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.     

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.     

Early C3A hydration in the presence of different kinds of calcium sulfate

Hydration reactions of C₃A with various amounts of calcium sulfate hemihydrate, gypsum or a mixture of the two, were investigated by isothermal microcalorimetry, and a monitoring of the ionic concentrations of diluted suspensions. This study shows that sulfate type used modifies the early C₃A–CaSO₄ hydration products and the rate of this hydration. The fast initial AFm formation observed before ettringite precipitation in the C₃A–gypsum system is avoided as soon as hemihydrate is present in the suspension. This was attributed to higher super saturation degrees and then higher nucleation frequency with regard to the ettringite obtained in the presence of hemihydrate. Moreover, replacement of gypsum by hemihydrate also leads to an increase of the ettringite formation rate during at least the five first hours under experimental conditions.     

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.     

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.     

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.     

The mechanism of the hydration in the system C3S-pozzolana

A mixture of five kinds of Japanese pozzolanas and synthesized pure C₃S were hydrated. The hydration mechanism in the system C₃S-pozzolana was investigated. The hydration of C₃S was accelerated by the addition of pozzolanas. The reasons for the acceleration increase of the precipitation sites of hydrates and the increase of the dissolution speed of C₃S caused by the depression of Ca²⁺ ionic concentration in the liquid phase was due to the addition of pozzolanas. The reaction between pozzolana and formed Ca(OH)₂ is pronounced after 1 to 3 days. Zonal hydrates existing between C₃S and pozzolana grains have Ca ionic concentration gradient from C₃S to pozzolana. It was often observed that in intact pozzolana grains which had no precipitated hydrates, there was clearance between pozzolanas and hydrates, and cast of trace. That tendency was pronounced in pozzolanas which had substantial alkalies. The mechanism of the hydration in the system C₃S-pozzolana was considered from those results.     

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.     

Effect of cement C3A content, temperature and storage medium on thaumasite formation in carbonated mortars

The purpose of this study is to determine the effect of cement C₃A content, temperature and composition of the immersion medium (water, gypsum and magnesium sulphate solution) on the rate of thaumasite formation in cement mortars. It also aims to ascertain how the C₃A content influences the composition of the salt formed.The mortar prisms for this study were made with two different cements, one with low and the other with high Al₂O₃ content, with or without gypsum and/or calcium carbonate. After hydration, curing and carbonation, the prisms were partially immersed in distilled water and stored at temperatures ranging from 0 to 5 °C for up to 5 years. Some of the prisms were immersed in a 2% (w/w) gypsum solution or in 1.4% (w/w) magnesium sulphate solution at ambient temperature. Samples were taken at different ages and mineralogical and micro-structurally characterised.Some of the specimens tested were observed to expand, in a process concurring with the formation of thaumasite or a solid solution of thaumasite and ettringite, at both ambient and cooler temperatures. A correlation was found between cement C₃A content and the composition of the deterioration product involved in the expansive process: thaumasite forms in mortars made with low C₃A cement, whereas mixed crystals or solid solutions of thaumasite and ettringite form in mortars made with high C₃A content cement.     

Study of C3A - gypsum - Tl2CO3 - H2O system

Effect of thallous carbonate on the hydration of tricalcium aluminate containing 20% gypsum has been studied using isothermal microcalorimeter. The calorimetric results indicate that the hydration reaction is retarded in the presence of Tl₂CO₃. X-ray diffraction and differential thermal analyses indicate the formation of calcium carboaluminate and CaCO₃. The non-evaporable water content of the samples containing Tl₂CO₃ are found to be lower than those without Tl₂CO₃ for the same intervals of time.     

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.     

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.     

Influence of lignosulphonate,glucose and gluconate on the C3A hydration

The influence of desugarized sodium lignosulphonate, glucose and sodium gluconate on the C₃A hydration has been examined using DTG analysis. At relatively low concentration levels of admixtures in the aqueous phase (1 to 3 g/l), such as those practically used for concrete mixes, lignosulphonate and glucose retard only slightly the C₃A hydration. Sodium gluconate is significantly more efficient in retarding C₃A hydration than lignosulphonate and glucose.     

Suspension hydration of C3S at constant pH. I. Variation of particle size and C3S content

Calcium and silicate ion concentrations during suspension hydration of C₃S indicate that at pH 11.5 an equilibrium is established between one of the hydrates and the solution during about 80 minutes. The concentrations found in this period are indipendent of the particle size of the C₃S and (within limits) of the C₃S content of the suspension.     

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.     

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.     

C4A3 hydration ettringite formation, and its expansion mechanism: I. expansion; Ettringite stability

C₄A₃ hydration, the mechanism of C₄A₃ expansion, ettringite formation and its thermal stability are examined in a 3-part study. In this part, the amount of ettringite formed and the C₄A₃ degree of hydration are interrelated, and correlated with the degree of expansion beyond an induction period for the initiation of expansion. Effects of hydration temperature are examined. Finally, phase equilibria describing ettringite stability are determined as a function of pressure.     

The effects of Cr2O3, Cu(OH)2, ZnO and PbO on the compressive strength and the hydrates of the hardened C3A paste

The purpose of this paper is to investigate the effects of Cr₂O₃, Cu(OH)₂, ZnO or PbO on the hydration of C₃A and characterization of its hydrates. C₃A pastes adding the above compound were examined on the basis of the hydration products and their structure, compressive strength and rate of early hydration.     

Hydration evolution of C3S-EVA composites analyzed by soft X-ray microscopy

The effect of poly(ethylene-co-vinyl acetate) or EVA on the hydration of a pure C₃S was investigated by transmission soft X-ray microscopy in highly diluted systems. EVA particles are readily recognizable by their spherical shape and semitransparency to soft X-rays. Polymeric particles adsorb on the surface of hydrating C₃S grains, and act as nucleation agents in the development of the composite microstructure. The formation of a polymeric film was observed after a few hours of hydration. Fourier-transform infrared spectroscopy supported the conclusions that a highly soluble phase is formed as a result of EVA-hydrating C₃S chemical interactions.