Interaction of individual meta clays with polycarboxylate (PCE) superplasticizers in cement investigated via dispersion, zeta potential and sorption measurements

Polycarboxylate (PCE) comb polymers were selected as potential dispersant for suspensions prepared from individual calcined clay samples. It was anticipated that mechanistically their dispersing effect relies on adsorption onto positively charged surfaces of the meta clays which was to be proven. Furthermore, the structural differences between two specifically selected PCE kinds were assumed to result in different dispersing performance. Using spread flow tests, the dispersing effectiveness of the PCEs on four different meta clay samples (meta kaolin, meta muscovite, meta illite and meta montmorillonite) was assessed. Moreover, the surface chemistry of the meta clays dispersed in synthetic cement pore solution (SCPS) was captured via zeta potential and sorption measurements to elucidate the interaction of the superplasticizers with the calcined clay samples. In accordance to common knowledge, the results confirmed the high water demand of calcined clays which made them asking for superplasticizers when used in mortar and concrete. It was found that the zeta potential (surface charge) of the meta clays initially was negative, but converted to positive via uptake of Ca2+ ions from the pore solution, especially in Ca2+-rich cementitious systems. Sorption measurements revealed that the PCE superplasticizers adsorbed onto these positively charged surfaces and thus achieved dispersion. The methallyl ether (HPEG) based PCE vastly outperformed the methacrylate ester (MPEG) based PCE. Surprisingly, meta illite could not be dispersed satisfactorily, in spite of high PCE adsorption. The results contribute to understand at a glance the dispersing mechanisms of PCE superplasticizers in naturally occurring calcined clays with various clay mineral compositions and to select the adequate superplasticizer easily.