Effect of Phosphoric Acid and Soluble Phosphate on the Properties of Magnesium Oxychloride Cement

This study investigates the effects of phosphoric acid (H₃PO₄), potassium dihydrogen phosphate (KH₂PO₄) and sodium dihydrogen phosphate (NaH₂PO₄) admixtures on the setting time, compressive strength and water resistance of magnesium oxychloride cement (MOC). MOC samples incorporating different admixtures are prepared, and their hydration products and microstructures are studied via X-ray diffraction (XRD) and scanning electron microscopy (SEM). The results indicate that the addition of H₃PO₄, KH₂PO₄ and NaH₂PO₄ reduces the initial and final setting times and decreases the compressive strength. However, the compressive strength of MOC is higher than 30.00 MPa with the addition of 2.0 wt.% phosphoric acid and its phosphate after 14 days of air curing. The water resistance of modified MOC slurries is significantly improved. The softening coefficient of MOC with 2.0 wt.% H₃PO₄ is 1.2 after 14 days of water immersion, which is 3.44 times higher than that of the neat MOC. The enhancement in water resistance is attributed to the formation of amorphous gel facilitated by H₃PO₄, KH₂PO₄ and NaH₂PO₄. Furthermore, the improvement in water resistance is manifested as H₃PO₄ > KH₂PO₄ > NaH₂PO₄.