A multi-method study of C3S hydration

C₃S pastes hydrated at 25°C have been studied using QXRD (to determine uncreacted C₃S), TG (to determine CH and water), and trimethylsilylation (to determinemonomeric and dimeric silicate) and the results compared with ones obtained with analytical electron microscopy. Monomeric silicate is accounted for by unreacted C₃S. The silicate in the C-S-H formed during the first few days is entirely dimeric, but at later ages dimer and polymer are both present. A new hypothesis for the reaction mechanism is tentatively proposed.     

ESCA and SEM studies on early C3S hydration

Electron Spectroscopy for Chemical Analysis (ESCA) and high-resolution Scanning Electron Microscopy (SEM) have been used to explore the phenomena occuring on the C₃S¹ surface in the very early stages of hydration. Definite changes in the surface morphology and Ca to Si ratio were observed already after a few seconds of hydration.     

Analyses of the aqueous phase during early C3S hydration

The concentrations of calcium and silica in solution during the first 4 hours of C₃S hydration were measured. The results of these analyses indicate that a solid calcium silicate hydrate forms within 30 seconds of the start of hydration and that an equilibrium between the solution and the solid hydrate is rapidly established. A strong dependence of the rate of early hydration on the w:C₃S ratio was observed, while the dependence on the surface area of the C₃S was minimal.     

The influence of VTES on the hydration process of C3S

This paper investigated the hydration process of tricalcium silicate (C₃S) in which a small amount of vinyltriethoxysilane (VTES) was added by using the techniques of ¹H NMR, ²⁹Si MAS-NMR and XRD. In comparison with the hydration process of C₃S without adding any additives, not only the average molecular weight of hydrated calcium silicate in C₃S paste, but also the ordering of the silicon nuclei in it increased. This indicates that the VTES has joined effectively into the real hydration process of C₃S. These results imply some possible reasons why the intrinsic properties of low porosity hardened cement paste (HCP) in which a small amount of VTES was added could be improved. Besides, it has been found that in early stage hydration of C₃S with or without VTES, Ca(OH)₂ crystal in the paste appears earlier than Q₁ which shows that in the first several hours of hydration, there only exists Si(OH)₄ and other basic salts and no dimer and polymer of silicate anion when Ca(OH)₂ crystal begins to form.     

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.     

Changes in C3S hydration in the presence of cellulose ethers

The influence of cellulose ethers (CE) on C₃S hydration processes was examined in order to improve our knowledge of the retarding effect of cellulose ethers on the cement hydration kinetics. In this frame, the impacts of various cellulose ethers on C₃S dissolution, C-S-H nucleation-growth process and portlandite precipitation were investigated. A weak influence of cellulose ethers on the dissolution kinetics of pure C₃S phase was observed. In contrast, a significant decrease of the initial amount of C-S-H nuclei and a strong modification of the growth rate of C-S-H were noticed. A slowing down of the portlandite precipitation was also demonstrated in the case of both cement and C₃S hydration. CE adsorption behavior clearly highlighted a chemical structure dependence as well as a cement phase dependence. Finally, we supported the conclusion that CE adsorption is doubtless responsible for the various retarding effect observed as a function of CE types.     

Preparation and hydration study of rich C2S cements

Three laboratory clinkers were made with variable C₂S content (28–57%), using industrial raw materials. These clinkers were cooled by air and studied by X-Ray Diffraction (XRD). It is concluded that by fast cooling, the active crystal forms of - and ′-C₂S were stabilized in the rich C₂S clinker.

The hydration phenomena were also studied in cements prepared from these clinkers by DTA-TG and XRD, at 2, 7, 28 and 90 days. The combined water and the liberated Ca(OH)₂ were quantitatively determined by means of thermogravimetry and the hydration rate was studied.

It is concluded that the hydration rate differs at the early ages, but it progresses with the same rate after 28 days. No significant differences in the formed hydration products of these cements were observed by XRD.     

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.     

