Pozzolanic activity of aluminium processing sludge (APS) waste for the decarbonisation of cements

As Climate Action Plans target the decarbonisation of cement, there is a universal search for cement alternatives. Cementing minerals can be generated from certain wastes. Aluminium profile processing plants continuously discharge vast amounts of waste. The APS waste studied is produced after surface treatments are applied to aluminium profiles. It is currently disposed of in landfill, but its high alumina content indicates potential for recovery in construction. The reactivity of APS waste for cement production has not yet been investigated. This paper studies the properties and reactivity of APS waste. The APS consists of SiO2 and Al2O3, with insignificant heavy metals. The raw APS is largely amorphous and highly pozzolanic. Calcination enhanced pozzolanicity. However, above 800 °C, crystallinity increased lowering reactivity. Calcination at 500 °C activates the APS, promoting participation of Si and Al in pozzolanic reactions, which caused significant strength improvement over 180 days. In the raw APS pastes, calcite and AFm-hydrocalumite are the main crystalline reaction phases. However, the calcined APS pastes have a denser matrix composed of cements with significant Si, Al and Ca (SEM/EDX) and C-S-H (FTIR) that produced substantial strength. APS waste can be used to decarbonise cement, either as an SCM or as a precursor in alkali activation technology and geopolymer production.