Comparative research on the application of slag as an alternative to cement in binder-bentonite cutoff wall backfills

Abstract

Slag, as a kind of industrial by-product, is a recognized substitute for cement to reduce carbon emissions and costs in engineering construction. This paper compares the differences in the performance of sand-sodium silicate-activated slag-bentonite (S-SM-S-B) and sand-cement-bentonite (S-C-B) cutoff wall backfills to investigate the feasibility of S-SM-S-B backfills for practical application. Engineering properties are tested by the hydraulic conductivity (k) test and unconfined compressive strength (UCS) test. The results show that compared with S-C-B backfills, S-SM-S-B backfills with high binder content have the characteristics of higher strength, faster curing rate and better impermeability. The S-SM-S-B backfills are hence more cost-effective for the sites with complex in-situ stress conditions. The strength of S-SM-S-B backfills with less than 5% ground granulated blast-furnace slag (GGBS) dosage is below the commonly suggested limit (100 kPa) and should be avoided in applications. In addition, the damage extent for bentonite particles during hydration of binders was evaluated using the methylene blue value test. Methylene blue values of S-SM-S-B backfills are larger than that of S-C-B backfills, which means that bentonite is less affected in S-SM-S-B backfills. It is hence more conducive to perform the adsorption capacity of added bentonite. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) experiments were carried out to determine the reaction products and microscopic properties of the samples, and the results can verify the above conclusions. At last, the interaction process among raw materials and the control factors for backfill properties are explained based on the double layer theory and hydration reaction mechanism.