Early Hydration of Tricalcium Silicate

The hydration of tricalcium silicate (C₃S) in the preacceleration stages was studied. The C₃S particles carry a positive charge during the early stages of hydration. Following a rapid hydrolysis of C₃S, calcium ions adsorbed on the Si-rich surface of C₃S particles, greatly reducing their further dissolution, thus initiating the induction period. The [Ca²⁺] and [OH^-] continue to increase at lower rates and, because Ca(OH)₂ crystal growth is inhibited by silicate ions, become supersaturated with respect to Ca(OH)₂. When the supersaturation reaches a value of ∼1.5 to 2.0 times the saturation concentration, nuclei are formed, and rapid growth of Ca(OH)₂ and C-S-H is initiated. These products act as sinks for the ions in solution, thus enhancing the further dissolution of C₃S.     

Hydration of Tricalcium Silicate

The kinetics of paste, bottle, and ball-mill hydration of 3CaO SiO₂ and the effects of additions of electrolytes and alcohols were studied. Paste and bottle hydrations proceed through periods of induction, acceleration, and decay. If 3CaO SiO₂ is hydrated in an excess of H₂O, as in bottle hydration, the reaction rate is lower than that for paste hydration. The ball-mill hydration rate is much the highest and is controlled by the removal of the hydrate layer coating the 3CaO SiO₂ particles. Electrolytes always accelerate and alcohols retard the reaction rate. Experimental results are discussed with reference to modern theories of the 3CaO SiO₂ hydration mechanism.     

Kinetics of the Hydration of Tricalcium Silicate

The hydration of tricalcium silicate was followed at a water/C₃S ratio of 0.7 between 5° and 50°C by determining free lime and combined water. Free lime was estimated by the o -cresol method, whereas combined water was calculated from the amount of free water remaining in the paste as determined by extraction with methyl ethyl ketone. The ratio of free lime to combined water was constant throughout the hydration; this ratio indicated that CSH(II) may be represented as 1.68CaO · SiO₂· 2.58H₂O. When maximum supersaturation of the solution with Ca^2+] is attained, the induction period terminates and the reaction proceeds rapidly, probably as the result of propagative surface nucleation-growth of CSH(II). Kinetic equations were derived for these reactions. When the surface of C₃S is entirely covered by CSH(II), the reaction becomes slow and is controlled by diffusion of water. Constants involved in the kinetic equations are evaluated and discussed.     

Hydration of Tricalcium Germanate–Tricalcium Silicate Solid Solutions

The hydration characteristics of solid solutions of composition 3CaO· x GeO₂·(1 − x )SiO₂ were investigated at 25°C by isothermal calorimetry. The compositions hydrated were for x = 0, 0.2, 0.4, 0.6, 0.8, and 1.0. Full hydration was achieved for compositions in which x < 0.2. Both the rate of hydration and the total heat evolved for complete hydration vary with composition. Hydrate compositions were determined, and these also show a compositional dependence. The hydration product for tricalcium germanate has a Ca/Ge ratio near 1.5. Although the hydration products of the solid solution are far more fibrous, their morphologies are reminiscent of that of calcium silicate hydrate.     

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 Sodium Carbonate on the Hydration of Tricalcium Silicate

The effect of sodium carbonate (2%) on the hydration of tricalcium silicate was studied. Free lime, nonevaporable water content, calorimetric measurements, analysis of the liquid phase for Ca²⁺ and OH⁻ ions, zeta potential measurements, and compressive strength measurements were made. The accelerating action of sodium carbonate can be interpreted in terms of adsorption of ions. Further, since sodium carbonate is a structure breaker for water, more free water will be available in the presence of sodium carbonate, which may accelerate the reaction.     

Early Hydration of Tricalcium Silicate: A Solid Reaction with Induction and Acceleration Periods

The hydration of tricalcium silicate was studied using a sample with a limited particle size distribution. The hydration reaction was analyzed as an example of general solid reactions having induction and acceleration periods. The first product, which may have a structure similar to the reactant, forms on the surfaces of reactant grains. The nuclei of the stable product are produced in the first product layer and act as reaction centers; the main reaction to form a stable product occurs rapidly in the acceleration period. In the hydration of calcium silicates, hydrates produced in this stage are poorly crystalline and metastable and change gradually into a more stable form.     

