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 Na 2 O/Al 2 O 3 and SiO 2 /Al 2 O 3 on geopolymers from low-calcium fly ash (CFB-FA1) and high-calcium fly ash (CFB-FA2) by using a mixture of Na 2 SiO 3 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 Na 2 O/Al 2 O 3 and SiO 2 /Al 2 O 3 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 Na 2 O/Al 2 O 3 ratio, forming N-A-S-H and resulting in the increased strength. A small amount of calcium mainly exists in the form of CaSO 4 , 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 SiO 2 /Al 2 O 3 ratio to increase the dissolution of silicon and aluminum, allowing CaSO 4 and CaO to react and form C-A-S-H and ettringite, with CaCO 3 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. • Adjusting Na/Al ratio to form N-A-S-H gel enhances compressive strength of low-calcium CFB-FA geopolymer. • Adjusting Si/Al ratio to form C(N)-A-S-H gel and ettringite enhances compressive strength of high-calcium CFB-FA geopolymer. • The reactive CaSO 4 was the main component for forming C-A-S-H gel in low- and high-calcium CFB-FA.