Sulfate Attack Resistance of Nano-SiO2-Modified Concrete for Corroded Tunnel Linings

Tunnel lining concrete exposed to sulfate-rich groundwater is prone to powdering, spalling, cracking, and loss of load-bearing capacity during long-term service. Taking the sulfate deterioration of the Fangdoushan Tunnel as the engineering background, C30 lining concrete was modified with nano-SiO2 sol at dosages of 0, 1.0%, 2.0%, and 3.0%. A 150-day single-sided accelerated corrosion test was conducted under the combined action of 5% Na2SO4 solution, wet–dry cycles, and pressure. Mass change, compressive strength, acoustic emission, sulfate ion concentration, SEM–EDS, and XRD were used to evaluate corrosion resistance and the associated mechanisms. Both mass and compressive strength first increased and then decreased, while an appropriate nano-SiO2 dosage delayed deterioration. The 2.0% mixture (NS-2) showed the best overall performance. At 150 days, its mass change, compressive strength, and strength ratio were 0.39%, 38.3 MPa, and 1.04, compared with −0.71%, 31.7 MPa, and 0.84 for the reference mixture. NS-2 also exhibited more stable crack propagation and a faster decrease in SO42− concentration with depth, indicating restricted sulfate ingress. Microstructural analyses showed that nano-SiO2 did not alter the principal sulfate attack products, but promoted denser C–S–H formation through micro-filling and pozzolanic reactions, thereby reducing sulfate transport and the accumulation of gypsum and ettringite. Field application of 2.0% nano-SiO2-modified C30 concrete further showed good construction adaptability and improved lining surface integrity. Overall, 2.0% nano-SiO2 sol was the most effective dosage within the investigated range and provides a practical reference for sulfate-resistant design and rehabilitation of tunnel linings.