Recently, alkali-activated slag (AAS) has attracted extensive attention in cemented paste backfill (CPB) due to its low cost/CO2 emissions and high strength benefits. However, a comprehensive analysis of the mechanical/rheological behavior and microstructure evolution of AAS-CPB using mineral admixtures is still lacking. In this study, metakaolin (MK), fly ash (FA), and silica fume (SF) were employed to replace ground granulated blast furnace slag (GGBS) at various levels to formulate an alkali-activated binder, and the corresponding mechanical, rheological, and microstructure properties of CPB were investigated. The results suggest that FA tends to reduce CPB’s rheological and strength evolution and this negative effect increases with the FA dosage. The replacement of MK or SF increases the rheological parameters and thus diminishes fluidity and has positive or negative effects on strength depending on the replacement level and curing age. This study’s findings will contribute to developing a new scheme for lucrative and environmentally responsive multi-solid waste-based AAS-CPB in the field.

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The significant difference in water-to-binder ratio, activator type and concentration between alkali-activated slag (AAS) paste/mortar/concrete and AAS-based cemented paste backfill (AAS-CPB) means that previous results related to the properties and mix optimization of AAS materials cannot be directly translated to AAS-CPB. This study statistically identifies the effect of key influential variables such as silicate modulus, slag fineness and activator concentration on 3- and 28 day unconfined compressive strength (UCS) of AAS-CPB by central composite design (CCD) established in response surface methodology (RSM). In this study, the prominence of independent variables and their relations are investigated by using ANOVA (analysis of variance) having a significant level of 0.05. ANOVA results certify that there is a strong link between the level of variable contribution on UCS performance of AAS-CPB and curing age. Obviously, silicate modulus and activator concentration are the most major variables influencing UCS at 3 and 28 days, respectively. Increased fineness of slag and higher pH of pore solution enhance 3 day UCS, but restrain the further hydration of unreacted slag and subsequent the gain in strength at advanced curing ages. The combination of independent variables of silicate modulus (0.295), slag fineness (12630.2), activator concentration (0.45) gives the optimum responses.

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