Synergistic Modification of Cement-Based Materials with Nano-ZnO and Nano-ZrO2 Under Carbonation Mixing: A Response Surface Methodology Study

Against the backdrop of global climate change and the urgent need to decarbonize the construction sector, cement production remains a major contributor to anthropogenic CO2 emissions, accounting for over 7% of the global total. Carbonation mixing has emerged as a promising green technology that integrates CO2 sequestration with performance enhancement of cement-based materials. However, the combined effects of carbonation mixing and binary nano-oxide modification on the multi-performance attributes of cement mortars have not been systematically explored. This study aims to address this gap by investigating the synergistic modification of cement-based materials with nano-zinc oxide (nano-ZnO) and nano-zirconia (nano-ZrO2) under carbonation mixing conditions. The results indicate that the quadratic polynomial models exhibit good-to-excellent goodness-of-fit, with the 28-day saturated water absorption model achieving an R2 of 0.9650 and an adequate precision of 16.47, confirming reliable predictive capability. Compressive strength ranged from 62.29 to 116.45 MPa, representing a 90.05% increase in the lower limit and a 22.37% increase in the upper limit relative to the reference group. Nano-ZnO generally reduced early-age strength due to its retarding effect, while nano-ZrO2 exhibited a continuous strengthening effect across the 0–10% dosage range. The optimal synergistic range for early-age strength was identified as 0–0.2% nano-ZnO and 0–2% nano-ZrO2, whereas nano-ZrO2 dominated long-term performance enhancement. Saturated water absorption decreased significantly from 3 to 28 days, reflecting progressive pore refinement through continued hydration and carbonation product filling. Autogenous shrinkage showed a non-monotonic trend with nano-ZnO content—initially increasing then decreasing—while drying shrinkage increased predominantly with nano-ZrO2 dosage, attributed to increased capillary tension resulting from pore structure refinement. The carbonation environment accelerates CO2 diffusion and reaction, generating stable calcium carbonate that partially fills microcracks and further refines the pore structure, which provides a viable technical pathway for developing low-carbon, high-performance cement-based composites, with the RSM-based optimization framework offering an optimization tool for tailored mix design in applications such as high-strength concrete, repair mortars, and prefabricated elements where early strength and dimensional stability are critical.