CO2 reduction pathways of the construction sector through calcium stock-flow dynamics

Despite its critical role in cement and concrete performance and CO 2 emissions, the circularity potential of calcium remains insufficiently explored due to the lack of linkage between laboratory-scale recovery and societal-scale material flow dynamics. This study quantifies calcium flows and stocks in Japan in 2020, encompassing reserves, extraction, consumption, accumulation in urban structures, waste, and recycling. Japan holds 4.6 billion tonnes of calcium reserves, primarily in limestone and dolomite, while urban stocks reached 5.5 billion tonnes. Annual calcium inflow was 60.9 million tonnes, of which 77% was used in construction. By systematically visualizing calcium dynamics, this study identified previously unquantified CO 2 -related flows at the societal scale and provided a basis for evaluating circularity and decarbonization strategies. Potential recycling strategies include cement paste reuse, carbonated concrete as supplementary cementitious material, and aggregate recycling, with hybrid approaches supporting scenario-based emission reduction and cross-sectoral resource management.