Comparative Assessment of Binary Alkali-Activated Mortars Incorporating Construction and Demolition Waste under Dry and CO₂ Curing Conditions

Abstract

[Abstract]: Construction and demolition waste (CDW) has attracted increasing attention as a potential precursor for alkali-activated materials, although its heterogeneous nature often limits mechanical performance. This study investigates the behaviour of binary alkali-activated mortars manufactured with different types of CDW: recycled concrete (RC), recycled brick (RB) and recycled tile (RT). The combination of these precursors with 30% fly ash (FA) as a supplementary precursor was also investigated. The mixes were subjected to two curing conditions—dry and CO₂—in order to evaluate the influence of precursor mineralogy and curing regime on their mechanical and physical performance. Compressive and flexural strengths, dynamic modulus of elasticity, ultrasonic pulse velocity (UPV), porosity, shrinkage and carbonation depth were determined. The results showed that precursor reactivity governs the overall behaviour. Tile-based systems (RT) achieved the highest strengths and lowest water-accessible porosity, followed by RB, whereas RC exhibited limited reactivity. Under CO₂ curing, RT70 reached 20.3 MPa at 28 days, representing a 53.9% increase compared to dry curing. In addition, CO₂ curing reduced water-accessible porosity by up to 15%. For a comparable compressive strength, CO₂-cured mixes exhibited higher flexural strength and dynamic modulus of elasticity than those cured under dry conditions. In contrast, RC systems showed a 21.5% reduction in compressive strength at 28 days under CO₂ curing, indicating that early carbonation may adversely affect low reactivity, calcium-rich residues. The incorporation of 30% FA mitigated this effect, increasing the 28-day compressive strength under CO₂ curing from 7.7 MPa (RC100) to 14.3 MPa (RC70). Correlation analysis confirmed an exponential relationship between mechanical performance and water-accessible porosity, highlighting the governing role of pore structure. Overall, the combined use of FA and controlled CO₂ curing offers a viable strategy for enhancing selected CDW-based alkali-activated systems.