Effects of nano-TiC and nano-CaCO3 on steel fiber-reinforced cementitious mortar: a study on mechanical properties and freeze-thaw performance

Steel fiber (SF) cement mortar, renowned for its dual advantages of strength and toughness, is widely used in construction, transportation, and other engineering fields. However, in actual service, it often faces challenges, such as weak bonding at the SF-cement matrix interface and insufficient freeze resistance in severe cold environments, which compromise the long-term durability of engineering structures. To optimize the performance of SF cement mortar, a composite cement mortar (TS) was developed. This study investigates the effects of single-blending nano-TiC(NT), nano-CaCO3 (NC), and their combined blending on the mechanical properties (flexural and compressive strength) and freeze-thaw resistance (mass and strength loss rates under freeze-thaw cycles) of the mortar. Analysis of variance was employed to examine the interactions among materials. Scanning Electron Microscope (SEM) and Energy Dispersive Spectroscopy (EDS) were also used to examine the microstructure of the mortar. Results indicate that the TS3 group with sole NT addition demonstrated stable mechanical property enhancement, achieving 28-day flexural and compressive strengths of 11.79 MPa and 38.21 MPa, respectively—representing 10.7% and 20.0% increases over the control group. The sole NC addition group exhibited significant performance fluctuations, while the TS6 group showed approximately 10% strength improvement at 28 days. Some groups experienced strength degradation due to NC agglomeration. Among the mixed-blended groups, the TS12 group exhibited the optimal “hydration-filling” synergistic effect, achieving a 28-day compressive strength of 38.89 MPa. Agglomeration occurred in most groups due to mismatched nanomaterial dosage or dispersion, resulting in strength reductions exceeding 14%. Under freeze-thaw cycles, the TS12 group demonstrated the best freeze resistance, with a compressive strength loss rate of 9.2% after 100 cycles. The TS3 group (single NT addition) and the TS6 group (single NC addition) also outperformed the control group, both suppressing freeze-thaw damage through optimized pore structure. The two-way ANOVA revealed that NT, NC, and their interaction exerted a highly significant influence on both the flexural strength and compressive strength of steel fiber cement mortar at 7-day and 28-day ages. SEM analysis revealed compact microstructures across all groups. EDS characterization results indicate that the elemental features in cement mortars with different additive combinations exhibit significant differences.