The influence of C3A on the dissolution kinetics of alite/gypsum mixtures in the presence of PCE’s

Portland cement is a multi-phase material, which can be simplified as a two-phase system with alite and C3A as the main constituents. Alite is the most abundant phase and in a first approximation it is responsible for the development of mechanical strength during hydration, while C3A mainly affects the plastic behavior before set. In portland cement, superplasticizers are preferentially adsorbed onto C3A rather than alite, due to the different interaction with the mineral surfaces. Three different polycarboxylate superplasticizers (PCEs) were studied, based on copolymers of methacrylic acid and MPEG-methacrylate and characterized by different side chain length and different charge density. Their affinity to C3A and alite surfaces was determined through adsorption measurements on alite/gypsum and alite/gypsum/C3A mixtures. The results of the adsorption tests indicated that the charge density of PCEs, expressed as the ratio between carboxylic groups to ester groups, is the main parameter affecting the adsorption of the PCEs: the lower the charge density, the lower the adsorption to both phases. The same parameter affects the induction period of alite phase, as demonstrated by in situ XRPD dissolution kinetics experiments, both in the presence and in the absence of C3A. These results can be put in relation with both the hindrance of adsorbed PCE molecules in the dissolution kinetic of alite and the concentration of PCE molecules in solution in conditions of saturation.