Characterizing and processing a kaolinite-rich water treatment sludge for use as high-reactivity pozzolan in cement manufacturing

This paper evaluates the potential of water treatment sludge ash (WTSA) as high-reactivity pozzolan using strength tests in mortars and techniques to evaluate the calcium hydroxide consumption in pastes with hydrated lime. The sample was dried in an oven and then calcined at 600, 700, and 800 °C in a muffle. A high-reactivity metakaolin sample was used as a pozzolan reference. The properties of the samples assessed were fineness (Blaine), density (Le Chatelier), and chemical and mineralogical composition using X-ray diffraction (XRD), X-ray fluorescence (XRF), and thermogravimetric analysis (TG). The assessment of the pozzolanic properties employed standardized tests such as compression tests with hydrated lime (NBR 5751) and Portland cement (NBR 5752) in mortars. Also, non-standardized techniques, such as XRD, TG, and electrical conductivity, were utilized to evaluate the consumption of calcium hydroxide in pastes and solutions with hydrated lime and pozzolan. The standardized tests showed that mortars with metakaolin (MK) presented higher compressive strength values, while mortars with ash calcined at 600 °C (SA600) obtained the lowest values among WTSA. This behavior occurred because the preparation of mortars was strongly influenced by their consistency, affecting the compressive strength. However, analyzing the consumption of calcium hydroxide in pozzolan-hydrated lime pastes, SA600 presented higher values. Unlike tests on mortars, consistency is not affected in preparing the pastes, allowing a better development of pozzolanic reactions. Based on Ca(OH) ₂ consumption, it was concluded that the samples of WTSA studied could be used as high-reactivity pozzolans.