Hydration and strength development in magnesium oxysulfate (MOS) cement incorporating silicic acid

The effect of silicic acid (SA) on the hydration and strength development of magnesium oxysulfate (MOS) cement containing citric acid (CA) was reported. Changes to the setting time, hydration reactions, mechanical strength, phase composition, microstructure and pore structure were investigated. The presence of SA promoted the formation of Mg(OH)2 center dot MgSO4 center dot 5H2O and 5 Mg(OH)2 center dot MgSO4 center dot 7H2O (phase 517), increasing the hydration rate and reducing the setting time of MOS cement containing CA. Magnesium silicate hydrate (MSH) gel with a silicate layered structure was formed after adding SA to MOS cement containing CA, and there was good co-existence between layered MSH gel and phase 517 whisker, which increased the compactness of MOS cement. Results show that the presence of SA enhanced the early mechanical strength of MOS cement by increasing the content and crystallite size of phase 517 whisker, while enhanced the later one by promoting the formation of MSH gel.