Performance Evaluation of Alkali-Activated Slag Cement for Preventing Crystallization Damage in Tunnel Linings

The persistent leaching of calcium ions from conventional shotcrete linings is a primary cause of crystalline blockage in tunnel drainage systems. This phenomenon, driven by the long-term hydration of ordinary Portland cement (OPC), leads to the precipitation of detrimental deposits that compromise structural integrity and operational safety. To address this issue at its source, this study investigates the use of low-calcium alkali-activated slag (AAS) as a sustainable alternative. A comprehensive evaluation was conducted, encompassing leaching behavior (pH and Ca²⁺ concentration), mechanical strength, and autogenous shrinkage. Microstructural evolution was characterized through XRD, SEM, solid-state NMR, and MIP. The results reveal that while the leachate pH of both OPC and AAS eventually stabilized after an initial decline, the AAS system exhibited a more rapid and pronounced decrease due to its lack of a portlandite buffer. Critically, ²⁹Si solid-state NMR spectroscopy confirmed a fundamental structural distinction: the AAS matrix comprises a highly cross-linked network dominated by Q³ and Q⁴ silicate units, characteristic of a geopolymeric gel, in contrast to the linear C-S-H (Q¹/Q²) chains predominant in OPC. This atomic-scale difference underpins the drastically reduced availability of free calcium, with AAS leachate Ca²⁺ concentrations remaining below 6.68 mg/L. Consequently, AAS mortar demonstrated superior mechanical performance, achieving a later-age compressive strength more than double that of OPC, a direct consequence of its robust, polymerized microstructure. However, this refined microstructure also contributed to higher wet-dry cycling shrinkage, reaching 1.4 times that of OPC, necessitating future strategies for shrinkage compensation. Mineralogical analysis confirmed the absence of portlandite and ettringite in AAS, with its reaction products consisting of low-alkali C-(A)-S-H gel and hydrotalcite. Pore structure analysis further confirmed a significant refinement in AAS, featuring a denser and more concentrated pore size distribution.