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.     

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.     

Calcium and silicon concentrations in solution during the early hydration of portland cement and tricalcium silicate

The solution chemistry of Portland cement and C₃S pastes has been studied with particular attention to the concentrations of calcium and silicon in the aqueous phase during early stages of hydration. Results are discussed in relation to solubility data available from studies of OPC and C₃S pastes and from the CaO---SiO₂---H₂O system. It is shown that under normal conditions the concentration of silicon in solution is extremely low (<2ppm) and this remains unchanged even when hydration is accelerated in the presence of CaCl₂. However, retarding admixtures such as oxalic acid and EDTA, which are strong calcium binding agents, release a flush of silicon into solution within the first few minutes of hydration. The results lend support to the osmotic membrane model of cement hydration and also provide insight into the mechanisms by which accelerating and retarding admixtures function.     

Effect of some liquefying agents on properties and hydration of portland cement and tricalcium silicate pastes

The effect of a melamine sulfonate resin, a naphthalene sulfonate resin and a sulfonated lignin on the rheological properties and the hydration of portland cement and tricalcium silicate pastes was studied. In addition to improving the flow properties of the pastes all three substances retarded the hydration of C₃S and altered the stoichiometric composition of the CSH-phase formed. The rate of ettringite formation was altered by the agents differently in two different cements studied.     

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.     

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 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.     

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 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.     

The effect of pressure on tricalcium silicate hydration at different temperatures and in the presence of retarding additives

The hydration of tricalcium silicate (C₃S) is accelerated by pressure. However, the extent to which temperature and/or cement additives modify this effect is largely unknown. Time-resolved synchrotron powder diffraction has been used to study cement hydration as a function of pressure at different temperatures in the absence of additives, and at selected temperatures in the presence of retarding agents. The magnitudes of the apparent activation volumes for C₃S hydration increased with the addition of the retarders sucrose, maltodextrin, aminotri(methylenephosphonic acid) and an AMPS copolymer. Pressure was found to retard the formation of Jaffeite relative to the degree of C₃S hydration in high temperature experiments. For one cement slurry studied without additives, the apparent activation volume for C₃S hydration remained close to ~ ? 28 cm³ mol^? 1 over the range 25 to 60 °C. For another slurry, there were possible signs of a decrease in magnitude at the lowest temperature examined.     

Morphology and microstructure of hydrating portland cement and its constituents I. Changes in hydration of tricalcium aluminate alone and in the presence of triethanolamine or calcium lignosulphonate

Morphological changes which occur during the hydration of C₃A^∗ paste were observed by electron microscopy from the age of 5 minutes to 3 months. X-ray analyses were used to identify phases. The effects of the admixtures, triethanolamine and calcium lignosulphonate, were investigated with and without gypsum. The dosage of admixture was 0.5%; the water to C₃A ratio was 1.0. The results showed clearly the morphological changes that took place as hydration proceeded, and showed the specific influences of each of the two admixtures.     

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.     

The influence of precuring on the autoclave hydration of quartz-tricalcium silicate mixtures

The hydration of quartz-tricalcium silicate mixtures at 175°C has been studied. The samples, precured at 25°C for different periods of time, in the range 0 – 16 hours, have been autoclaved for 8 hours. Upon addition of quartz to the system water-tricalcium silicate, some remarkable modifications have been observed in a) the degree of hydration of the silicate, b) the type and the relative amounts of the products obtained, and c) the influence of the precuring period on the following autoclave hydration.     

Dilation of portland cement grains during early hydration and the effect of applied hydrostatic pressure on hydration

An optical interference method shows that at early stages of hydration there is a significant increase in size of individual cement grains. The effect of applied hydrostatic pressure on cement pastes is an increase in the degree of hydration. Explanations for these effects are given in terms of the osmotic gel membrane model of hydration.     

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.     

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.     

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

The effect of sodium carbonate and/or sodium lignosulfonate on the hydration of C₂S alone and in the presence of C₃A has been examinated by DTG and TG curves and by zeta potential measurements. The combined addition of sodium carbonate and lignosulfonate retards the C₂S hydration to a lower extent than that observed for the C₃S hydration. The retarding effect on the C₂S hydration is significantly lower in the presence of 20% C₃A. On the other hand, the early C₃A hydration is completely blocked by admixtures simultaneously added. Addition of 0.9% sodium carbonate without lignosulfonate blocks the early hydration of both C₃A and C₂S. This effect was not found in the C₃SC₃A system.