Thermally activated drinking water treatment sludge as a supplementary cementitious material: Properties, pozzolanic activity and hydration characteristics

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Drinking water treatment sludge (DWTS), a hazardous municipal solid waste, is Al-rich and Si-rich. Although it can be used as a supplementary cementitious material (SCM), the appropriate treatment temperature of DWTS, the mineral phase transformation in the heating process and hydration characteristics are indefinite. This study explores the physicochemical characteristics and pozzolanic activity of DWTS after thermal treatment at seven different temperatures (105 °C, 200 °C, 400 °C, 600 °C, 800 °C, 1000 °C, 1200 °C) and its effects on cement hydration. The results show that DWTS calcined at 800 °C has the highest reactivity due to the presence of γ-aluminum (AlⅣ), which not only exhibits pozzolanic activity but also can undergo hydration reaction with cement to form calcium aluminosilicate hydrate (C-ASH). At lower temperatures (105 °C to 200 °C), DWTS exhibits non-pozzolanic activity due to the presence of a higher content of organic matter (i.e., TOC greater than 5 %). Although the Frattini test proves that the the DWTS treated at medium temperature (400 °C to 600 °C) has pozzolanic activity, it mainly achieves dehydroxylation (similar to the transition from kaolinite to metakaolin) in this temperature range. The reactivity is improved, but the crystal structure does not fundamentally change and the hydration product is calcium silicate hydrate (CSH). After DWTS is calcined at ultra-high temperature (greater than1000 °C), γ-aluminum (AlⅣ) is converted into α-aluminum (AlⅥ) and combines with amorphous quartz to form mullite, which acts as a filler in DWTS-cement samples.