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

WEN Jing (文静)^{1, 2}, YU Hong-fa(余红发)^{1, 3, 4}, LI Ying(李颖)^{1, 2},WU Cheng-you(吴成友)^{1, 2}, DONG Jin-mei(董金美)^{1, 2}, ZHENG Li-na(郑利娜)^{1, 2}

(1. Qinghai Institute of Salt Lakes, Chinese Academy of Sciences, Xining 810008, China;
2. Graduate School of Chinese Academy of Sciences, Beijing 100049, China;
3. Department of Civil Engineering, Qinghai University, Xining 810016, China;
4. Department of Civil Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing 210016, China)

Abstract: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 (K_n) 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.

Key words:thermally decomposed magnesium oxychloride cement (TDMOC); H₃PO₄; Ca(H₂PO₄)₂; strength; water resistance; hydration heat