Utilisation of calcium carbonate derived from pilot-scale indirect CO2 mineral carbonation of waste concrete: Impacts on mechanical and microstructural properties of mortar

Mineral carbon capture and utilisation provides a promising pathway for recycling waste concrete while reducing carbon dioxide emissions associated with cement production. This study investigates the mechanical and microstructural performance of mortar in which Portland cement was partially replaced with calcium carbonate produced via pilot-scale indirect carbon dioxide mineral carbonation of waste ready-mix concrete sludge. Calcium carbonate was generated using a 1000 L carbonation system and incorporated into mortar at replacement levels of 0%, 5%, 10%, and 15%. Fresh properties, including slump, slump flow, and bulk density, as well as hardened properties such as dry density, compressive strength, and flexural strength, were evaluated after 7 and 28 days of curing. Microstructural evolution was examined using scanning electron microscopy, X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and nitrogen physisorption techniques. Results show that increasing calcium carbonate content reduces workability but enhances matrix densification. At 15% replacement, compressive and flexural strengths increased by 33% and 21%, respectively, at 28 days, despite a reduction in the degree of reaction. Microstructural analyses reveal that strength enhancement is governed by a transition from hydration-controlled mechanisms at lower replacement levels to physically-controlled mechanisms at higher replacement levels, driven by improved particle packing, pore refinement, and reduced pore connectivity. Replacing 15% of Portland cement with waste-derived calcium carbonate enables the avoidance of 24.6 kg of carbon dioxide emissions per tonne of mortar. These findings demonstrate that calcium carbonate produced from pilot-scale mineral carbonation can enhance mortar performance while contributing to circular construction and carbon mitigation strategies. • Indirect carbonation enhances sustainability by recycling waste concrete into CaCO 3. • PC replacement with CaCO 3 improved mortar compressive and flexural strengths. • CaCO 3 acts as a filler, refining microstructure and reducing porosity in mortar. • XRD confirms CaCO 3 's non-involvement in crystalline phase formation of mortar. • 15% PC replacement with CaCO 3 reduces CO 2 emissions by 24.6 kg per tonne of mortar produced.