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

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.     

Hydration of dibarium silicate II. rate of hydration

The rate of hydration of dibarium silicate (Ba²SiO₄) in paste form at room temperature was investigated. The uncombined barium hydroxide produced in the reaction, determined by a modified Franke method, was used as a measure of the extent of hydration. The linear relationship obtained between the uncombined barium hydroxide and the chemically combined water confirms that similar hydration products are formed at all stages of hydration, and either may be used as a measure of the degree of hydration. The rate of hydration of dibarium silicate was found to be lower than that of tricalcium aluminate, but higher than those of alite, tricalcium silicate and β-dicalcium silicate. The effect of water/solid ratio on the rate of hydration was also investigated. Dibarium silicate was completely hydrated after 30 days when it was mixed with a water/solid weight ratio > 0.7:1.     

The early hydration of tricalcium silicate and β-dicalcium silicate in neat portland cement pastes

Quantitative X-ray diffraction analysis was used to study the early hydration of the tricalcium silicate and β-dicalcium silicate phases in neat Portland cement pastes. There is an initial ‘dormant’ period during which these phases hydrate only very slowly. In the case of the Ca₃SiO₅ phase in the cement used, this ‘dormant’ period lasts 3–4 hours. Of the Ca₃SiO₅ originally present 5% hydrated in 5 h; 22% in 10 h; 34% in 15 h; 45% in 24 h; and 63% in 72 h. No conclusive evidence of any β-dicalcium silicate hydration during the seventy-two hour period investigated was found.     

Influence of water activity on hydration of tricalcium aluminate-calcium sulfate systems

The hydration of tricalcium silicate (C₃S)—the major phase in cement—is effectively arrested when the activity of water (a_H) decreases below the critical value of 0.70. While it is implicitly understood that the reduction in a_H suppresses the hydration of tricalcium aluminate (C₃A: the most reactive phase in cement), the dependence of kinetics of C₃A hydration on a_H and the critical a_H at which hydration of C₃A is arrested are not known. This study employs isothermal microcalorimetry and complementary material characterization techniques to elucidate the influence of a_H on the hydration of C₃A in [C₃A + calcium sulfate (C$) + water] pastes. Reductions in water activity are achieved by partially replacing the water in the pastes with isopropanol. The results show that with decreasing a_H, the kinetics of all reactions associated with C₃A (eg, with C$, resulting in ettringite formation; and with ettringite, resulting in monosulfoaluminate formation) are proportionately suppressed. When a_H ≤0.45, the hydration of C₃A and the precipitation of all resultant hydrates are arrested; even in liquid saturated systems. In addition to—and separate from—the experiments, a thermodynamic analysis also indicates that the hydration of C₃A does not commence or advance when a_H ≤0.45. On the basis of this critical a_H, the solubility product of C₃A (K_C3A) was estimated as 10^−20.65. The outcomes of this work articulate the dependency of C₃A hydration and its kinetics on water activity, and establish—for the first time—significant thermodynamic parameters (ie, critical a_H and K_C3A) that are prerequisites for numerical modeling of C₃A hydration.     

Hydration of cement — role of triethanolamine

Following addition of 0.1, 0.25, 0.35, 0.5 and 1.0 per cent triethanolamine, studies have been made of the hydration and hardening characteristics of (a) tricalcium aluminate, (b) tricalcium aluminate + gypsum, (c) tricalcium silicate, (d) dicalcium silicate, and (e) portland cement. Triethanolamine (TEA) accelerated the hydration of 3CaO.Al₂O₃ and 3CaO.Al₂O₃-CaSO₄.2H₂O systems and extended the induction period of the hydration of 3CaO.SiO₂. In portland cement paste TEA decreased the strength at all ages and setting characteristics were drastically altered, especially at higher TEA contents. Evidence was obtained also of the formation of a complex of TEA with the hydrating silicate phase.     

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.     

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.     

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.     

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.     

