Reaction mechanism and mechanical properties of geopolymer based on kaolinitic coal tailings

Abstract

Valorization of coal tailings as supplementary cementitious material is vital for green development, including their application as aluminosilicate in geopolymer materials. Coal tailings from southern Brazil were activated mechanically and thermally. This work analyzed the effect of calcium addition via Portland cement and flue gas desulfurization on the reaction mechanism of geopolymer using setting times and semi-adiabatic calorimetry. Additionally, un-calcined coal gangue was added to geopolymer cement to evaluate the effect of sulfur in the microstructure of geopolymer. The experimental results showed kaolinitic coal sludge was transformed into a metastable phase with higher reactivity, while X-ray pyrite reflection was observed for un-calcined coal gangue. The findings showed that calcium modified the reaction mechanism by accelerating the monomers' condensation and increasing the energy released, promoting the hardening of the geopolymer at 24 h. In addition, a solid matrix of geopolymer was developed compounded mainly by an amorphous inorganic polymer and C-A-S-H gel as a secondary phase. The thermodynamic simulation confirmed the addition of coal gangue reduced the pH of the pore solution and increased the content of SO42− and Fe2+. These results evidenced that partial oxidation of pyrite in an alkaline medium releases iron and sulfate ions and this process can lead to acidification of the pore solution resulting in internal degradation.