Influence of three types of superplasticizers on tricalciumaluminate hydration in presence of gypsum.

International audienceDifferent 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 different types of superplasticizers: polynaphtalene sulfonates (PNS), polycarboxylate-polyox (PCP) and diphosphonate terminated polyoxyethylene. Whereas diphosphonate terminated polyoxyethylene does not seem to modify tricalcium aluminate hydration carried out in presence of gypsum, PCP and even more PNS slow down ettringite formation. This effect seems to be largely due to a decrease of the C3A dissolution rate and might be connected to an adsorption of PCP. or PNS observed from the early C3A hydration. Such an adsorption does not happen with diphosphonate terminated polyoxyethylene superplasticizer. Moreover the presence of PCP superplasticizer causes a decrease in the size of the ettringite crystals formed