Interaction of Superplasticizers with Model Powders in a Highly Alkaline Medium

Synopsis: It is broadly recognized that the adsorption of superplasticizers on cement particles is a key factor in determining the rheology of concrete. In order to avoid the problems linked to the hydration of cement, the adsorption of superplasticizers is often studied on unreactive model powders. However, in order for the model system to remain as close as possible to cement, the surface should have a similar charge and a similar chemical nature. Furthermore, the pH of the solution should be close to that of the hydrating cement (about 12.5). Under these conditions, cement has been shown to have a positively charged surface. The model powders used in this study were Mg(OH) 2 and dead burnt MgO, which have nominal isoelectric points of 12.0 and 12.4 respectively, and which are chemically similar to Ca(OH) 2 and CaO. The surface charge of such model suspensions was studied as a function of added superplasticizer. These were either commercially available or currently under development, ranging from strongly to very weakly ionic. Adsorption isotherms for two polymeric superplasticizers, with similar structures but with different ionic group spacing, have been measured for both MgO and Mg(OH) 2 at pH 12 and 11.3 respectively and between 10 and 40°C. Results showed a strong temperature dependence for the adsorption of the less ionic polymer on MgO.