Microstructure and mechanical properties of sustainable cemented-paste backfill materials with the substitution of alkali-activated rice husk ash

Rice husk ash (RHA) is currently utilized as a supplementary cementitious material in cement products due to its pozzolanic properties. This work aims to investigate the pozzolanic effect (PE) and the filler effect (FE) of RHA on the mechanical properties and microstructure of cemented paste backfill (CPB). RHA and inert quartz sand were ground to similar particle sizes and used to prepare CPB, partially replacing the Portland cement traditionally used in these mixtures. Unconfined compressive strength (UCS) tests and scanning electron microscopy (SEM) were used to analyze the mechanical properties and microstructure of the CPB, respectively. The effects of the RHA content, cement-to-tailings ratio (C/T), and mass concentration (MC) on the UCS of the CPB samples were investigated. The results show that RHA can enhance the strength of the CPB even at concentrations of less than 20 wt.%. It was also found that the MC has a greater effect on the strength of CPB produced with RHA compared to the C/T. The CPB sample with the highest pozzolanic activity index (PAI) of 101.63% had a C/T of 1:4 and an MC of 74%. The proportion of the UCS of the CPB that could be attributed to the PE was 67.30–87.92% higher than could be attributed to the FE in samples with RHA contents ranging from 10–20%. The FE of RHA exerted a stronger influence at lower curing times due to the early hydration of blended materials, while the influence of the PE of RHA was limited at curing times of 3 d. In contrast, the PE played a more crucial role at curing times of more than 3 d. SEM analysis showed that CPB produced with 10–20% RHA exhibits a dense and homogeneous microstructure. Thus, the influence of the PE of RHA on the mechanical properties and microstructure of CPB was far more significant than the FE of RHA at a curing time of 28 d. These results provide new insights into the potential use of RHA as a cementitious material for use in backfilling during mining operations.