Effects of H3PO4 and Ca(H2PO4)2 on mechanical properties and water resistance of thermally decomposed magnesium oxychloride cement

The effects of H3PO4 and Ca(H2PO4)2 on compressive strength, water resistance, hydration process of thermally decomposed magnesium oxychloride cement (TDMOC) pastes were studied. The mineral composition, hydration products and hydration heat release were analyzed by XRD, FT-IR, SEM and TAM air isothermal calorimeter, etc. After being modified by H3PO4 and Ca(H2PO4)2, the properties of the TDMOC are improved obviously. The compressive strength increases from 14.8 MPa to 48.1 MPa and 37.1 MPa, respectively. The strength retention coefficient (Kn) increases from 0.38 to 0.99 and 0.94, respectively. The 24 h hydration heat release decreases by 10% and 4% and the time of hydration peak appearing is delayed from 1 h to about 10 h. The XRD, FT-IR and SEM results show that the main composition is 5Mg(OH)2·MgCl2·8H2O in the modified TDMOC pastes. The possible mechanism for the strength enhancement was discussed. The purposes are to extend the potential applications of the salt lake magnesium resources and to improve the mechanical properties of TDMOC.