Calcium silicate hydrates investigated by solid‐state high resolution H and Si nuclear magnetic resonance

This work focuses on phases formed during cement hydration under high pressure and temperature: portlandite Ca(OH)₂ (CH); hillebrandite Ca₂(SiO₃)(OH)₂ (β‐dicalcium silicate hydrate); calcium silicate hydrate (C-S-H); jaffeite Ca₆(Si₂O₇)(OH)₆ (tricalcium silicate hydrate); α‐C₂SH Ca₂(SiO₃)(OH)₂ (α‐dicalcium silicate hydrate); xonotlite Ca₆(Si₆O₁₇)(OH)₂ and kilchoanite Ca₆(SiO₄)(Si₃O₁₀). Portlandite and hillebrandite were synthesized and characterised by high resolution solid‐state ¹H and ²⁹Si Nuclear Magnetic Resonance. In addition, information from the literature concerning the last five phases was gathered. In certain cases, a schematic 3D‐structure could be determined. These data allow identification of the other phases present in a mixture. Their morphology was also observed by Scanning Electron Microscopy.     

Hydration of tricalcium silicate at elevated temperatures

The early hydration of tricalcium silicate, Ca₃SiO₅, has been studied at temperatures of 25, 50, 75 and 100 °C. Changes in the chemical composition of hydration products formed at different temperatures were followed by means of chemical, thermal and X-ray analyses, and correlated with surface area and pore structure measurements reported previously.     

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.     

Influence of nucleation seeding on the hydration kinetics and compressive strength of alkali activated slag paste

Addition of pure calcium silicate hydrate (C–S–H) to alkali-activated slag (AAS) paste resulted in an earlier and larger hydration rate peak measured with isothermal calorimetry and a much higher compressive strength after 1 d of curing. This is attributed to a nucleation seeding effect, as was previously established for Portland cement and tricalcium silicate pastes. The acceleration of AAS hydration by seeding indicates that the early hydration rate is controlled by nucleation and growth. For the experiments reported here, the effect of C–S–H seed on the strength development of AAS paste between 1 d and 14 d of curing depended strongly on the curing method. With sealed curing the strength continued to increase, but with underwater curing the strength decreased due to cracking. This cracking is attributed to differential stresses arising from chemical and autogenous shrinkage. Similar experiments were also performed on Portland cement paste.     

A study of the microstructure and hydration characteristics of tricalcium silicate in the presence of calcium chloride

The morphological and hydration characteristics of tricalcium silicate treated with 0, 2 and 5 per cent calcium chloride were followed by employing Scanning Electron Microscopy (SEM), Differential Thermal Analysis (DTA), and X-ray diffraction techniques. The water: solid ratios used were either 0.5 or 0.3. In terms of Ca(OH)₂ estimation CaC?₂ accelerated hydration, except with 5 per cent CaC?₂ at a 0.3 w/s ratio. The maximum amount of Ca(OH)₂ and calcium silicate hydrate was formed at 2 per cent CaC?₂ and a w/s ratio of 0.5. Except at very early times, the rate of reaction was slower at 0.3 w/s with or without CaC?₂ compared with the corresponding samples at 0.5 w/s.     

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.     

The filler effect: The influence of filler content and type on the hydration rate of tricalcium silicate

The partial replacement of ordinary portland cement (OPC) by fine mineral fillers accelerates the rate of hydration reactions. This acceleration, known as the filler effect, has been attributed to enhanced heterogeneous nucleation of C‐S‐H on the extra surface provided by fillers. This study isolates the cause of the filler effect by examining how the composition and replacement levels of two filler agents influence the hydration of tricalcium silicate (T1‐Ca₃SiO₅; C₃S), a polymorph of the major phase in ordinary portland cement (OPC). For a unit increase in surface area of the filler, C₃S reaction rates increase far less than expected. This is because the agglomeration of fine filler particles can render up to 65% of their surface area unavailable for C‐S‐H nucleation. By analysis of mixtures with equal surface areas, it is hypothesized that limestone is a superior filler as compared to quartz due to the sorption of its aqueous CO₃^2? ions by the C‐S‐H—which in turn releases OH^? species to increase the driving force for C‐S‐H growth. This hypothesis is supported by kinetic data of C₃S hydration occurring in the presence of CO₃^2? and SO₄^2? ions provisioned by readily soluble salts. Contrary to prior investigations, these results suggest that differences in heterogeneous nucleation of the C‐S‐H on filler particle surfaces, caused due to differences in their interfacial properties, have little if any effect on C₃S hydration kinetics.     

