Effect of recycled concrete powder on the performance of alkali-activated slag

This study investigated the effect of recycled concrete powder (RCP) content on the hydration and hardened properties of alkali-activated slag, including hydration heat evolution, hydration products, micromorphology, pore structure, setting time and mechanical strength. The results showed that the hydration heat evolution of alkali-activated slag blended with varying RCP contents followed four stages: induction, acceleration, deceleration, and dormant. As the RCP content increased from 0% to 40%, the peak heat release was delayed from 3 to 4 hours, while the peak heat flow and total cumulative heat over 24 hours increased. Both the initial and final setting times of alkali-activated slag significantly increased with the RCP content increasing. The incorporation of RCP reduced the amount of amorphous hydration products (C-S-H/C-A-S-H) due to the dilution effect, leading to more prominent crystalline phases. The total porosity increased from 8.35% to 24.3% as the RCP content increased from 0% to 40%, while the pore size distribution remained a slight effect, with newly formed pores having diameters smaller than 20 nm. The compressive strength of mortar decreased progressively with higher RCP content; however, the compressive strength reduction remained acceptable when the content of RCP in AAS ≤20%. This study provides important evidences to the potential application of RCP in the system of alkali-activated slag.