Search results for "Ettringite"
showing 10 items of 11 documents
Surface Relaxivity of Cement Hydrates
2014
Numerous aspects of the physical chemistry of colloidal systems are conditioned by the solid–liquid interface, and this is also the case for hydrated cement systems. Estimating the surface area is thus essential for studying the kinetics of cement hydration and admixture adsorption. Proton nuclear magnetic resonance (NMR) relaxation techniques have already proven useful for this objective, but, for hydrating cements at early ages, it is necessary to know the surface relaxivities of all of the individual phases present to correctly interpret the relaxation data. This paper reports the results of a comparison of NMR relaxometry and Brunauer–Emmett–Teller gas adsorption measurements on various…
Modification of the rate of formation and surface area of ettringite by polycarboxylate ether superplasticizers during early C3A–CaSO4 hydration
2015
Abstract Early C3A–CaSO4 hydration was studied in the presence of various amounts of two polycarboxylate ether superplasticizers differing in their grafting degree. Hydration and surface area developments were investigated by in-situ NMR relaxometry coupled with BET and DSC during the first 2 h after mixing. This study enables a quantitative comparison of the amount and the specific surface area of the ettringite precipitated along the C3A–CaSO4 hydration with or without PCE. The main effect of PCE is to strongly increase ettringite specific area for a variable period. These effects are clearly dependent on the PCE charge and dosage and are reduced when using delayed addition. In this conte…
Ettringite surface chemistry: Interplay of electrostatic and ion specificity
2011
International audience; This paper presents a detailed experimental study combined with Monte Carlo (MC) simulations within the primitive model of the physical chemistry at the ettringite-water interface over a wide range of pH and bulk conditions for which ettringite exists thanks to its solubility in aqueous solutions. Ettringite, which is an important phase in hydrated cement-based systems, bears a permanent and positive structural charge. In contrast with previous studies, electrokinetic measurements together with the careful chemical analysis of the equilibrium solutions of the dispersions have brought strong support to designate sulfate as being the ion determining the potential. Simu…
Influence of temperature on the hydration products of low pH cements
2012
International audience; The chemical evolution of two hydrated "low pH" binders prepared from binary (60% Portland cement + 40% silica fume) or ternary (37.5% Portland cement +32.5% silica fume + 30% fly-ash) mixtures was characterized over one year at 20 degrees C. 50 degrees C, and 80 degrees C. The main hydrates were Al-substituted C-S-H. Raising the temperature from 20 to 80 degrees C caused a lengthening and cross-linking of their silicate chains. Ettringite that formed in pastes stored at 20 degrees C was destabilized. Only traces of calcium sulfate (gypsum and/or anhydrite) reprecipitated after one year in some materials cured at 50 degrees C and 80 degrees C. The sulfates released w…
Influence of three types of superplasticizers on tricalciumaluminate hydration in presence of gypsum.
2006
International audience; Different types of superplasticizers have been widely used over the past few decades in order to produce a more fluid or very high strength and durable concrete. These chemical admixtures interfere with the various physico-chemical processes occurring in early cement paste. In this paper we present results from a study on the influence of superplasticizers on pure tricalciumaluminate hydration in presence of gypsum. The suspensions hydration has been investigated by conductimetry, isothermal calorimetry and total organic carbon analysis of the liquid phase. The time taken for ettringite formation has been determined without superplasticizer and in presence of three d…
Mechanisms and parameters controlling the tricalcium aluminate reactivity in the presence of gypsum.
2007
International audience; 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 i…
Early C3A hydration in the presence of different kinds of calcium sulfate.
2009
International audience; Hydration reactions of C3A with various amounts of calcium sulfate hemihydrate, gypsum or a mixture of 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. More…
The removal of sulphate from mine water by precipitation as ettringite and the utilisation of the precipitate as a sorbent for arsenate removal.
2016
Abstract The aim of this research was to investigate sulphate removal from mine water by precipitation as ettringite (Ca6Al2(SO4)3(OH)12·26H2O) and the utilisation of the precipitate as a sorbent for arsenate removal. The mine water sulphate concentration was reduced by 85–90% from the initial 1400 mg/L during ettringite precipitation depending on the treatment method. The precipitation conditions were also simulated with MINEQL + software, and the computational results were compared with the experimental results. The precipitated solids were characterised with X-ray diffraction and a scanning electron microscope. The precipitated solids were tested as sorbents for arsenate removal from the…
Influence of superplasticizers structure on the hydration, the initial surface creation and the fluidity of cement model systems
2014
Nowadays the use of superplasticizers admixtures becomes unavoidable for concrete. It allows enhancing the rheological properties at the fresh state: the initial flow and slump retention during the 2-3 first hours of the life of a concrete. The understanding of this mechanism is still partly elucidated for PCE (grafted copolymers). The aim of this thesis was to challenge the assumption of the evolution of the adsorbed amount of PCE per instantaneous mineral surface unit as origin of the fluidity temporal evolution during the workability period.On an inert system (calcite), we confirmed that the fluidity is mainly governed by the adsorption level. Thus for a same surface adsorption density, …
Study of nucleation and growth processes of ettringite in diluted conditions
2020
Abstract Ettringite is a key compound of cementitious materials, formed during hydration that influences the rheological behavior of the cement paste. Despite its importance, ettringite formation has been poorly studied. In this work, nucleation and growth processes of ettringite were separately investigated. Ettringite nucleation was described using the Classical Nucleation Theory and an interfacial crystal-solution energy was determined. After the initial primary homogeneous nucleation, when the surface of already formed crystals was still low, the precipitation rate was shown to depend almost exclusively on the solution supersaturation. When the surface of crystals in suspension was gett…