Aglomerantes a base de cimento, cinza de lodo de ETA e calcário: influência na hidratação, resistência à compressão e estrutura de poros

Abstract

Increases in population and consequent increases in the volume of treated water have increased the amount of sludge produced in water treatment plants (WTPs), which can introduce risks to the environment when its final disposal is not adequate. WTP sludge contains some of the main constituents of Portland cement, such as silica, aluminum, and iron, and has potential for use as a mineral addition, contributing to the reduction of cement consumption and environmental impacts resulting from its production. The objective of this study was to investigate the compressive strength and hydration of mixtures produced from the partial replacement of cement with WTP sludge ash (WTPSA) and ground limestone. WTPSA has chemical and physical characteristics similar to those of calcined clays, and their use combined with finely ground limestone has shown positive synergistic effects on the characteristics of cementitious mixtures. The use of chemical activation was also evaluated to potentiate the effects of the additions. To achieve the proposed objectives, this study was divided into three stages: evaluation of the pozzolanic activity of WTPSA submitted to different thermal and mechanical treatments, evaluation of the compressive strength and evaluation of the hydration and pore structure. The results showed that the best pozzolanic performance was obtained by WTPSA calcined at 700°C for 1 hour and ground for 1 hour. Depending on the substitution content, it was possible to obtain a compressive strength increase of up to 45% compared to the reference concrete. The use of WTPSA and/or limestone changed the hydration kinetics, the types of hydration products formed, and the pore structure of the mixtures. The consumption of calcium hydroxide by the pozzolanic reaction and the formation of carboaluminates from the reaction between WTPSA and limestone were detected in all of the mixtures with additions. In general, the mixtures containing 15% WTPSA and 15% WTPSA + 7.5% limestone showed a higher compressive strength, higher chemically combined water content and lower pore volume, indicating the beneficial effect of these additions on the composition of the binder. The use of higher contents resulted in slower reaction rates, smaller strength increases and higher pore volumes, although the reaction of the additions promoted a refined pore structure. The use of gypsum as a chemical activator was effective in improving the mechanical performance and hydration of the mixtures with higher cement substitution content, although the general performance did not match that of the mixtures containing 15% WTPSA or 15% WTPSA+ 7.5% limestone.