Enhancement of surface properties of cementitious materials by phosphate treatments

This study aims at investigating the ability of aqueous phosphate solutions to improve the surface properties of cement pastes via the formation of low-solubility calcium phosphate (CaP) phases. Diammonium hydrogen phosphate (DAP), dipotassium hydrogen phosphate (DPP) and disodium hydrogen phosphate (DSP) were investigated based on thermodynamic modeling and experiments. For DPP, the influence of concentration and cement age was evaluated. A 1 M DPP + 1mM CaCl2 solution applied to cement hydrated for 7 days was most promising. The CaP coating formed after 24 hours of exposure resulted in significant improvements in the cement pastes’ resistance to abrasion, surface indentation and staining. Preliminary durability tests by immersion in water were successfully carried out to ascertain that no efflorescence due to soluble salts appeared and that mechanical improvement was retained. Compared to DAP, DPP has the advantage of not releasing NH3 into the atmosphere, thus preventing unpleasant odor and improving the treatment’s environmental sustainability.