Hydration and carbonation behaviour of selected recycled materials from Slovenia

The European Union’s shift towards a circular economy emphasizes the substitution of virgin materials with recycled alternatives, particularly in the construction sector, which can accommodate large volumes of industrial by-products. Ashes from coal, biomass, paper sludge, and co-combustion processes are abundant secondary materials whose variable chemical and mineralogical compositions necessitate careful assessment to enable safe and effective reuse.This study investigates the hydration behaviour and early carbonation potential of ashes of different origin. The ashes were characterized using X-ray fluorescence, X-ray diffraction, and scanning electron microscopy coupled with energy-dispersive spectroscopy. Hydraulic reactivity was evaluated by preparing ash pastes at a 1:1 water-to-ash ratio and monitoring hydration product formation over time using XRD. Carbonation of co-combustion ash was studied under controlled CO₂ conditions (2 % CO₂, 50 % RH, 20 °C ± 1 °C) at different moisture contents (0 wt.%, 10 wt.%, 20 wt.%, and 40 wt.%) and early curing times (0 hours, 1 hours, 4 hours, and 24 hours).Results indicate distinct differences among the ashes. Paper sludge ash exhibited the most extensive formation of calcium aluminate hydrates, coal and co-combustion ashes showed moderate hydration, while biomass ash produced only minor secondary phases. Carbonation of co-combustion ash proceeded concurrently with hydration, with lime depletion and calcite formation enhanced by higher moisture and longer curing. These coupled processes influenced both the kinetics and composition of hydration products. The findings demonstrate the potential of diverse combustion ashes for valorisation in construction materials and provide insight into their reactivity under early-age hydration and carbonation conditions, supporting circular economy initiatives.