Mechanical properties, durability and microstructure of cementitious materials with low-calcium circulating fluidized bed fly ash

To enhance the utilization of circulating fluidized bed fly ash in construction materials, this study investigated the influence of low-calcium circulating fluidized bed fly ash (LCFA) on the properties of cement-based materials. In addition, an alkali activator was added to improve the properties of cement-based materials mixed with LCFA and the synergistic effect was also investigated. The results show that an appropriate amount of LCFA can improve the compressive strength of cement-based materials in underwater curing conditions. The addition of LCFA can improve the physical properties and carbonization resistance of cement-based materials. Whereas, the addition of water glass (WG) can appropriately shorten the setting time and effectively increase the compressive strength of cement mixtures with LCFA. In addition, adding WG can effectively improve the resistance of chloride ion penetration and carbonation. It is speculated that adding appropriate amount of LCFA enhanced the amount of hydration products generated, filled the coarse pores, and made the pore structure of samples more compact. Appropriate addition of LCFA and WG increases the concentration of hydroxyl groups in the system, accelerates the rupture of Si-O and Al-O bonds on the surface of LCFA particles, and forms free and unsaturated active bonds on the surface of LCFA, which is easier to react with Ca(OH)2 to form hydration products such as C-S-H and C -A-S-H. At the same time, gelatinous hydration products migrate and diffuse with the change of ion concentration in the system, and gradually fill and compact the internal structure.