Alkali-Activated Mortar for Tunnel-Lining Structure Repair

With the aging of tunnel structures, rehabilitation and repair has become an increasingly important part of tunnel maintenance. This paper investigates the use of alkali-activated mortar for tunnel-lining structure repair. First, the effects of NaOH concentration and ordinary portland cement (OPC) content on the fresh and hardened states of alkali-activated repair mortar (ARM) were studied by investigating its setting time and compressive strength. Second, the bond strength of ARM made from the optimum mix proportion was compared with that of cement repair mortar (CRM) using a self-design tunnel-lining-crack-treatment platform (TLCTP). Finally, scanning electron microscopy (SEM) and X-ray diffraction (XRD) analyses were undertaken to study the morphology, mineral composition, and hydration products of ARM and CRM. It is found that the setting times of ARM are greatly shortened and its compressive strength is noticeably increased with increasing OPC content. Increasing the NaOH concentration from 10 to 12 M induces decrease in setting times and increase in compressive strength of ARM, but further increasing NaOH concentration to 14 M results in a slight increase in setting times and decrease in the compressive strength of ARM. Bond strength test results show that ARM made from an optimum mix proportion exhibits a bond strength superior to that of CRM in dry curing conditions. Although the presence of water has a negative effect on bond strength, ARM still shows better bond strength than CRM. XRD characterization indicates that ARM consists of sodium aluminosilicate hydrate (NASH) gel and calcium silicate hydrate (CSH) gel that are responsible for increasing its strength. SEM characterization reveals that ARM has a dense structure with voids filled with cementing agents, whereas the CRM shows a rough structure with small cracks and unfilled pores. The dense microstructure benefits the reduction in crack propagation and water absorption, leading to high compressive and bond strengths.