Waste concrete powder (WCP), derived from construction and demolition waste, often remains unrecycled due to its fine particle size. However, the small particle size and high calcium hydroxide content of WCP offer potential for efficient carbon uptake. This study investigates carbonation of WCP for CO 2 sequestration and use of carbonated WCP together with fly ash (FA) to produce alkali-activated materials (AAM). The results indicate that one ton of WCP can sequester 84.50 kg of CO₂ after 56 h’ carbonation. The carbonation of WCP reduces chemically bound water and promotes the formation of poorly crystalline calcium carbonate. More importantly, it provides additional sites through the precipitation of fine carbonates, which in turn enhances the generation of C–N–A–S–H gel. XRD and FTIR analyses of AAM show increased calcite crystallinity and peak intensity with more WCP incoporated and higher WCP carbonation degrees. The mechanical performance is enhanced by incorporating up to 40% WCP subjected to relatively mild carbonation. However, incorporating excessive amounts of highly carbonated WCP may hinder the formation of C–N–A–S–H gel, because carbonation converts Ca(OH)₂ in WCP into CaCO₃, thereby reducing the amount of soluble Ca 2 ⁺ available to participate geopolymeric gel formation, which negatively impacts the mechanical properties of AAM. SEM–EDS analysis confirms this by showing a decrease in the Ca/Si ratio of C–N–A–S–H gel and essentially unchanged Al/Si and Na/Si ratios for the AAM containing a high content of highly carbonated WCP. The findings from this study will promote complete recycling of waste concrete.
Zeng et al. (Wed,) studied this question.