The hydration characteristics of MSWI fly ash slag present in C3S

This paper reports on an investigation of the hydration characteristics of C₃S and the mixing of C₃S with municipal solid waste incinerator (MSWI) fly ash slag. The results can be summarized as follows: TGA observations show lower amounts of CSH and Ca(OH)₂ in samples that incorporated MSWI slag into C₃S, possibly due to the partial replacement of the C₃S by slag with less activity. In general, the incorporation of slag into C₃S decreases the initial hydration reactions, but it increases the pozzolanic reactions at a later stage by consuming Ca(OH)₂. As the hydration precedes, the C₃S peaks decrease, up to 90 days, due to the activation of the slag by liberated Ca(OH)₂. As well, the amount of hydration products (Ca(OH)₂) from the pure C₃S pastes are more than for the C₃S-slag pastes. Moreover, the degree of hydration of the C₃S pastes increases with the curing time, the C₃S-slag paste also shows that lower hydration degree values at all ages.     

The hydration of Portland cement,C3S and C2S in the presence of a calcium complexing admixture (EDTA)

The effect of EDTA, a calcium chelating agent, on the early hydration of Portland cement, C₃Sand β-C₂S has been studied by solution analysis and electron microscopy. EDTA is a retarded of cement hydration. Under normal conditions of hydration, the silica levels in solution are very low (<0.05 M) but in the presence of EDTA an initial flush of silica appears in the bulk aqueous phase. On continued hydration, following the saturation of EDTA with calcium, the appearance of ‘free’ calcium causes precipitation of C-S-H gel from the bulk solution and changes in microstructure of the colloidal gel around clinker particles in C₃S and β-C₂S pastes are observed. The action of EDTA as a retarding admixture is explained in terms of the membrane model of cement 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 of polymorphic modification C3S

Various types of triclinic, monoclinic and rhombohedral polymorphic modifications of C₃S were prepared by firing homogenized mixtures of pure C₃S with the addition of 0.75, 1.50 and 4.50 wt % of La₂O₃ or ZnO at 1600°C and by subsequent cooling of the crystalline specimens at varying rates. The heat treated specimens were then submitted to a hydrothermal process in an autoclave at 180°C for 24 hours. The types of polymorphic modifications of C₃S and the products of hydration were identified by x-ray diffraction. After compressive strength measurement of the autoclaved pieces the topography and morphology of the frature surfaces were examined by stereoscan microscope.     

Hydration reactions in pastes C3S+C3A+CaSO4.2aq+H2O at 25°C.I

A characteristic retardation of the hydration of C₃A is found in pastes C₃S+C₃A+CaSO₄.2aq+H₂O of weight ratios 1:3:z:4 at certain values of z, when sulphate concentration becomes insufficient for monosulphate formation. This retardation is ascribed to precipitation of amorphous Al(OH)₃, when C₃A dissolves in a limited amount of the aqueous phase shielded from the rest by C₄AH₁₃ or C₄AH₁₉. Evidence for the conversion of C₃AH₆ into C₄AH_n in supersaturated Ca(OH)₂ solution is found.     

Intrinsic strength and microstructure of hydrated C3S

In previous work it was shown that addition of gypsum to hydrating C₃S changes the intrinsic strength of the CSH gel. In the present work an attempt was made to find whether this change could be correlated with variations in pore structure, measured by nitrogen adsorption. The results indicated that at the stage where intrinsic strength is independent of gypsum content (about 40% hydration) the analysis of the adsorption and desorption curves could not reveal any changes in the microstructure. At the stage where the intrinsic strength decreases with increase in gypsum content (about 60% hydration) the pore analysis indicated changes in microstructure. The pure C₃S could be described better by a cylindrical pore model while the highest gypsum content paste was better accounted for by a parallel plate model.     

Gas-phase and liquid-phase hydration of C3A

In cement manufacturing an important problems is the tendency of cement to combine with water vapour during the grinding, transport and storage. Prehydrated cement may result in retardation of the strength development of the concrete.As it is mainly the clinker mineral C₃A which reacts with acqueous vapour, some experiments concerning hydration of C₃A in the gas phase and liquid phase have been carried out. Variable parameters were temperature, relative humidity and hydration time. The morphology and composition of the hydration products were characterized by using scanning electron microscopy, XRD and thermal analysis.During gas-phase hydration gel, hexagonal and cubic phases were formed. The liquid hydration products were shown to be identical whether the C₃A was almost pure or contaminated with minor components such as C₁₂A₇, free lime or chemically bound water formed during water-vapour hydration. However, if the amount of chemically bound water exceeds 3% the hydration products were anomalous showing rounded, irregular C₃AH₆ aggregates regardless of hydration conditions.The properties of the water-vapour hydrated C₃A might be connected with the retardation of strength when using prehydrated cement, but no possible mechanism has been found.     

