Effect of calcium chloride-assisted calcined coal gangue as a supplementary cementitious material (SCM) on cement strength and microstructure

The accumulation of coal gangue (CG) has led to severe environmental pollution. Using calcined coal gangue (CCG) as a supplementary cementitious material (SCM) is an effective strategy for recycling CG. However, the low reactivity and high energy consumption of CCG present significant challenges. This study introduces an improved calcination method by incorporating a small amount of calcium chloride (CaCl₂) into CG before calcination, which significantly increases the efficiency of CG calcination. The product obtained from this improved method (CGC) greatly enhances the strength and fluidity of cement. The study examines the impact of CaCl₂ on CG during calcination and its subsequent effects on the properties of cement. The results indicate that adding CaCl₂ during calcination effectively facilitates the transformation of minerals such as kaolinite, clinochlore, and illite into reactive forms of silicon and aluminum. CG calcined at 650 ℃ (CG650) exhibits increased reactivity, significantly enhancing its performance as a SCM. CG calcined at 850 °C (CG850) demonstrates even better reactivity, producing more hydration products with a higher degree of polymerization. CG650C, obtained by adding CaCl₂ to CG650, promotes the formation of Friedel's salt, further contributing to the strength of the cement paste. CGC calcined at 650 °C (CGC650) combines the higher reactivity similar to CG850 with the filling effect of Friedel's salt, achieving the highest strength. Furthermore, when used as a SCM, CGC650 exhibits the lowest indices for CO₂ emissions and cost.