Strength and Water Resistance of Low-Grade Fly ASH Incorporated Magnesia-Phosphate Cement Based Materials

Developed countries are suffering from aging/deterioration of infrastructure, and repair/rehabilitation is thus a long-term work. Magnesia-phosphate cement (MPC) is a promising binder material for utilization in repair works of degraded infrastructure. However, the cost of MPC is much higher than normally used cementitious materials. The incorporation of industrial wastes, such as low-grade fly Ash which is not suitable for use as supplementary cementitious materials, can largely reduce the material cost of MPC and make the color of the resulting material consistent with the substrate. It will be a surprise if specific properties can be improved. In this paper, an MPC paste with the magnesia-to-phosphate molar (M/P) ratio of 8 and the water-to-solid (W/S) ratio of 0.2 is selected as the reference, and a low-grade fly Ash is used to replace 20%, 40% and 60% of the solid phase by volume, respectively. The compressive strengths of the four pastes are measured following the increasing ages, and the water resistance indices of them are also evaluated. The results show that the higher the fly Ash replacement, the lower the compressive strength at all ages, but the better the long-term water-resistance. Based on a three-limit theory, the effect of fly Ash replacement on compressive strength is explained, and it is proved that the fly Ash is more than an inert filler. The influence of fly Ash on water resistance is also explained through a microstructural analysis.