The effect of CaO in fly ash on the hydration mechanism and properties of magnesium phosphate cement

Fly ash (FA) is often used to modify MPC, playing a role in delaying setting time, changing color, enhancing mechanical properties, and reducing costs. Current research mainly focuses on higher quality FA, in which the CaO content is low and mainly of F-type. This type is also widely used to modify traditional cement, leading to a shortage in the supply of F-type FA and limiting its engineering applications. Therefore, this study aims to use FA with a higher CaO content and higher stack volume to solve this problem, systematically studying the effect of high CaO content FA on the hydration mechanism and properties of the MPC system, and comparing it with low CaO content FA. The results show that the CaO in FA undergoes a secondary hydration reaction in the MPC, reacting with the phosphate radical to form Ca3(PO4)2. In addition, Ca(OH)2 is only generated at high CaO content and finally reacts to form CaCO3 with the increase of curing age, while CaHPO4 is generated at low CaO content. As the MPC sets and hardens quickly, the newly generated CaCO3 not only enhances compressive strength but also damages the matrix. However, the addition of FA still enhances the compressive strength of MPC, with the highest increase reaching 14.4%.In addition, the incorporation of FA significantly improves the water resistance of MPC, the compressive strength retention rate of the FA/MPC system after 28d of water immersion is significantly higher than that of MPC. This study provides a theoretical basis for the application of high CaO content FA.