Ternary sustainable geopolymer matrices containing metakaolin, water treatment sludge, and porcelain tile polishing residue

Geopolymeric matrices allow for the incorporation of a wide variety of waste materials. Therefore, this study aimed to evaluate ternary mixtures composed of metakaolin, water treatment plant sludge (WTPS), and porcelain tile polishing residue (PPR). Firstly, the chemical and physical properties of the materials were investigated using scanning electron microscopy (SEM) and thermogravimetric analysis (TG). Afterwards, the geopolymeric pastes were assessed by rotational rheometry, compressive strength, X-ray diffraction (XRD), and Fourier transform infrared spectroscopy (FTIR) analysis. Estimations of the geopolymer energy embodied (EE) and carbon equivalent emissions (ECO2-eq) were also calculated. The results showed that yield stress increased progressively with fresh WTPS and, to a lesser extent, calcined WTPS due to the kaolinite content. The reduction in viscosity in the formulations suggested lower reactivity, impacting the geopolymerization reactions. FTIR and XRD results align with calorimetric results, revealing minimal changes and reduction in geopolymerization with high metakaolin substitution. The compressive strength was maintained, even with only 35 % MK, possibly attributed to physical effects compensating for the lower reactivity of the residues. The reductions in ECO2-eq and EE were not significant due to the high environmental burden of the activators. A cost reduction of ∼16 % was achieved by replacing 65 % metakaolin. Finally, more eco-efficient geopolymer cements were produced, with only 35 % MK and resistance similar to the control mixture. [Display omitted] •65 % of metakaolin was replaced by waste.•Compressive strength greater than 80 MPa, with only 35 % MK.•30 % calcined WTPS + 15 % PPR resulted in 90 MPa compressive strength.•Replacing 65 % of MK with waste reduced costs by ∼16 %.