Waste-based alkali-activated mortars containing low- and high-halloysite kaolin nanoparticles

Recently, with the aim of decreasing the environmental impact of concrete, developing an environmentally friendly construction material using industrial by-products has received great attention. This study aims for developing an eco-friendly construction material using industrial by-products as alternatives to cement, namely fly ash (FA) and ground granulated blast furnace slag (GGBS), and green nanoparticles (un-calcined high- (HH) and low-halloysite kaolin (LH)). 12 batches of cement-based and geopolymer mortars were manufactured, and their setting time, efflorescence, and compressive and splitting tensile strength were evaluated. Scanning electron microscopy (SEM) was performed to characterize the microstructure of the mortars. Production cost and embodied CO₂ of the mortars were also investigated. The results reveal that increasing kaolin content increases the setting time of the geopolymer, with mortars containing LH having a higher setting time than those containing HH. Incorporating HH leads to an increase (up to 6% at 6% HH) in the compressive strength of geopolymer, whereas incorporating LH results in a decrease (up to 6% at 9% LH) in the compressive strength of geopolymer. The results also show that both geopolymers containing HH and LH exhibit a higher splitting tensile strength (30–32% and 11–18%, respectively) than that of the companion geopolymers with no kaolin. Although the production cost of geopolymers is currently higher (∼40%) than that of the cement-based mortars, it is shown that their embodied CO₂ is significantly lower (∼134%), even when un-calcined kaolin is used in the geopolymer. These findings are promising and indicate the significant potential of using un-calcined kaolin materials in geopolymers towards cleaner production of structural grade construction materials.