Effect of the CO2 curing on a certain recycled concrete with the mineral-added binary and ternary cementitious systems

Demolition waste and cement production account for 36 % of the total waste generated on earth and 8 % of the world’s CO₂ emissions, respectively, while considering that studies on CO₂ curing of recycled concrete (RC) containing ternary cementitious systems (fly ash (FA) and slag) are limited. In this paper, the effects of adding FA and slag as binary and ternary cementitious systems on the unit carbon sequestration, carbon sequestration rate, compressive strength and microstructure of RC under CO₂ curing were investigated. Finally, the overall benefit is evaluated by combining three indicators: compressive strength, CO₂ emission and cost. The results showed that the unit carbon sequestration of RC with binary cementitious systems peaked at 45 % mineral replacement, reaching 16.69 g/kg (FA + cement) and 14.46 g/kg (slag + cement), respectively, while compressive strength peaked at 30 % mineral replacement. At the same replacement rate, the unit carbon sequestration (17.56 g/kg) and compressive strength of ternary cementitious system RC were superior to those of binary systems when FA and slag were incorporated at ratios of 7.5 % and 37.5 %, respectively. From a microstructural perspective, after CO₂ curing, the addition of FA does not significantly affect the calcite diffraction peak intensity in RC, while the calcite diffraction peak in slag RC is significantly enhanced. In ternary cementitious system RC, the calcite diffraction peak no longer shows significant changes once the FA content reaches a certain level. In addition, the ternary cementitious system offers better overall benefits in terms of CO₂ emissions, cost, and compressive strength.