Mechanism and kinetics of hydration of C3A and C4AF. Extracted from cement

The early hydration of C₃A + C₄AF extracted from cement and mixes with quartz, gypsum and pulverised fuel ash (PFA) has been studied by x-ray diffraction. The investigation has shown that the hydration of both aluminates is essentially a mechanism which obeys a modified diffusion equation. The values obtained for the reaction rates show that the hydration of C₃A takes place at seven times the rate of hydration of C₄AF. PFA was shown to be a very effective retarder. A mechanism to explain retardation is also proposed.     

The influence of CaCl2 on the crystallization of calcium silicates in autoclave hydration of C3S

The influence of CaCl₂ on the autoclave hydration C₃S (5 hours at 190°C) after different precuring times (0–16 hours) at room temperature has been studied. The addition of calcium chloride retards the autoclave hydration of C₃S and prevents completely the formation of the crystalline hydrated silicates (-C₂SH and C₃SH_1.5). This result has been compared with that observed in the ball-mill hydration of C₃S in the presence of CaCl₂, i.e., the complete absence of crystalline afwillite.

Abstract

On a étudié l'influence du CaCl₂ sur l'hydratation en autoclave du C₃S (5 heures à 190°C) après différents temps de conservation (0–16 heures à température ambiante. L'addition de chlorure de calcium retarde l'hydratation en autoclave du C₃S et empêche complétement la formation des silicates cristallins hydratés -C₂SH et C₃SH_1.5. Ce résultat a été comparé avec le phenomene observé en étudiant l'hydration du C₃S dans un moulin à boulets en presence de CaCl₂, c'est à dire la disparition complète du composé crystallin afwillite.     

A two-stage reaction sequence for C3S formation

Reaction mechanism and kinetics were investigated for the formation of C₃S from C₂S obtained as a byproduct from the Ames Lime-Soda Sinter Process for the recovery of Al₂O₃ from the mixed oxides found in power plant fly ash. A change in the rate of reaction was found to occur after about 25 minutes at burning temperatures of 1300 to 1500 C. The change was attributed to a two-stage reaction sequence in which the reaction rate for the first stage is phase boundary controlled and for the second phase diffusion controlled. A modified version of the Ginstling-Brounshtein solid-state reaction rate equation was found to describe the diffusion-controlled portion of the process. Apparent activation energies determined for this part of the process agreed with reported activation energies for Ca diffusion in clinker melt and for the self-diffusion of Ca in CaO.     

The morphology and composition of an immature C3S paste

The morphology of a one month old hydrated C₃S paste has been studied using the scanning electron microscope (SEM) and preferential etching techniques. The C/S ratios of the calcium silicate hydrates (CSH) have been determined using the SEM with an energy dispersive spectrometer. The hydrate rims covering the partially hydrated C₃S grains were found to consist of two layers; an outer porous product with an acicular morphology having a C/S ratio of 1.6 ± 0.1 and an inner product with a C/S ratio of 1.90 ± 0.05.

A region of increased C/S ratio in the C₃S core at the C₃S-CSH interface is attributed to a depletion of SiO₂ resulting from the diffusion of Si⁴⁺ ions into the hydrate.     

Current understanding of cellulose ethers impact on the hydration of C3A and C3A-sulphate systems

The impact of cellulose ethers (CE) on C₃A hydration was examined to support the understanding of the retarding effect of CE on cement hydration. In this sense, we successively studied the CE adsorption on ettringite and calcium hydroaluminates, and then the CE influence during C₃A hydration in presence or absence of calcium sulphate. We emphasized a phase-specific adsorption of CE depending on CE chemistry. Besides, in addition of CE, we highlighted a gradual slowing down of C₃A dissolution as well as ettringite and calcium hydroaluminates precipitation. Again, a great impact of CE chemistry and CE adsorption behaviour were noticed. Thus, HECs induce always a stronger adsorption on calcium hydroaluminates and a longer C3A hydration delays than HPMCs.