Influence of superplasticizer adsorption on the rheology of cement paste

The production of high performance concrete requires the development of new superplasticizers with increased efficiencies. The knowledge of the mechanisms by which these admixtures act is of primary importance for the design of new polymers. They are believed to be linked to the dispersion cement agglomerates by electrostatic, steric or entropic repulsion, or improved wetting (capillary effect). The two first are directly linked to adsorbed polymers. The third is linked to both adsorbed and solvated polymers, while the last is mainly linked to polymers remaining in solution. To elucidate the effects of the superplasticizers, two approaches were used. On one hand, adsorption isotherms of polymers were measured by solution depletion. On the other hand, rheological testing leading to the determination of the amount of superplasticizer needed to reach a minimum yield stress. A comparison of both results reveals the relative importance of the possible mechanisms contributing to superplasticizer efficiency. It is shown that molecular mass distribution effects polymer adsorption. It is also shown how a cement obtained by grinding with and without a grinding aid has different adsorption properties and different polymer requirements. Results are discussed in the light of understanding by which mechanisms polymers are acting.