Strength optimization and mechanism of carbon-negative mortars incorporating CaO-activated binders and chemically-strengthened carbonated recycled fine aggregates

To advance decarbonization in construction, this study develops and optimizes a novel carbon-negative mortar (CNM) by fully replacing ordinary Portland cement with a CaO-activated low-carbon binder and incorporating chemically strengthened carbonated recycled fine aggregates (CRFA). An L9 orthogonal design was employed to evaluate the impact of water/binder ratio (W/B), sand/binder ratio (S/B), and three CRFA strengthening treatments (sodium silicate, nano-SiO₂, and silane) on compressive strength. The optimal mix (W/B = 0.40, S/B = 1.50, and sodium silicate-treated CRFA) achieved a 28-day compressive strength of 48.6 MPa, exceeding that of all trial mixtures. SEM–EDS analysis revealed that chemical strengthening treatments narrowed the interfacial transition zone (ITZ) thickness from about 7∼17 μm to about 4∼7 μm. X-ray computed tomography (X-CT) further confirmed enhanced pore structure, with total porosity reduced by over 50% and harmful pore fraction decreased from 2.01% to 0.97%. These microstructural improvements directly correspond to strength gains. The findings demonstrate a synergistic mechanism between mix design optimization and pore/ITZ refinement, offering a feasible approach to achieve high-performance, carbon-negative mortars aligned with the "dual-carbon" goals. • A novel carbon-negative mortar was developed using CaO-activated binders and carbonated recycled fine aggregates (CRFA). • Orthogonal experimental design identified W/B ratio and CRFA treatment as key strength-determining factors. • Sodium silicate–treated CRFA significantly refined the interfacial transition zone (ITZ) and improved bond strength. • X-ray computed tomography (X-CT) revealed over 50% reduction in total porosity and harmful pore fraction. • The optimized mortar achieved 48.6 MPa compressive strength with simultaneous CO₂ sequestration and low emissions.