High-Early-Strength Magnesium Phosphate Cement with Fly Ash

The microstructure and properties of a novel magnesia phosphate cement (MPC) with large amounts of fly ash were studied in the present work. For comparison purposes, reference specimens without the incorporation of fly ash were also investigated. The effect of fly ash content on the properties of MPC was studied with two dead burnt magnesia materials with different MgO contents and fineness. The results showed that fly ash can improve the bonding and compressive strength of MPC, even at very early ages. It was found that a fly ash content of 30 to 50% has the best improving effect on MPC, despite the two types of magnesia. Due to the difference of MgO content and particle fineness of two dead burnt magnesia materials, however, MPC mortars with finer magnesia demonstrated higher compressive strength. The incorporation of fly ash does not retard the setting reaction of MPC, but decreases the total heat evolution of MPC. The hydrates and microstructure of MPC paste were examined by an x-ray diffractometer, scanning electron microscopy-energy dispersive x-ray analysis (SEM-EDX), and Fourier transform infrared (FTIR) spectroscopy. The main products formed in MPC paste were crystal magnesium potassium phosphate hexahydrate and amorphous species. It is believed that the particles of fly ash fill the voids of MPC paste and strongly bond together with hydrates of MPC.