Effects of fly ash, phosphoric acid, and nano-silica on the properties of magnesium oxychloride cement

This study investigates the effects of fly ash, phosphoric acid, nano-silica additives on the hydration process, setting time, compressive strength, water resistance, and thermal stability of magnesium oxychloride cement (MOC). MOC samples incorporating different combinations of additives are prepared, and their hydration products and microstructures are studied via X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), thermogravimetry-differential scanning calorimetry (TGA-DSC), and scanning electron microscopy (SEM). Results indicate that the addition of nano-silica to MOC containing fly ash and phosphoric acid reduces initial and final setting times, decreases the thermal stability, and increases compressive strength. Furthermore, the water resistance of modified MOC pastes is significantly improved through the combined use of additives. Hydration mechanisms arising in MOC are elucidated, and the remarkable enhancement of water resistance is attributed to secondary hydration of the 5 Mg(OH)(2)center dot MgCl(2 center dot)8H(2)O (5.1.8 phase) and the formation of amorphous gel facilitated by nano-silica inclusion.