Philippe Maitrasse

Influence of Three Types of Superplasticizers on Tricalciumaluminate Hydration in Presence of Gypsum

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 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.

Changes in Cement Paste and Mortar Fluidity after Mixing Induced by PCP: A Parametric Study

The interaction mechanism between polycarboxylate-type superplasticizer (PCP) and cement hydration is not fully understood and incompatibilities between concrete and additive are sometimes observed. In some cases, the fluidity tends to increase (“overfluidification”) few minutes after mixing. This is a problem because the overfluidification leds to bleeding of the concrete which could be critical on job site. Our study consisted first in highlighting the phenomenon of “over-fluidification” by slump flow tests on mortar. Next, the time evolution of the rheological behaviour of cement pastes in the presence of PCP was analysed thanks to a rheometry protocol in order to quantify the phenomenon. Later on, a parametric study was undertaken using this methodology. The operating conditions such as temperature and mixing process were studied as well as the effect of PCP structural parameters and the chemical characteristics of cement. In order to understand the origin of the phenomenon, adsorption measurements of PCP on cement particles were performed in the same conditions as those in the rheological measurements. Indeed, the phenomenon of “over-fluidification” could be related to the rate of the initial adsorption and the adsorption kinetics, both of which depend on the parameters of the process, the PCP structure and the cement reactivity.