Hydration of dibarium silicate I. Stoicheiometry of hydration

Dibarium silicate, Ba₂SiO₄, was hydrated in two ways: in paste form at 25° using a water/solid weight ratio of 0.7:1, and in a polyethylene bottle rotated on a wheel at 5°, 25° and 50°, using a water/solid weight ratio of 9.0:1. When Ba₂SiO₄ is hydrated in paste form, the stoicheiometry of the reaction at 25° is the same as in bottle-hydration at 50°: 2BaO.SiO₂+2.2H₂O = 1.2BaO.SiO₂.1.4H₂O+0.8Ba(OH)₂. The stoicheiometry of bottle-hydration at 5° and 25° is represented by the equation: 2BaO.SiO₂+2.2H₂O = BaO.SiO.1.2H₂O+Ba(OH)₂. Barium silicate hydrate, 1.2BaO.SiO₂.1.4H₂O, is well crystallised and has a specific surface area of ? 3m²/g. The crystals are plate-like and have a tendency to form clusters. The low-baria hydrate, BaO.SiO₂.1.2H₂O, is poorly crystallised and consists of thin platelets. It has a specific surface area of ? 35m²/g. The thermal dehydration of fully hydrated barium silicate and of the barium silicate hydrates was investigated by thermogravimetric and differential thermal analysis techniques. The similarities and differences between the barium silicate hydrates obtained in the hydration of barium silicate and the calcium silicate hydrates obtained in the hydration of β-dicalcium silicate and Ca₃SiO₅ are discussed. A mechanism of hydration of barium silicate is proposed which involves solution, precipitation and crystallisation steps.     

Influence of size-classified and slightly soluble mineral additives on hydration of tricalcium silicate

Early-age hydration of cement is enhanced by slightly soluble mineral additives (ie, fillers, such as quartz and limestone). However, few studies have attempted to systematically compare the effects of different fillers on cementitious hydration rates, and none have quantified such effects using fillers with comparable, size-classified particle size distributions (PSDs). This study examines the influence of size-classified fillers [ie, limestone (CaCO₃), quartz (SiO₂), corundum (Al₂O₃), and rutile (TiO₂)] on early-age hydration kinetics of tricalcium silicate (C₃S) using a combination of experimental methods, while also employing a modified phase boundary and nucleation and growth model. In prior studies, wherein fillers with broad PSDs were used, it has been reported that between quartz and limestone, the latter is a superior filler due to its ability to partake in anion-exchange reactions with C-S-H. Contrary to prior investigations, this study shows that when size-classified and area matched fillers are used—which, essentially, eliminate degrees of freedom associated with surface area and agglomeration of filler particulates—the filler effect of quartz is broadly similar to that of limestone as well as rutile. Results also show that unlike quartz, limestone, and rutile—which enhance C₃S hydration kinetics—corundum suppresses hydration of C₃S during the first several hours after mixing. Such deceleration in C₃S hydration kinetics is attributed to the adsorption of aluminate anions—released from corundum's dissolution—onto anhydrous particulates’ surfaces, which impedes both the dissolution of C₃S and heterogeneous nucleation of C-S-H.     

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.     

煅烧磷石膏对蒸压硅酸盐制品水化过程的影响

磷石膏经高温煅烧改性后与粉煤灰、砂粉、石灰及水泥熟料等制备蒸压硅酸盐制品,研究了不同温度煅烧的磷石膏对蒸压硅酸盐制品水化过程的影响,用蒸压制品中未反应的Ca(OH)₂量及结合水量分析它们的反应速率,用XRD测定蒸压硅酸盐制品的水化产物,并结合SEM分析,结果表明,经煅烧的磷石膏对蒸压硅酸盐制品的水化有明显的促进作用,托勃莫来石与C-S-H(1)等水化产物的迅速生长而形成密实的水化产物结构是其增强蒸压硅酸盐制品的根本原因。