Cold-sintered Carbonated Concrete Waste Fines: A Calcium Carbonate Concrete Block

Cementitious materials generally have large carbon footprints because of the high CO₂ emitted during Portland cement production. This is because limestone is used as an essential CaO resource, and its decomposition by calcination emits CO₂. From this perspective, the concrete in urban buildings can be considered an urban mine of CaO resources. In this study, we propose obtaining a solidified product by crushing all the waste concrete, carbonating it, pressurizing it with a calcium bicarbonate solution, and drying it. The experimental results show that the bicarbonate solution, high-temperature conditions, and extended loading period produce a higher strength. In addition, neck growth at the contact surfaces of the carbonated concrete fines was confirmed using scanning electron microscopy. Consequently, the proposed method indicates that the hardening mechanism is the cold sintering of calcium carbonate on the surface of fine-carbonated concrete particles. This method allows the developed blocks to be used semi-permanently with relatively low energy consumption through repeated crushing and re-pressurization.