The mechanism for soluble phosphates to improve the water resistance of magnesium oxychloride cement

Abstract This paper discusses the reasons why the addition of some soluble phosphates to magnesium oxychloride cement (MOC) can greatly improve its water resistance. With the XRD analysis and the determination of the strength retention coefficients (a judging index for the water resistance), it has been shown that the addition of small quantity of the soluble phosphates, such as H 3 PO 4 , NaH 2 PO 4 ·2H 2 O, and NH 4 H 2 PO 4 , to MOC pastes does not influence the phases formed in the MOC pastes, but can result in the great increases of the strength retention coefficients of hardened MOC pastes, i.e., the great improvement of the water resistance of MOC. It was considered that the key components being responsible for the improvement of the water resistance could be the anions H 2 PO 4 − , HPO 4 2− , and PO 4 3− yielded by the ionization of these phosphates in the solution of MOC pastes. These anions can decrease the lowest concentration of Mg 2+ ions in the solution required by the formation of 5Mg(OH) 2 ·MgCl 2 ·8H 2 O (5-phase) or 3Mg(OH) 2 ·MgCl 2 ·8H 2 O (3-phase) in the MOC pastes and increase the stability of these phases in water. Thus, when the hardened MOC pastes with small quantity of the soluble phosphates are immersed in water, the 5-phase or 3-phase in them will not be decomposed by water, which makes the strength of the hardened MOC pastes remain unchanged in water.