The influence of Na2O on the hydration of C3A. II. Suspension hydration

The influence of Na₂O on the hydration of C₃A was studied in suspensions from the start of the reaction onwards. The heat evolution rate in very early stages of the hydration, measured at varying NaOH concentrations, and SEM, indicate that at NaOH concentrations larger then 0.1 M the reaction mechanism differs from that in water. In these solutions the hydration is thought to be controlled at first by a more or less amorphous Ca(OH)₂ layer.     

The influence of an ion-exchange resin on the kinetics of hydration of tricalcium silicate

The addition of a finely-ground ion-exchange resin makes it possible to modify the hydration kinetics of C₃S pastes. Analyses of the liquid phase in pastes and more dilute suspensions show that the resin exchanges calcium ions for sodium ions very rapidly during the early stage of hydration and therefore the concentration of silica in solution increases. The resin impacts the hydration of C₃S by other mechanisms which depends on the resin quantity added. For a high resin quantity, the induction period is very short, but the longer-term hydration is enhanced compared to a reference sample without resin. We hypothesize that the surface of the resin can provide sites for the nucleation and growth of C-S-H hydrates and/or portlandite far away from the surface of the C₃S grains. This consequently increases the quantity of hydrates that can precipitate before a continuous hydrate layer forms over the surfaces of C₃S particles.     

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.     

C3A hydration

C₃A is hydrated with time and temperature as variable parameters. The solid hydration products were observed using the scanning electron microscope and determined by XRD. The heat development was followed by means of isothermal microcalorimetry.The first hydration product is a gel-like material with no detectable XRD lines. The hexagonal phases which follow have a better crystallinity when formed below than above room temperature. In the latter case distinct XRD lines are only obtained after some time. C₃AH₆ as single crystals or aggregates develops earlier at high than at low temperatures. The morphology of C₃AH₆ varies with hydration time and temperature.This sequence of reactions occurs slowly in suspensions if a small amount of C₃A is used. In pastes, and in suspensions if a larger amount of C₃A is used, C₃AH₆ is formed very quickly and no hexagonal hydrates were detected by XRD.     

Etude de l'effet retardateur du zinc sur l'hydratation de la pate de ciment Portland

The retarding effect of zinc on the hydration of C₃S and C₃A, the two principal Portland cement components, has been investigated by X - ray diffraction. The results show that the C₃S retardation is more important than that of C₃A. This retardation is due to the precipitation of an amorphous layer of zinc hydroxide around the anhydrous grains. The effect of this coating depends on its permeability. The hydration reaction starts again through the transformation of the zinc hydroxide into the crystalline calcium zinc hydroxide Ca Zn₂ (OH)₆, 2H₂ O.     

Hydration reactions in pastes C3S+C3A+CaSO4.2aq+water at 25°C. II

Variation of C₃A/C₃S ratio in pastes if C₃S+C₃A+CaSO₄.2aq+water influences the hydration reactions in a way compatible with retardation of C₃A hydration by amorphous Al (OH)₃, but not compatible with retardation by dissolved ions or by a “C₄AH₁₃” retarding layer.     

Influence of amorphous Al(OH)3 on the hydration of tricalcium aluminate

The influence of amorphous Al(OH)₃ on the hydration of tricalcium aluminate was investigated in pastes and in suspensions. In suspensions it was found that amorphous Al(OH)₃ started to dissolve almost immediately, and a stage was passed in which most of the anions present in the aqueous phase were polymeric aluminate units. Paste hydration results could be understood by assuming that highly soluble amorphous Al(OH)₃ dissolved and, by so doing, adjusted the concentrations in the aqueous phase so as to cause precipitation of some less soluble form of Al(OH)₃, thereby retarding 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.     

Physicochemical effects of nanosilica on C3A/C3S hydration

In this paper, C₃A-gypsum and C₃A-C₃S-gypsum model cement systems with and without nanosilica were studied. The effects of nanosilica on the early stage cement hydration, particularly C₃A hydration, were assessed through the heat of hydration (isothermal calorimetry), phase assemblage (quantitative X-ray diffraction), zeta potential, ion concentration measurements, and morphology (scanning electron microscopy) examinations. The results indicate that while promoting C₃S hydration, nanosilica retarded C₃A hydration in both the systems studied. The retardation was caused by the adsorption and coverage of nanosilica on C₃A surfaces through the electrostatic interaction, thus decreasing the C₃A dissolution rate and hindering the precipitation of hydration products. Consequently, the reduced gypsum consumption rate and the seeding effect of nanosilica further promoted C₃S hydration. These findings suggest that nanosilica and other silica-based nanoparticles can physicochemically influence hydration of cement-based materials, and a better understanding of these influencing mechanisms can help optimize performances of nanoparticle-modified cement-based materials.     

Mechanisms and parameters controlling the tricalcium aluminate reactivity in the presence of gypsum

To understand the mechanisms and the parameters controlling the reactivity of tricalcium aluminate in the presence of gypsum at an early age, a study of the hydration of the “C₃A-sulphate” system by isothermal microcalorimetry, conductimetry and a monitoring of the ionic concentrations of diluted system suspensions have been carried out with various gypsum quantities. The role of C₃A source and its fineness were also studied. This work shows the fast initial formation of AFm phase followed by ettringite formation during the period when the sulphate is consumed. It has been highlighted that the time necessary to consume all the gypsum varies with the type of C₃A and it has been attributed to the intrinsic reactivity of each one and mainly to the change of fineness from one C₃A to another. Results are discussed alongside hypothesis from the literature to explain the slowing down of C₃A hydration process in the presence of calcium sulphate.     