Polymorphism and Hydration of Tricalcium Silicate Doped With ZnO

Up to 4.7% ZnO can be incorporated into the crystalline lattice of C₃S. Five allotropic forms (i.e. T_I, T_II, M_I, M_II, and R) can be stabilized at room temperature, depending on the amount of ZnO. When heated to high temperature, the C₃S-ZnO solid solution decomposes and ZnO escapes from the crystalline lattice. The hydration and strength development of C₃S are altered by ZnO doping.     

The Influence of Water Activity on the Hydration Rate of Tricalcium Silicate

Tricalcium silicate does not undergo hydration at relative humidities (RH's) below 80%. But, the rate at which its hydration rate decreases as a function of the RH has not yet been elucidated. By invoking correspondence between RH and water activity (a_H, unitless), both of which are related to the chemical potential of water, the reaction evolutions of triclinic tricalcium silicate (i.e., T1‐Ca₃SiO₅ or C₃S) are tracked in water + isopropanol (IPA) mixtures, prepared across a wide range of water activities. Emphasis is placed on quantifying the: (a) rate of hydration as a function of a_H, and (b) the critical (initial, a_H0c or the achieved) water activity at which hydration effectively ceases, i.e., does not progress; here identified to be ≈ 0.70. The hydration of tricalcium silicate is arrested even when the system remains near saturated with a liquid phase, such that small, if any, capillary stresses develop. This suggests that changes in chemical potential induced via a vapor‐phase or liquid‐phase route both induce similar suppressions of C₃S hydration. A phase boundary nucleation and growth (pBNG) model is fit to measured hydration rates from the onset of the acceleration period until well beyond the rate maximum when the water activity is altered. The simulations suggest that for a fixed hydrate nucleation density, any water activity reductions consistently suppress the growth of hydration products. Thermodynamic considerations of how water activity changes may influence reactions/hydrate evolutions are discussed. The outcomes improve our understanding of the chemical factors that influence the rate of Ca₃SiO₅ hydration.     

Thermal Study of the Effect of Additives on the Hydration of Tricalcium Silicate

The rate of paste hydration of 3C_aO·SiO₂ (C₃S) and the effects of additions of CaCl₂, CdI₂, and CrCl₃, were studied by differential thermal analysis and thermogravimetry. X-ray analyses were used to identify the synthesized C₃S. The salts CaCl₂, CdI₂, and CrCl₃, accelerated the hydration of C₃S. The degree of hydration was estimated by the amount of Ca(OH)₂, formed, as determined by TG.     

Effects of Particle Size Distribution on the Kinetics of Hydration of Tricalcium Silicate

An analysis of the validity of approximating selected particle size distributions by small numbers of size classes was carried out. It was determined that the selection of a small number of particle size classes is not adequate to describe a particle size distribution. One consequence of this is that the particle size distribution, if not properly accounted for, can mask the kinetics of a hydration process. The hydration rates of two finnesses of tricalcium silicate, each with known particle size distributions, were measured by isothermal calorimetry for a period of 28 d. These data were integrated, normalized, and represented as α-time curves as a basis for comparison with kinetic models of hydration. Agreement with kinetic models was found to be strongly influenced by the effect of particle size distribution. However, the rate-limiting mechanisms appear to be independent of particle size distribution No single kinetic model was adequate to describe C₃S hydration over the first 28 d. A kinetic model that assumes initial surface-area control with diffusion control dominating subsequently provided an excellent fit to the experimental data.     

Kinetics of Tricalcium Silicate Hydration in Diluted Suspensions by Microcalorimetric Measurements

Investigations of tricalcium silicate (C₃S) suspensions using different techniques have shown the initial hydration kinetics of C₃S are strongly dependent on the water/C₃S ratio. In order to provide new experimental data on the hydration, we have adapted a Tian-Calvet heat flow isothermal microcalorimeter to study thermal flow variation released by C₃S stirred suspensions in water and saturated lime water. We observed three exothemic peaks and one endothermic peak. Their relative magnitude and duration depend on the W/C ration and the lime solution concentration. The protective layer theory appears to be consistent with our results for high W/C ratio. Moreover, portlandite precipitation seems to be a consequence and not the cause of the acceleratory period.     

