Japan’s Cities as “Massive Calcium Reservoirs”: Visualizing Resource Circulation for Strategic Concrete Recycling and CO₂ Reduction

Calcium, as a resource, is often stored in stable forms bound to CO₂, such as in limestone. Despite its critical role in industrial processes and CO₂ emissions, its circularity potential has not been systematically examined. In particular, the linkage between laboratory-scale recovery technologies and society-wide material flow dynamics has not been sufficiently explored.

A research team has quantified calcium flows and stocks in Japan for 2020, covering the entire lifecycle from raw material extraction and production/consumption of calcium-containing materials to accumulation in urban structures, as well as waste generation and recycling. The study revealed that Japan contains approximately 4.6 billion tonnes of calcium in limestone and dolomite reserves, and about 5.5 billion tonnes accumulated in urban structures. Annual calcium input was estimated at 60.9 million tonnes, of which 77% was used in construction. By visualizing calcium dynamics, the study identified previously unquantified CO₂-related calcium flows and provides a quantitative foundation for circular economy and decarbonization strategies. Hybrid recycling approaches that integrate cross-sectoral and multi-stage resource recovery may significantly contribute to CO₂ reduction and sustainable resource management.

This research was conducted collaboratively by Assistant Professor Naho Yamashita, Professor Ippei Maruyama, Professor Shinsuke Murakami, and Mr. Kenji Nabeshima of the Graduate School of Engineering, The University of Tokyo, together with Dr. Yuya Yoda and Dr. Keiichi Yano of Shimizu Corporation.

Simplified conceptual diagram of calcium flows and stocks identified in this study

Papers

Journal: Resources, Conservation & Recycling Advances

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

Authors: Naho Yamashita, Shinsuke Murakami, Keiichi Yano, Yuya Yoda, Kenji Nabeshima, Ippei Maruyama

DOI: 10.1016/j.rcradv.2026.200322