Utilizing CO2 to improve plastic shrinkage and mechanical properties of 3D printed mortar made with recycled fine aggregates

Reducing carbon footprint in the construction industry by leveraging construction waste aligns well with the concept of sustainable development in this sector. In this study, recycled fine aggregates (RFA) as the raw material for 3D printed mortar (3DPM), and devised three carbonation techniques: elevated temperature accelerated carbonation of RFA, injecting CO2 as an admixture for the fresh 3DPM mixing (CO2-injecting), and carbonation curing of 3DPM, to improve properties of 3DPM. The study employed the digital image correlation method to evaluate plastic shrinkage of 3DPM specimens, and compared their mechanical properties. Additionally, SEM-EDS results were integrated to analyze the mechanisms of improvement associated with the carbonation methods. The findings indicated that using RFA resulted in a minor reduction in the plastic shrinkage of 3DPM. After carbonation treatments, the plastic shrinkage of mortar exhibited a significant decrease, while alleviating in moisture loss of 3DPM specimens. In the horizontal direction, all specimens displayed a "trapezoidal compression", while a "slope settling" behavior was exhibited in the vertical direction. The strength of the casting specimens experienced the most substantial improvement. Furthermore, the interlayer structure of 3DPM was enhanced by carbonation treatments, resulting in the refinement of interlayer microcracks and a strengthening of interlayer bonding.