Unraveling the interactive effects of Na2O/Al2O3, SiO2/Al2O3 and calcium on the properties of geopolymers from circulating fluidized bed fly ashes

Circulating fluidized bed fly ash (CFB-FA), generated as solid waste from the thermal power plants with different contents of calcium. The reuse of CFB-FA for construction materials is the most promising method for the large-scale transformation of waste to resource. This paper examines the ratio effects of Na2O/Al2O3 and SiO2/Al2O3 on geopolymers from low-calcium fly ash (CFB-FA1) and high-calcium fly ash (CFB-FA2) by using a mixture of Na2SiO3 and NaOH as the alkali activator. The detailed mechanistic study with XRD, FTIR, SEM, and MAS NMR is also conducted for the roles of silicon, aluminum, and calcium in the reaction process and their mutual interaction mechanism, along with the regulating mechanism for the compressive strength and setting time of geopolymers. The results indicate that the increased ratios of Na2O/Al2O3 and SiO2/Al2O3 are favorable for the reactions and reduce the setting time of CFB-FA1 and CFB-FA2 based geopolymer. In CFB-FA1, the higher silicon and aluminum content is conducive to their dissolution by regulating the Na2O/Al2O3 ratio, forming N-A-S-H and resulting in the increased strength. A small amount of calcium mainly exists in the form of CaSO4, which completely participates in the reaction. In CFB-FA2, the silicon and aluminum contents are relatively low, while the calcium content is high. The compressive strength is achieved by regulating the SiO2/Al2O3 ratio to increase the dissolution of silicon and aluminum, allowing CaSO4 and CaO to react and form C-A-S-H and ettringite, with CaCO3 as unreactive. The low calcium is favorable for the long-term strength, but the high calcium acts oppositely, primarily attribute to the decomposition of ettringite, a critical component for maintaining the compressive strength of CFB-FA2 based geopolymers.