Hydration characteristics of cement blended with thermally reactivated recycled concrete demolition waste

Massive amounts of concrete demolition waste are generated due to the demolition of old concrete buildings and the rapid expansion of urban infrastructure, posing environmental and resource recovery challenges. This study investigates the hydration behavior of cement pastes blended with concrete demolition waste (CDW) that has been thermally activated at temperatures ranging from 500 to 900 °C. Thermal activation alters the physicochemical nature of CDW, with progressive decomposition of hydrates and carbonates leading to the formation of reactive CaO and amorphous silica above 700 °C. FTIR and XRD analyses reveal that CDW activated at 500 °C acts as a mild pozzolan, delaying early hydration but enhancing C-S-H formation and matrix densification over time. Increasing activated CDW content enhances pozzolanic reactivity. At 700 °C, activation improves silica and alumina availability, promoting denser C-S-H networks, while activation at 900 °C increases particle fineness but yields less gel volume due to phase crystallinity. SEM observations confirm that low to moderate activated CDW replacement levels (10-30%), particularly at 500-700 °C, can refine pore structure and improve long-term matrix performance. Optimal mechanical and durability properties are achieved at 20-30% CDW substitution. These results confirm that thermal activation enhances the pozzolanic potential of CDW, enabling its use as a sustainable supplementary cementitious material in blended systems.