Effects of phosphoric acid and phosphates on magnesium oxysulfate cement

Magnesium oxysulfate (MOS) cement is a promising material for fire-proofing and insulation applications. Here, we have studied the effects of phosphoric acid and phosphates (H3PO4, KH2PO4, K3PO4 and K2HPO4) on the setting time, mechanical strength and water resistance of MOS cement. X-ray diffraction was used to examine phase composition, and analytical reagents were used to prepare samples of the new phase found so that it would be examined by chemical and thermogravimetric analyses. Adding phosphoric acid and phosphates can extend the setting time and improve the compressive strength and water resistance of MOS cement by changing the hydration process of MgO and the phase composition. A new subsulfate phase 5Mg(OH)2·MgSO4·7H2O (517 phase) is formed, which is needle-like and insoluble in water. Phosphoric acid or phosphates ionize in solution to form H2PO4 −, HPO4 2− and/or PO4 3−, and these anions adsorb onto [Mg(OH)(H2O) x ]+ to inhibit the formation of Mg(OH)2 and further promote the generation of a new magnesium subsulfate phase, leading to the compact structure, high mechanical strength and good water resistance of MOS cement. The improvement produced by adding phosphoric acid or phosphates to MOS cement follows the order of H3PO4 = KH2PO4 ≫ K2HPO4 > K3PO4.