimensional throughout the course of both the acceleratory and post-acceleratory periods.     

Influence of precuring on high pressure steam hydration of tricalcium silicate II. Effect of fineness of tricalcium silicate

C₃S samples of different fineness were precured at room temperature and subsequently autoclaved. It was found that, as the fineness is increased: 1) the effect of precuring on the hydration rate of C₃S in autoclave is less evident; 2) the precuring time corresponding to the highest amount of C₃SH_1.5 becomes longer; 3) particularly for short precuring times, the quantity of C₃SH_1.5 decreases and the formation of α-C₂SH is favored; 4) the amounts of the two crystalline hydrated silicates are reduced while the formation of CSH is favored.     

Structural and hydration properties of amorphous tricalcium silicate

Mechanical milling was carried out to synthesize amorphous tricalcium silicate (Ca₃SiO₅) sample, where Ca₃SiO₅ is the most principal component of Portland cement. The partial phase transformation from the crystalline to the amorphous state was observed by X-ray and neutron diffractions. Moreover, it was found that the structural distortion on the Ca-O correlation exists in the milled Ca₃SiO₅. The hydration of the milled Ca₃SiO₅ with D₂O proceeds as follows: the formation of hydration products such as Ca(OD)₂ rapidly occurs in the early hydration stage, and then proceeds slowly after about 15 h. The induction time for the hydration of the milled Ca₃SiO₅ is approximately one half shorter than that for the hydration of the unmilled one. This result means that the mechanical milling brings about the chemical activity of Ca₃SiO₅ for hydration, and may be particularly useful for increasing the reactivity in the early hydration stage.     

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.     

Influence of inorganic salts on the hydration of tricalcium silicate

The influence of various chlorides and potassium salts on the hydration of alite (3CaO·SiO₂ solid solution) has been studied by conduction calorimetry and an explanation based on diffusion experiments in hardened Portland cement is presented. The mechanism of the action of inorganic electrolytes on cement hydration was also investigated. In hardened Portland cement the diffusion rate of the Cl^? ion was greater than that of the coexisting cations. The accelerating effect of inorganic electrolytes was dependent mainly on the mobility of anions. The higher the anion mobility, the greater was the accelerating effect on the hydration. It is shown that the hydration of alite is a topochemical reaction and that the rate of hydration of alite is controlled by the rate of the dissolution of Ca²⁺ or OH^? ions into a liquid phase. It is concluded that the dissolution of OH^? ions from the hydrate layer around the cement particle is increased when the reciprocal diffusion action of the anion accelerates the hydration.     

Influence of triethanolamine on the hydration characteristics of tricalcium silicate

The hydration characteristics of 3CaO.SiO₂ or β2CaO.SiO₂ are studied by an addition of 0.0, 0.1, 0.5 or 1.0% triethanolamine. The amount of Ca(OH)₂ found at 1, 3, 7 or 28 days was in the order C₃S + 0% TEA > C₃S +0.1% TEA > C₃S + 0.5% TEA > C₃S+1.0% TEA, irrespective of whether lime was estimated by X-ray, DTA, TGA or chemical analysis. The rate of hydration, in terms of the disappearance of 3CaO.SiO₂, showed that hydration proceeded faster in the presence of TEA after 1 day. Additions of TEA increase the induction period, promote the formation of a C-S-H with higher CaO/SiO₂ ratio, increase the formation of non-crystalline Ca(OH)₂ and enhance the surface area of the hydrated silicate product.     

Effect of calcium formate on the hydration of tricalcium silicate

The effect of 0, 0.5, 1.0, 2.0, 4.0 and 6.0% of calcium formate on the hydration of C₃S has been studied. Free lime determinations, non-evaporable water content, pH of the liquid phase, zeta potential, thermal analysis and infrared spectral studies have been made for understanding the mechanism of action of calcium formate. Results indicate that calcium formate acts as an accelerator up to 2%. Above this concentration, the excess of it has practically no effect.     

The tricalcium silicate hydration in the presence of active silica

The influence of active silica upon the tricalcium silicate hydration was studied. On the microcalorimetric curves a considerable acceleration of the process with dormant period elimination was observed. The reason for this is the lowering of calcium ions concentration in the solution as they are consumed, rapidly by the CSH phase formation of a low C/S ratio. Basing on these results the authors presume that the delaying factor of C₃S hydration process is the quasi-stationary layer supersaturated with calcium ions surrounding the anhydrous grains of tricalcium silicate.     

A new approach on the hydration mechanism of tricalcium silicate

A new interpretation on the hydration mechanism of the tricalcium silicate is given. This interpretation is dependent on the total released lime extraction, free, interlayer and “bound” limes, by the modified Franke's method in which the lithium chloride is used as accelerator and to increase the solubility of the complex formed (1). The chemical studies as well as the infrared spectra of the hydrated tricalcium silicate after complete hydration (3.5 years) is identical with the natural and synthetic mineral tobermorite, 5CaO.6SiO₂.5H₂O and is far from the tobermorite-like structure, 3CaO.2SiO₂.3H₂O as stated earlier. The hydration mechanism is divided into five stages which are discussed in full detail.     

Analysis of the surface of tricalcium silicate during the induction period by X-ray photoelectron spectroscopy

X-ray photoelectron spectroscopy allows the analysis of surface layers with a thickness of a few nanometers. The method is sensitive to the chemical environment of the atoms since the binding energy of the electrons depends on the chemical bonds to neighboring atoms. It has been applied to the hydration of tricalcium silicate (Ca₃SiO₅, C₃S) by analyzing a sample after 30 min of hydration. Also two references have been investigated namely anhydrous C₃S and intermediate phase in order to enable a quantitative evaluation of the experimental data. In the hydrated C₃S sample, the analyzed volume (0.2 mm² surface by 13 nm depth) contained approximately 44 wt.% of C₃S and 56 wt.% of intermediate phase whereas CSH was not detected. Scanning Electron Microscopy data and geometric considerations indicate that the intermediate phase forms a thin layer having a thickness of approximately 2 nm and covers the complete surface instead of forming isolated clusters.