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.     

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.     

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.     

Influence of aluminium hydroxide and lime on the hydration of tricalcium silicate

Experiments by isothermal calorimetry, indicate that the hydration of 3CaO·SiO₂ (C₃S) is influenced very little by gibbsite; it is influenced by bayerite to a somewhat larger extent. In the presence of amorphous Al(OH)₃ the reaction of C₃S with water shows a very complicated course and gives four heat peaks. If CaO is added in addition to this Al(OH)₃, the third and the fourth heat peaks are more pronounced. From qualitative d.t.a., infra-red, electron-microscope and X-ray investigations, as well as from quantitative X-ray analysis, a reaction mechanism is proposed. The quantity of C₃S reacted, determined by means of quantitative X-ray analysis, is greater during the reaction of 2·00 g C₃S with 0·40 g amorphous Al(OH)₃, 0·08 g CaO and 2·00 ml water, than during the reaction of 2·00 g C₃S with 2·00 ml water.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

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.     

Studies on the hydration of tricalcium silicate pastes II. Strength development and fracture characteristics

The fracture of hardened tricalcium silicate pastes has been studied using optical microscopy and scanning electron microscopy. The path of fracture changes with the age of the pastes. In young pastes, fracture passes primarily through high porosity C-S-H gel avoiding low porosity areas where calcium hydroxide has crystallized in the pores. In more mature pastes, this discrimination is lost as the matrix becomes more homogeneous. The different modes of fracture appear to have no significant influence on tensile strength, the tensile strength depends primarily on porosity and is proportional to the cube of Powers' gel-space ratio.     

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.     

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.