Synergistic effect of processing water treatment sludge rich in kaolinite for the sustainable production of LC3

The increasing population and urban development intensify the need for potable water, resulting in higher production of sludge in water treatment plants (WTPs). To mitigate the improper disposal of sludge and reduce carbon dioxide emissions from Portland cement production, the pozzolanic activity of sludge and its ashes, derived from WTPs and referred to as WTPA, is being investigated. These materials are being evaluated as supplementary cementitious materials (SCMs), enabling their application in the development of limestone calcined clay cement (LC³). The WTPA samples calcined at 600, 700, and 800 °C were analyzed using X-ray fluorescence (XRF), X-ray diffraction (XRD), thermogravimetric analysis (TGA), scanning electron microscopy (SEM), Brunauer-Emmett-Teller (BET) surface area analysis, Blaine fineness, and laser diffraction techniques. Compressive strength, electrical conductivity, and R³ tests were performed to evaluate pozzolanic reactivity of WTPA. The influence of calcination temperature, the proportion of WTPA, and the water/cement ratio were studied using the Box-Behnken design in LC³ mixtures. The results suggest that the calcination temperature of 700 °C is ideal for producing pozzolana, standing out with an SAI of 140 % and 19.09 MPa in the lime test. This temperature also favored a higher combined water content (15.8 g/100 g of paste), indicative of resistant hydrated compounds. Additionally, the multi-objective analysis indicated that the optimal formulation for WTPA use in LC³ involves a calcination temperature around 700 °C, a replacement rate of 17.62 %, and a water/cement ratio of 0.54. This formulation provides a fluid consistency, while compressive strength reaches 35.57 MPa, demonstrating the effectiveness of sludge as pozzolana. This study provides new insights into the use of water treatment plant sludge ash as a sustainable material for the development of LC³, offering a promising alternative to reduce the environmental impact of the cement industry.