As the critical weak link in grouting reinforcement systems, the interfacial adhesion between cementitious grout and rock minerals is highly susceptible to performance degradation under high-temperature and water-rich conditions. In this paper, molecular dynamics simulations were performed across a temperature range of 293 K to 368 K to systematically investigate the effects of high-temperature and water-rich environments on the mechanical response, bonding structure, and dynamic behavior of the grout–rock interface. All simulations were performed using the LAMMPS package with the ClayFF force field. Two interface models, including a CSH/SiO₂ direct-contact model and a CSH/H₂O/SiO₂ water-containing model, were constructed and subjected to uniaxial tensile tests. Key findings are as follows: (i) The tensile strength and interaction energy of the CSH/SiO₂ interface exhibit distinct thermal degradation characteristics. The tensile strength decreases by 32.57%, and the interaction energy by 15.78% when the temperature rises from 293 K to 368 K. High temperatures induce expansion of the interface transition zone from 2.74 Å to 4.60 Å and loosening of the interface structure. (ii) High temperatures intensify atomic diffusion at the interface. The number and stability of Ca-O bonds and hydrogen bonds formed between CSH and SiO₂ are reduced, leading to a decline in interfacial adhesion. (iii) The presence of an interfacial water layer significantly impairs the tensile strength and interaction energy of the interface. Compared with the direct-contact interface, the interaction energy is reduced by 38% at 293 K, and the tensile strength decreases by 73.58%. Water molecules in the solution compete for bonding sites of hydrogen bonds and Ca-O bonds at the interface, weakening the direct interaction between CSH and SiO₂ and transforming it into an indirect interaction mediated by water molecules.

Cement slurry is a staple grouting agent, yet its properties can weaken when exposed to heat. Studying grouting materials for use in high-temperature tunnels is therefore a matter of considerable importance. To enhance the applicability of cement-based slurry in high-temperature tunnels, multi-walled carbon nanotubes (MWCNTs), polycarboxylate ether superplasticizer (PCE), and hydroxypropyl methylcellulose (HPMC) were added to improve their performance at elevated temperatures. Various experimental methods were employed to investigate the properties of the modified slurry at different temperatures, including flowability, setting time, compressive strength, and dynamic water retention ratio. Additionally, X-ray diffraction (XRD), thermogravimetric analysis (TG), and scanning electron microscopy (SEM) were used to study the effects of temperature on hardened slurry. Experimental results indicate that the optimal MWCNTs content is 0.32%. At this content, the compressive strength of the hardened slurry after 28 days of curing at 80 °C increases by approximately 20%, reaching 26.4 MPa. PCE improves the fluidity of the slurry, while HPMC enhances its water dynamic water retention ratio. The optimal proportion was found to be 0.3% PCE and 0.2% HPMC. At this ratio, the fluidity of the slurry increased by about 8%, reaching approximately 17.7 cm; the dynamic water retention ratios of 0.8 m/s and 1.0 m/s improved by approximately 22% and 38%, respectively, achieving 35.8% and 18.1%. Furthermore, multi-walled carbon nanotubes significantly enhance the compressive strength of the hardened slurry primarily by suppressing the formation of ettringite during the later stages of hydration, as well as by providing nucleation sites, encapsulating hydration products, and bridging hydration product clusters within the microstructure. This investigation lays a theoretical groundwork for formulating and choosing grouting materials suited to high-temperature tunnel environments.

This study employs molecular dynamics simulations to construct a CSH/SiO₂ nanochannel model, systematically investigating the dynamic transport behavior of pure water and Na₂SO₄ solution within the pores across temperatures ranging from 293 K to 368 K. The results elucidate the fluid intrusion mechanism under the combined effects of elevated temperature and sulfate ions. Results show that: (i) Water migration within the CSH/SiO₂ channel exhibits wall-dependent differences. The interaction between water and the CSH surface is approximately eight times stronger than with the SiO₂ side at 293 K, causing water molecules to preferentially advance along the CSH side. As the penetration depth difference between the two sides increases, local water molecules extend toward the SiO₂ side, forming liquid finger-like protrusions that lead to rapid filling on the SiO₂ side. (ii) Elevated temperature enhances water molecule mobility; at 368 K, the mean square displacement of water in the pure water system is about 3.7 times greater than at 293 K, and the critical time for liquid finger formation on the SiO₂ side decreases from 700 ps at 293 K to 100 ps at 368 K. (iii) The introduction of Na₂SO₄ solution alters the transport mechanism within the nanochannel. Na⁺ and SO₄²⁻ form ionic coordination structures with oxygen and calcium sites on the CSH surface, establishing Na-O_CSH and S-Ca_CSH ionic bonds, which reduces the solution’s transport rate. Na⁺ and SO₄²⁻ adsorb and accumulate near the CSH/SiO₂ interface; heating weakens their hydration shells and strengthens the ion association between Na⁺ and SO₄²⁻, promoting the formation of local ion clusters.