Effect of Nano-TiO₂ Dioxide on the Hydration Process of Tunnel Construction in Low-Temperature Environments

To address winter construction challenges such as slow early-stage strength development, inhibited hydration processes, and pore structure defects in concrete under low-temperature conditions, this study employs nano-TiO₂ as a modifying agent. It is incorporated into concrete through cement replacement methods; the study systematically investigates the influence of different admixture dosages (1%, 2%, 3%, by cement mass) on the mechanical properties, hydration process, and micro-pore structure of concrete. The test employed an electro-hydraulic servo universal testing machine to measure compressive and splitting tensile strengths. Differential thermal analysis (DTA) characterized the formation of hydration products (Ca(OH)₂). Micro-CT technology and pore network modeling were utilized to quantify micro-pore parameters. Results indicate that (1) nano-TiO₂ regulates the setting time of pure paste, with increased dosage shortening both initial and final setting times. At a 3% dosage, initial setting time plummeted from 5.5 min in the control group to 3.3 min; (2) nano-TiO₂ significantly enhances early-age (1–3 days) strength of low-temperature concrete, with optimal effect at 1% dosage. Compressive strength and splitting tensile strength at 1 day increased significantly by 20% and 26%, respectively, compared to the control group. Strength differences among groups gradually narrowed at 28 days; (3) DTA indicates that nano-TiO₂ accelerates early cement hydration; (4) micro-CT results show that the 1% dosage group exhibits significantly reduced porosity at day 1 compared to the control group, with notable decreases in Grade 0 and Grade 1 interconnected porosity resulting in the most optimal pore structure density. In summary, the optimal dosage of nano-TiO₂ in low-temperature environments is 1% by mass of cement. Through the synergistic “nucleation-filling effect,” it promotes early-stage hydration and optimizes pore structure, providing technical support for winter concrete construction.