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.     

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.     

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.     

The effect of inorganic salts on tricalcium silicate hydration

Hydration of C₃S in salt solutions having ions in common with its hydration products was investigated by calorimetry and aqueous phase analyses. Soluble calcium salts, which depress hydroxyl ion concentrations in solution by promoting Ca(OH)₂ precipitation, were observed to accelerate hydration. Acceleration did not occur prior to Ca(OH)₂ precipitation. A saturated CaSO₄ solution, which delayed Ca(OH)₂ precipitation, was initially retarding but subsequently accelerated hydration as the hydroxyl ion concentration in solution decreased. Of the solutions investigated, a 0.2M CaCl₂ solution was the most effective in depressing the hydroxyl ion concentration and caused the greatest acceleration.     

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.     

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.     

Influence of precuring on high pressure steam hydration of tricalcium silicate

Samples of C₃s, hydrated at room temperature for 0, 2, 4, 8 and 24 hours, were steam cured at 130, 160 and 190°C for 5, 15 and 24 hours, in order to assess the influence of preliminary curing on autoclave hydration. The room temperature curing duration affects the autoclave hydration of C₃S, mainly at low temperatures. The types and relative amounts of the obtained products are also markedly affected by the preliminary treatment.     

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.     

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.     

Influence of precuring on high pressure steam hydration of tricalcium silicate in the presence of other constituents of portland cement

The influence of precuring at room temperature on the autoclave hydration of C₃S in the presence of other constituents of clinker and of gypsum was studied. C₃A, C₄AF and β-C₂S hampered the formation of C₃SH_1.5 and, especially for short precuring times, favored the formation of α-C₂SH. Gypsum hampered the formation of both the crystalline hydrated silicates. When the steam treatment took place after a long precuring, C-S-H was the prevailing hydrated silicate formed.     

The effect of water-to-cement ratio on the hydration kinetics of tricalcium silicate cements: Testing the two-step hydration hypothesis

Recent efforts to model tricalcium silicate based cements assume a two-step hydration mechanism where, in the first step, the empty space between cement particles rapidly fills with a low density calcium silicate hydrate (C-S-H) and in the second step, the thus formed C-S-H densifies slowly. This gives rise to models that nicely mimic the shape of experimentally observed hydration calorimetry curves and explains the transition between Stage 3 (acceleration) and Stage 4 (decreasing rate) hydration as well as post Stage 4 continued slow reaction. If this mechanism is correct, however, one would expect that the amount of heat energy released should increase with available space between particles, i.e. with water-to-cement ratio. A series of experiments, wherein the water-to-cement ratio was systematically varied showed little or no increase in total heat released when the water-to-cement ratio was doubled or tripled suggesting that the two-step mechanism may need revision.     

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.     

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.     

Combined effect of lignosulfonate and carbonate on pure portland clinker compounds hydration. III. Hydration of tricalcium silicate alone and in the presence of tricalcium aluminate

The effect of carbonate and/or lignosulfonate on the hydration of C₃S alone and in the presence of C₃A has been examined by DTG and TG curves and by zeta potential measurements. The combined addition of sodium carbonate and lignosulfonate strongly retards C₃A hydration. However by mixing 20 % C₃A with C₃S the retarding effect is significantly lower. On the other hand the early C₃A hydration is completely blocked by sodium carbonate and lignosulfonate simultaneously added. It seems that the fluidifying effect of the combined addition of those admixtures could be ascribed to both the dispersing action and the completely blocking effect on the early C₃A hydration.     

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.     

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 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.     

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.     

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.     

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.