Effect of heat treatment and vitrification on the reactivity, strength, and leaching behaviour of waste-derived supplementary cementitious materials

The increasing demand for cement has intensified the search for SCMs. In this study, five waste streams (recycled-concrete sand, recycled-concrete washing sludge, municipal solid waste incineration (MSWI) bottom ash from grate incineration (RA), MSWI bed ash from fluidised bed incineration (BA), and a glass-ceramics-stones mixture) were evaluated as SCMs. Each material was investigated in three states: untreated and milled, heat-treated at 600°C, and vitrified followed by quenching and milling. Chemical and mineralogical characterization was performed using XRF and XRD. Reactivity was assessed using the strength-activity-index (SAI) at 1, 7, 28, and 90 days and the R³ test via isothermal calorimetry and TG/DTG, while leaching behaviour was evaluated against Austrian regulatory limits. Untreated materials exhibited low to moderate reactivity, with 90-day SAI values of 0.55–0.90 and cumulative 7-day R³ heat-release of approximately 14–62 J/gSCM. Heat treatment at 600°C resulted in moderate enhancement, increasing SAI by up to ∼40% and R³ heat release to 44–69 J/gSCM. Vitrified materials showed substantially higher reactivity, with 90-day SAI values of 1.03–1.10 and cumulative 7-day R³ heat release ranging from approximately 140–365 J/gSCM. A clear distinction was observed between MSWI ashes, with bed ash (BA) consistently outperforming bottom ash (RA) in terms of compressive strength, reflecting the influence of incineration technology on reactivity development. This study highlights the variable efficiency of vitrification across chemically diverse wastes and the favourable leaching performance of the resulting matrices. While vitrification is energy-intensive, it represents a promising high-performance upcycling route for selected waste-derived SCMs.