Al3+掺杂对硅酸三钙结构及水化活性的影响

利用X射线衍射仪、差示扫描量热-热重分析仪、红外光谱及微量热分析等,研究了Al3+掺杂对硅酸三钙C3S结构及活性的影响.结果表明:Al3+在C3S中固溶同时取代Ca和Si,并伴随少量Ca空位的形成保持电荷平衡.当Al2O3掺量高于0.5％(质量百分数,下同)时,Al取代Si比例增加.Al2O3掺量≤0.5％时仅使C3S晶胞参数改变,当掺量达1％时,可稳定T3晶型,符合离子稳定C3S多晶态规律.Al3+在C3S中固溶形成大量非本征缺陷,显著提高C3S早期水化反应活性.     

Hydration and Strength Development in Tricalcium Silicate Pastes Seeded with Afwillite

The formation of afwillite in tricalcium silicate pastes initiated by ding with crystals was studied to determine the effects on development of tensile strength. Afwillite forms in pastes only when fresh surfaces of both tricalcium silicate and afwillite seed are exposed by initial grinding. Higher early strengths reflect increased rates of hydration in seeded pastes, but strengths at later ages are lower than those of untreated pastes. At all ages tensile strength is lower at a given degree of hydration in seeded pastes than in unseeded ones; this behavior is considered to result from changes in pore-size distribution which occur as afwillite forms. Afwillite initially forms concomitantly with calcium silicate hydrate gel but later arises from conversion of the gel.     

A New Approach to Modeling the Nucleation and Growth Kinetics of Tricalcium Silicate Hydration

The hydration kinetics of tricalcium silicate (C₃S), the main constituent of portland cement, were analyzed with a mathematical "boundary nucleation" model in which nucleation of the hydration product occurs only on internal boundaries corresponding to the C₃S particle surfaces. This model more closely approximates the C₃S hydration process than does the widely used Avrami nucleation and growth model. In particular, the boundary model accounts for the important effect of the C₃S powder surface area on the hydration kinetics. Both models were applied to isothermal calorimetry data from hydrating C₃S pastes in the temperature range of 10°–40°C. The boundary nucleation model provides a better fit to the early hydration rate peak than does the Avrami model, despite having one less varying parameter. The nucleation rate (per unit area) and the linear growth rate of the hydration product were calculated from the fitted values of the rate constants and the independently measured powder surface area. The growth rate follows a simple Arrhenius temperature dependence with a constant activation energy of 31.2 kJ/mol, while the activation energy associated with the nucleation rate increases with increasing temperature. The start of the nucleation and growth process coincides with the time of initial mixing, indicating that the initial slow reaction period known as the "induction period" is not a separate chemical process as has often been hypothesized.     

Effects of Saccharide Set Retarders on the Hydration of Ordinary Portland Cement and Pure Tricalcium Silicate

The effects of aliphatic sugar alcohols (e.g., threitol, xylitol, sorbitol) on the hydration of tricalcium silicate (C₃S) and ordinary portland cement (OPC) were investigated and compared with those of sucrose, a well-established cement set retarder. Only sugar alcohols which contain threo diol functionality retarded the setting of C₃S and OPC, their efficacy increasing with the number of threo hydroxy pairs and, to a smaller extent, with the overall population of hydroxy groups. None, however, were as effective as sucrose. The initial and final setting times increased exponentially with the concentration of saccharide, although the hydration of OPC was less inhibited than that of C₃S. Saccharides function as "delayed accelerators," that is, cement hydration is first inhibited and then proceeds faster than in saccharide-free cement. This behavior is consistent with the theory that the induction period is controlled by slow formation and/or poisoning of the stable calcium silicate hydrate (CSH) nuclei. The early inhibiting influence of saccharides on CSH precipitation is apparently stronger than on the growth of crystalline calcium hydroxide. Saccharides did not negatively affect the degree of hydration and compressive strength of fully set OPC paste; on the contrary, sorbitol yielded modest increases.     

Hydration of Tricalcium Silicate in the Presence of Lignosulfonates, Glucose, and Sodium Gluconate

Two commercial calcium lignosulfonates were separated into their main constituents, i.e. fractions high in salts of lignosulfonic acids and fractions high in sugars and sugar-acid salts. Hydration of C₃S in the presence of these fractions was studied. The fractions high in sugar acids caused a delay in hydration with a subsequent acceleration of hydration at additions of 0.1%. Fractions high in lignosulfonates caused little delay, whereas additions of 0.5% caused complete inhibition. Hydration was also studied in the presence of glucose and sodium gluconate. Adding 0.1 % glucose delayed hydration for about 11 days, whereas adding 0.1% sodium gluconate caused complete inhibition. The delay in hydration is discussed in terms of poisoning of nucleation sites.