The role of bauxite and fly-ash on the water stability and microstructural densification of magnesium phosphate cement composites

The novel physical and strength properties of magnesium phosphate cement (MPC) composites have been capturing the attractiveness in the last three decades. But the porous microstructure and poor strength performance in water region are limiting the applications. The current study added the fly-ash (FA) and bauxite as additive in MPC mixtures for improving the structural density and water permanency. Six different combinations were designed based on pure MPC, MPC with FA and MPC blended with FA and bauxite. The analytical results showed that MPC mortars prepared with FA (one third of magnesium oxide, MgO) and bauxite (12% of MgO) presented the superior compressive strength (CS) around 92.7 MPa and 90.4 MPa, and flexural strength (FS) about 13.7 MPa and 10.3 MPa at air and water environment, respectively which were nearby 25% higher strength than control sample. In addition, specimens containing FA and bauxite exposed the less and small sizes pores, and formations of more intermediate hydration products in the microstructures. Moreover, the XRD and EDS results showed the development of struvite along with secondary crystals phases which was the prime reason of solidity through decreasing the porosity. Furthermore, thermogravimetric analysis displayed the mass loss resisting ability of specimen incorporation with FA (one third of MgO) and bauxite (12% of MgO) in heat conditions. The major findings of the study might be a potential in the practical applications of MPC compounds, especially for water structures.