Mechanical-Low Carbon Synergy in Nano-CaCO₃/Ultrafine Cement Grouting Composites: Mechanisms and Study

To address the issues of low mechanical strength, long setting time, and high carbon emissions associated with cementitious grouts in deep roadway grouting projects, a high-performance, low-carbon nano-calcium carbonate (NC) modified superfine cement grouting material (SCGM) was developed. By compounding varying amounts of NC, superfine cement, and multi-component additives (water reducer, expansion agent, and accelerator), the effects of NC content on the material's workability, mechanical strength, microstructure, and full-length anchoring characteristics were systematically investigated using isothermal calorimetry, XRD, SEM, and FTIR. A Pearson correlation coefficient matrix for key performance indicators was constructed, and its life-cycle carbon emissions were assessed. The results showed that the fluidity, bleeding rate, and setting time of the SCGM slurry exhibited a negative correlation with NC content, while the expansion rate and mechanical strength of the hardened grout first increased and then decreased with increasing NC content. At an NC content of 3%, SCGM exhibited the optimal comprehensive performance, with an initial setting time reduced by 38.4%, a bleeding rate decreased by 75%, and 7 d and 28 d compressive strengths increased by 57% and 42%, respectively, compared to the reference group. Microstructural analysis revealed that the nucleation effect of NC in the superfine cement system provided additional nucleation sites, promoting the formation of C-S-H gel and participating in the formation of Mc, thereby influencing the hydration product formation process, effectively refining the pores of SCGM, and enhancing matrix densification. In full-length anchorage pull-out tests, the system modified with 3% NC exhibited increases of 49.5% in peak load and 125% in corresponding displacement, demonstrating enhanced mechanical load-bearing capacity and deformation resistance. This superior performance was primarily attributed to the synergistic "physical filling-chemical promotion-structural enhancement" mechanism of NC in the superfine cement system. Life-cycle carbon emission accounting revealed that NC, produced via a CO₂ mineralization process, exhibited net negative carbon characteristics, reducing the CO₂ emissions per unit volume of SCGM3 by 4.7% and the carbon emission intensity per unit strength by 31.8%, demonstrating an excellent synergy between mechanical performance enhancement and carbon emission reduction.