Carbonation Behaviors of Recycled Concrete Fines Containing Supplementary Cementitious Materials: Compositions, Microstructures, and Mechanisms

Waste concrete has been recycled and crushed to produce sustainable alternatives to natural aggregate, and accelerated carbonation has been applied to improve the qualities of crushed waste concrete aiming at promoting their reutilization in new concrete production. However, investigations into the impact of carbonation on alternative aggregates derived from blended concrete containing supplementary cementitious materials (SCMs) have been scarce. Therefore, in this study, recycled concrete fines (RCFs), the fine fractions of crushed waste concrete with particle sizes between 0.3 and 2.36 mm, were investigated for their responses toward carbonation. First, it was found that RCFs containing low-calcium SCMs particularly fly ash had a higher carbonation rate than plain RCFs. Meanwhile, RCFs comprising calcium-rich SCMs such as slag exhibited an opposite trend. Second, RCFs attained less microstructural refinement after carbonation due to the presence of SCMs, and adverse impacts such as pore coarsening may even take place. Third, RCFs exhibited different surface morphologies after carbonation. Because vaterite formation was favored in the presence of fly ash, the size of calcite grains was restrained by silica fume, and vesuvianite formed when slag or fly ash that contained rich alumina was present. After decalcification and polymerization, different pozzolanicities were developed. The RCFs with the pure silica-based SCM released the maximum amount of heat while reacting with portlandite, suggesting the highest reactivity. Overall, the behaviors of RCFs toward carbonation were both SCM- and dosage-dependent, and they were intrinsically associated with the initial amount of portlandite, the Ca/Si ratios of C-S-H, the presence of aluminate hydrates, as well as the compactness of the initial microstructures. The understanding gained from this study revealed the carbonation behaviors of RCFs derived from different sources of waste concrete, which is helpful for the recycling of RCFs and the sustainability of the construction industry.