Enhanced reactivity of depolymerized Class F fly ash: Mechanism and its effect on the performance of high-volume fly ash cementitious materials

The highly polymerized aluminosilicate framework in Class F fly ash limits its reactivity and poses a significant constraint on its use in high-volume fly ash cementitious materials (HVFC). In this study, a novel co-sintering method with Na 2 SO 4 at temperatures of 600, 700, 800 and 900 °C was employed to enhance reactivity by depolymerizing the aluminosilicate network within fly ash. The introduction of externally resourced Na 2 SO 4 reduced the content of 4-connected tetrahedra and accelerated the dissolution of fly ash. As a result, the reaction degree of fly ash in 28 days HVFC was promoted from 3.1% of the raw fly ash to 19.3% of that co-sintered with Na 2 SO 4 at 900 °C. The initial and final setting time of HVFC reduced from 437 and 580 min to 295 and 418 min, respectively. Additionally, more hydration products were observed in HVFC containing sintered fly ash, leading to an increase in compressive strength by up to 20%. A mechanistic model based on the shielding effect of peripheral tetrahedra linked to 4-connected tetrahedra was developed to account for the enhanced reactivity of depolymerized Class F fly ash.