Effect of Sodium Orthophosphate on Rheology and Hydration of MgO/Nesquehonite Cement

Magnesium oxide-magnesium carbonate cements are promising alternatives to Portland cement due to their ability to sequester CO2. This study investigates the effect of sodium orthophosphate on workability, hydration kinetics and hydrate assemblage in MgO/nesquehonite binder to assess its possible use as an admixture. Sodium orthophosphate decreases the yield stress of pure MgO pastes. In case of MgO/nesquehonite blends, the yield stress is much lower than for the pure MgO system and is not significantly further decreased by sodium orthophosphate. Isothermal calorimetry results indicate a delay in the hydration of MgO pastes, increasing with the sodium orthophosphate content, while for MgO/nesquehonite blends, only a moderate retardation is observed. Mortars containing up to 1 mass-% of sodium orthophosphate show similar flexural and compressive strength values as mortars without sodium orthophosphate. Phosphate species sorb onto the surface of MgO resulting in a strongly negative surface charge and retarding the reaction of MgO to brucite. During hydration, magnesium phosphate phases precipitate and phosphate is incorporated into brucite-like phases. Thus, despite a mixed effect of sodium orthophosphate on MgO/nesquehonite cement, it is an encouraging additive to control the kinetics and workability times of MgO-only systems.