Vertical Cutoff Walls with Novel Alkali-Activated Slag–Fly Ash Backfills: Hydraulic and Mechanical Behavior

Vertical cutoff walls are widely used as in situ barriers for the containment of contaminated water migration in the subsurface and groundwater. Conventional materials, such as soil–bentonite and soil–cement–bentonite backfill, exhibit the inherent swelling and shrinkage behavior of bentonite, as well as low strength and chemical incompatibility. These characteristics can lead to the development of preferential flow paths, which in turn can reduce long-term containment performance. Hence, in the present study, a novel alkali-activated slag–fly ash (AASF) backfill material was developed for application in the vertical cutoff walls. The workability, hydraulic conductivity, and unconfined compressive strength (UCS) tests of various mixes were evaluated to determine the optimum mix proportions. Further, the hydration products and microstructural development were characterized to elucidate the reaction mechanism underlying the hydraulic and mechanical properties. From the results, the optimum mix proportion is identified to be a mix of 1 ∶ 2 fly ash to ground granulated blast-furnace slag (GGBS) with an alkali activator dosage of M1.6N8, having the lowest hydraulic conductivity of 1.3 × 10 − 10 m / s . For a fly ash–to-GGBS ratio of 1 ∶ 2 , with increasing the activator dosage from M1.2N4 to M1.6N8, the hydraulic conductivity is decreased from 1.24 × 10 − 8 to 1.3 × 10 − 10 m / s , and UCS is increased from 10.31 to 28.5 MPa. The highest unconfined compressive strength of 33.28 MPa was observed for the binder with only GGBS and an alkali activator dosage of M1.6N8 at 28 days. From microstructural characterization, a binder ratio of fly ash to GGBS of 1 ∶ 2 , with a modulus of 1.6 and an 8% Na 2 O content, has resulted in the formation of an aluminosilicate gel, which contributes to reduced pore structure and enhanced matric densification.