The heats of hydration of cement constituents

Integral heats of hydration and time courses of heat power function were directly measured for a series of nine cements in a heat-conduction calorimeter. Both the heat power and the integral heat of hydration of cement constituents C₃S, C₂S, and C₃A+C₄AF were evaluated by the thermochemical analysis method. The rate of heat evolution courses for the cement constituents, evaluated by the thermochemical analysis method, were found to differ from those recorded for the cement-constituting minerals hydrated individually. This fact may be helpful in the interpretation of the mechanism of cement hydration processes.     

Simultaneous study of mechanical property development and early hydration chemistry in Portland cement slurries using X-ray diffraction and ultrasound reflection

A sample cell for the simultaneous measurement of synchrotron X-ray powder diffraction and ultrasound shear-wave reflection data from cement slurries is described. White cement slurries at 25 and 50 °C with 0–3% bwoc CaCl₂ were studied to illustrate the potential of the apparatus. The decrease in reflected S-wave amplitude, in dB, showed a linear correlation with C₃S hydration. CaCl₂ retarded the development of G′ and G″ relative to the extent of C₃S hydration. At short times, there was a correlation between the time evolution of both G′ and G″, and the amount of precipitated CH seen by diffraction, which was almost independent of CaCl₂ concentration and temperature. CaCl₂ addition resulted in a decrease in the amount of CH visible to X-rays, relative the degree of C₃S hydration. This may indicate a change in C–S–H gel C:S ratio or the presence of nanoscale CH that could not be seen by diffraction.     

Hydration reactions in pastes C3S + C3A + CaSO4 .2aq. + water at 25°C.III

Influences exerted by various additives, by changes in the water/solids ratio and by variations in C₃S/C₃A ratio at constant CaSO₄.2aq./C₃A ratio, are consistent with a retardation of the hydration of C₃A by local precipitation of amorphous Al(OH)₃.     

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.     

Influence of the availability of calcium on the hydration of tricalcium aluminate (C3A) in seawater-mixed C3A–gypsum system

This work examined the effects of seawater (SW) on the hydration of tricalcium aluminate (C₃A) in C₃A–gypsum and C₃A–gypsum–Ca(OH)₂ systems through the characterization of hydration heat release, the evolution of aqueous phase composition and hydration products with the hydration time. It was found that SW increased the dissolution driving force of C₃A and solubility of gypsum, which accelerated the early hydration of C₃A and the formation of ettringite (AFt), leading to a higher hydration degree of C₃A at an early age compared with the deionized (DI) water–mixed pastes. After gypsum depletion to form AFt, and in the absence of Ca(OH)₂, the formation of chloroaluminate hydrates was slower due to the insufficient Ca resulted in an accumulation of Al in solution. This would delay the subsequent transformation of AFt to monosulfate (SO₄–AFm) and the formation of hydrogarnet (C₃AH₆), which would further reduce the hydration degree of the C₃A at the later ages. However, in the presence of Ca(OH)₂, the hydration degree of C₃A–gypsum–Ca(OH)₂ at later ages was increased, which was similar to that of the corresponding DI pastes. This can be inferred that the amount of Ca available in SW-mixed cement concrete can affect the hydration degree of C₃A in cement.     

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.     

The influence of Na2O on the hydration of C3A. I. Paste hydration

The influence of Na₂O on the hydration of C₃A was studied both by following the hydration of xNa₂O. (3?x) CaO.Al₂O₃ (0<x<0.25) in water, and of C₃A in solutions of NaOH. Low NaOH concentrations prevent a very early appearance of the second heat evolution peak, indicating a more controlled formation of C₃AH₆ nuclie. Higher NaOH concentrations advance the second peak; this is ascribed to a decreased stability of the hexagonal hydrates with increasing NaOH concentrations.     

Hydration and microhardness of mineral trioxide aggregate depending on ratio between di- and tricalcium silicates

Tricalcium silicate (C₃S) and dicalcium silicate (C₂S) are the main components related with the hydration process of mineral trioxide aggregates (MTAs) for endodontic materials. In this study, we investigate the influence of different ratios of C₃S and C₂S in a series of MTA samples with (100-x)C₃S-xC₂S-18ZrO₂ (x = 0, 10, 15, 34, and 100) on their physical and chemical characteristics, hydration process, and microhardness properties. The chemical compositional properties of different samples are measured using X-ray diffraction, X-ray photoelectron spectrometry, and scanning electron microscopy. The physical and microhardness properties are also investigated after the standard hydration process (ISO 6876:2021). Generally, the sample with higher C₃S ratio induces the faster hydration, which results in decreased fluidity as well as shorter working and setting times. The microhardness generally decreases with larger C₃S ratio.     

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.     

Alkaline Hydration Of C2S and C3S

This study analyzed the behavior of two laboratory‐synthesized calcium silicates, C₃S and C₂S, after hydration in 8‐M NaOH and in water as a control. Two‐ and 28‐d mechanical strength values were determined and the products were characterized with XRD, TEM, and ²⁹Si and ²³Na MAS NMR. The results showed that hydrating C₃S in a highly alkaline medium had no significant effect on the mechanical development of the material, whereas in C₂S hydration, that medium hastened hydration substantially, impacting setting and hardening times. This finding has technological implications, given the low early‐age reactivity of dicalcium silicate under normal hydration conditions.