Unlocking the role of Al(OH)3 in the evolution of mechanical properties of magnesium oxysulfate (MOS) cement

Abstract

Magnesium oxysulfate (MOS) cement is widely used in façade panels and building components, but its broader application is limited by inadequate water resistance. This study incorporated Al(OH)₃ into MOS cement to improve its mechanical properties and water resistance and, more importantly, to elucidate the underlying hydration-regulation mechanism. The effects of Al(OH)₃ were systematically investigated through mechanical testing, water immersion, phase and thermal analyses, pore structure characterisation, solid-state ²⁷Al NMR, SEM, and HR-TEM. MOS cement containing 3% Al(OH)₃ achieved a 28-d compressive strength of 81.6 MPa, 10.8% higher than that of the control. After 56 d of water immersion, the specimen containing 6% Al(OH)₃ exhibited the highest compressive strength of 59.1 MPa, representing a 12.3% improvement over the control. Mechanistically, Al(OH)₃ participated in the hydration process by consuming OH⁻ released during MgO hydration, thereby suppressing Mg(OH)₂ formation and regulating the content and crystal growth of the strength-bearing 5Mg(OH)₂·MgSO₄·7H₂O (5·1·7) phase. Meanwhile, amorphous aluminium hydroxide formed and became distributed within the matrix and interwoven with the 5·1·7 phase at the nanoscale. The coupled evolution of these hydration products refined the pore structure and densified the cement matrix, compensating for the reduction in 5·1·7 phase content and enhancing mechanical performance. After water immersion, the 5·1·7 phase underwent partial transformation accompanied by the formation of plate-like and flower-like basic magnesium sulfate phases. Amorphous aluminium hydroxide associated with the 5·1·7 phase mitigated water-induced deterioration, contributing to improved strength retention. These findings reveal that Al(OH)₃ enhances MOS cement through coupled regulation of OH⁻ availability, hydration-product evolution, and microstructural densification, providing a mechanistic basis for designing water-resistant MOS cement.