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

抄録

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.