Alkali activation of construction and demolition waste for sustainable building materials

Rapid population growth and urban development have intensified construction activity, increased construction and demolition waste (CDW) generation and contributing to rise in global solid waste. In this context, the present study investigated the production of cement free alkali-activated mortars (AAMs) using CDW derived brick waste (BW) and concrete waste (CW) as the sole solid constituents. Eleven BW/CW blending ratios were prepared by increasing BW from 0% to 100% in 10% increments with CW reduced proportionally. Sodium hydroxide (NaOH) was used as the alkaline activator at 0%, 5%, 7%, and 10% (by mass). Specimens were water cured under controlled conditions for 7, 28, and 60 days. Mechanical performance was assessed using unconfined compressive strength (UCS) and non-destructive ultrasonic pulse velocity (UPV) to determine shear modulus (G₀). Microstructural characterization was conducted using scanning electron microscopy (SEM), and X-ray diffraction (XRD) to examine reaction products and matrix development. NaOH activation substantially improved both UCS and G₀ compared with untreated mixtures. The optimum performance among all tested mixtures was achieved by the 90BW/10CW blend in the 10% NaOH series at 60 days, reaching a maximum UCS of 28.7 MPa and G₀ of 30.4 GPa. Across activated mixes, UCS and G₀ increased with BW content up to approximately 90BW, followed by a decrease at 100BW, consistent with insufficient calcium availability for effective gel development. Analysis of variance (ANOVA) identified NaOH dosage as the most influential factor, followed by BW/CW blending ratio and curing age, and SEM indicated a denser, more homogeneous matrix in activated mixes. Overall, the results demonstrate that optimized BW/CW combined with NaOH activation can produce high performance cement free mortars from CDW, supporting circular construction practices. Future research should focus on long term durability, and life cycle analysis to quantify the environmental benefits and tradeoffs associated with large scale implementation.