Investigation of the hydration kinetics and microstructure formation mechanism of fresh fly ash cemented filling materials based on hydration heat and volume resistivity characteristics

Fly ash cemented filling materials (FCFM) have received a great deal of attention in recent years as more sustainable solutions to the large-scale utilization problems of fly ash and coal gangue. Based on the hydration heat and the volume resistivity characteristics, this study examined the changes in the fresh FCFM properties from the thermodynamic and electrical perspectives, which reflected the characteristics of the hydration kinetics and microstructure formation mechanism of the FCFM. The results showed that the hydration exothermic process of the FCFM exhibited a rapid reaction stage, an induction stage, an acceleration stage, a deceleration stage and a stable stage. Compared with cement, the mixture of fly ash and coal gangue resulted in a substantially lower hydration heat. The volume resistivity of the FCFM changed over time, initially increasing, then decreasing, then increasing again. The hydration kinetics of the FCFM could be described by three processes: nucleation and crystal growth (NG), interactions at the phase boundaries (I) and diffusion (D). The NG process dominated in the early stage of hydration, and the I and D processes gradually became dominant as the hydration degree increased. Compared with cement, changing the hydration kinetics required a higher degree of hydration in the mixture of fly ash and coal gangue. The improvement in the hydration reaction rate of FCFM due to additives was primarily reflected in the early hydration stage. The calculated data on the C-S-H gel content of the FCFM paste agreed well with the changes in the uniaxial compressive strength (UCS) and plane porosity over time. Remarkable negative correlations between the plane porosity and the UCS and between the plane porosity and the C-S-H gel content were found, along with a significant positive correlation between the C-S-H gel content and UCS. A microstructure formation mechanism model of the FCFM was constructed, and the hydration process of the filling material was divided into a dissolution period, a hydration period and a flocculated-structure formation and growth period. These findings provide new insights into the hydration kinetics and microstructure formation mechanism of fresh FCFM.