Sulfate Resistance of Novel Alkali-Activated Filling Grout for Shield Tunnels: Comparison with Typical OPC-Based Filling Grout

The novel alkali-activated filling grout (AAG) for shield tunnels requires detailed understanding of its sulfate resistance prior to practical application. This paper presents a comparative study of the sulfate resistance of a typical ordinary portland cement (OPC)-based grout and two groups of representative AAG formulated with industrial by-products including fly ash, ground granulated blast-furnace slag, and steel slag as the solid precursors. In the study, the grout samples after 28 days of standard curing were exposed to the 5% by weight Na2SO4 (pH = 7.92) and 5% by weight MgSO4 (pH = 7.25) solutions, respectively, to simulate different cases of external sulfate erosion. The sulfate resistance and deterioration mechanism of various grout samples were evaluated and analyzed via multitechnical examinations including visual appearance, mass loss, compressive/flexural strength, strength retention coefficient, mineral composition, microstructure morphology, and pore-size distribution at different test ages. The experimental results indicated that whether under Na2SO4 or MgSO4 erosion, AAG always exhibited more slight deterioration than the OPC-based grout in terms of visual appearance, mass loss, and strength reduction. The reasons for these observations are as follows. The main gel products of AAG were mainly C-(A)-S-H and N-A-S-H, which interweaved and coexisted to form a denser matrix structure than that of the OPC-based grout. The resulting poorer permeability of the AAG matrix was not conducive to the ion exchange between the matrix and the environmental solution. In addition, compared with the OPC-based grout, the lower calcium content and more stable bonded aluminum phases in AGG limited the formation of expansive products responsible for microcracking under sulfate erosion. MgSO4 could cause more severe deterioration than Na2SO4, mainly due to the promotion of decalcification of calcium-containing phases by the presence of Mg2+ and the resulting gel products harmful to the matrix. This study demonstrated the excellent sulfate resistance of AAG, which can provide scientific basis for its future engineering application.