Coupled effects of fly ash and calcium formate on strength development of cemented tailings backfill

Cemented tailings backfill (CTB) is widely adopted to ensure the safety of underground goafs and mitigate environmental risks. Fly ash (FA) and calcium formate (CF) are common industrial by-products that improve the mechanical performance of CTB. How the coupling of the two components affects the strength development is not yet well-understood. Neural network modelling was conducted to predict the strength development, including the static indicator of uniaxial compressive strength (UCS) and the dynamic indicator of ultrasonic pulse velocity (UPV). Sobol’ sensitivity analysis was carried out to reveal the contributions of FA, CF and curing time to CTB strength. SEM microstructure investigation on CTB samples was implemented to reveal the mechanism of strength development and justify the predictions by neural network modelling and sensitivity analysis. Results show that the combination of FA content, CF content and curing time can be used to predict both UCS and UPV while providing adequate accuracy. The maximum of UCS of 6.1215 MPa is achieved at (FA content, CF content, curing time) = (13.78 w%, 3.76 w%, 28 days), and the maximum of UPV of 2.9887 km/s is arrived at (FA content, CF content, curing time) = (11.67 w%, 3.08 w%, 10 days). It is also implicated that prediction of UCS using UPV alone, although common in field application is not recommended. However, UPV measurement, in combination with the information of FA dosage, CF dosage and curing time, could be used to improve UCS prediction. The rank of variable significance for UCS is curing time > FA content > CF content, and for UPV is FA content > curing time > CF content; variable interaction is strongest for FA with CF for UCS development, and for FA with curing time for UPV evolution. Influence of FA on CTB strength development is due to improved polymerisation and consumption of Ca(OH)₂. Influence of CF on strength development is a result of accelerated hydration and increased combined-water content in calcium silicate hydrate (CSH). Effect of curing time is attributed to the evolution of CSH product and pore-water content during cement hydration.