Binary alkali-activated systems obtained by the valorisation of calcined kaolin sludge and bottom ash

This paper assesses the use and valorisation of two industrial wastes generated at a large scale, which are currently disposed in landfills, as raw materials to produce geopolymers. Specifically, a kaolinitic sludge from the mining industry (CKS), and bottom ash (BA) generated during coal combustion in a thermal power station, were used as aluminosilicate precursors in geopolymer synthesis. The geopolymers were synthesised at 50°C, with a sodium oxide/aluminium oxide (Na₂O/Al₂O₃) molar ratio of 1.0, and different silica/aluminium oxide (SiO₂/Al₂O₃) molar ratios adjusted by manipulating the content of the soluble silicate solution used as the activator. The mechanical strength and reaction products formed during the geopolymerisation process were assessed up to 90 days of curing. The use of CKS as the main component of the precursor blend provides a geopolymer with better mechanical properties due to its higher reactivity than BA. The content of soluble silicates in the alkali activator plays an important role during geopolymerisation, improving the mechanical properties due to the formation of a more reticulated and dense structure. The mortars show a compressive strength higher than 55 MPa after 28 days and low water absorption by capillarity. This elucidates the feasibility of valorising these industrial residues as precursors for geopolymer cements.