Unraveling the role of magnesium salts in liquid alkali-free accelerators: Insights into setting and early-age properties of accelerated cement pastes

Abstract

Magnesium salts are key components of liquid alkali-free accelerator; however, their influence on the setting and hardening of accelerated cement pastes remains unclear. This study systematically investigates the role of different magnesium salts by preparing and examining aluminum sulfate (AS)-based accelerators at different dosages. The effects of the accelerators on the setting and early age mechanical properties of the cement pastes were comparatively studied. Results revealed that the type and dosage of magnesium salts significantly affected these properties. Specifically, MgSiF₆ substantially accelerated the cement paste setting, albeit at the expense of early age mechanical strength, owing to its considerable delay in C₃S hydration. Conversely, both Mg(NO₃)₂ and MgSO₄ slightly shortened the setting time while improving the 6 h compressive strength, which was attributed to the facilitation of C₃S hydration and refinement of the cement pore structure. Further analysis of ettringite formation, growth and chemical composition indicated that the low pH environment induced by MgSiF₆ promoted the incorporation of magnesium and fluoride ions into ettringite, leading to the formation of higher-aspect-ratio crystals. This morphological change enhanced the percolation network, thus accelerating cement setting. These findings advance our understanding of the setting and hardening mechanisms of accelerated cement pastes and offer valuable guidelines for optimizing accelerator formulations.