Unlocking the role of silica gel in enhancing mechanical properties and water resistance of magnesium oxysulfate cement

The continuous hydration of residual periclase to form brucite with expansive stress remains an issue for the utilization of hardened magnesium oxysulfate (MOS) cement in humid environments. This study explored converting residual periclase and brucite into magnesium silicate hydrate (M–S–H) gel to enhance the mechanical properties of MOS cement after water immersion. Changes to the hydration process, strength development, phase composition, microstructure, and pore structure of MOS cement with silica gel (SG) before and after immersion in water were investigated. Results show that M–S–H gel with encapsulation and cohesiveness formed at very early ages reduced the fluidity and initial setting time of MOS slurry and accelerated the hydration of periclase to form 3 Mg(OH)₂·MgSO₄·8H₂O, thereby shortening the final setting time. The addition of SG complicated the forming process of 5 Mg(OH)₂·MgSO₄·7H₂O (Phase 517), extending its formation period but increasing its content and crystallite size. Synergistic growth between layered M–S–H gel and Phase 517 whiskers optimized the pore structure and densified the matrix, enhancing the early and later mechanical strength of MOS cement by 40%–100 %. When MOS cement with SG was immersed in water, the conversions of residual periclase and brucite into M–S–H gel occurred. Under conditions where the formation rate of M–S–H gel exceeded that of brucite and the consumption of brucite surpassed its formation, SG allowed the mechanical strength and microstructure of MOS cement to develop further, despite MOS cement being immersed in water. However, this enhancement mechanism was effective only when SG dosage was in the range of 5–10 wt%.