Influence of CNC modification and CO2 curing on the macroscopic, microstructural, and durability properties of foamed concrete grouts

CNC–CO 2 curing shows a clear synergistic effect. • Optimum at 0.1 wt% CNC and 24 h carbonation. • Strength increases by 44.7% (compressive) and 38.5% (flexural). • Improved durability and CO 2 sequestration capacity. This study aims to improve the inadequate mechanical strength and limited durability of foamed concrete grouting materials. Mixtures with varying cellulose nanocrystals (CNC) dosages and carbonation durations were prepared, and the relationships between macroscopic performance and microstructural evolution were elucidated through multiscale characterization. Durability was further evaluated via wet–dry cycling in a 3.5% NaCl solution. The results indicate that the incorporation of CNC increases the flow time by 2.2%–15.1%, while remaining within acceptable construction limits at dosages below 0.1 wt%. Carbonation reduces the 24 h water absorption by up to 2.78% relative to the control group, while significantly slowing the water release process. Optimal performance was achieved at 0.1 wt% CNC with 24 h carbonation, where the 28 d compressive and flexural strengths increased by 44.7% and 38.5%, respectively. After 120 saline wet–dry cycles, strength retention exceeded 80%, markedly higher than the control group (64.38%). Microstructural analysis revealed that carbonation promoted continuous CaCO 3 precipitation, resulting in a reduction in average pore size and total porosity, and transforming the pore structure from interconnected macropores to a finer and denser configuration. However, excessive CNC dosage or prolonged carbonation reduces compressive strength by approximately 9.6% compared to the optimum and accelerates long-term deterioration. Overall, CNC modification combined with CO 2 curing exhibits a clear synergistic effect, characterized by a distinct threshold behavior. Proper parameter control enabled simultaneous enhancement of mechanical performance, moisture resistance, durability, and CO 2 sequestration potential.