The engineering properties and reaction mechanism of MgO-activated slag cement-clayey sand-bentonite (MSB) cutoff wall backfills

An innovative cutoff wall backfill consisting of reactive MgO, ground granulated blast furnace slag (GGBS), bentonite and local clayey sand (MSB) was developed recently for land remediation applications. This paper investigates the engineering characteristics (e.g. strength and permeability) and reaction mechanisms of the MSB backfills with various MgO-activated GGBS (i.e., the binder) and bentonite contents. A series of analytical techniques are employed to identify the hydration products in this complex system. The engineering properties are tested via the unconfined compressive strength (UCS) test and flexible-wall permeation test. Results show that UCS and dry density decrease with increasing bentonite content, while the opposite trends are observed when increasing the binder content. The UCS values increase with curing time and become plateaued after ∼90 days. Meanwhile, the hydraulic conductivity (k) decreases distinctly with the increase of the binder content and bentonite content. All backfills reach UCS of >100 kPa UCS and k <10−8 m/s at 28 days while curing for 90 days leads to increase of UCS by >1.5 times and reduction of k by nearly one order of magnitude. The major hydration products of MSB backfills are identified as hydrotalcite-like phases (Ht), calcium silicate hydrates (C-S-H), monosulfate (AFm) and portlandite (CH). The hydration products, binding adjacent soil particles, filling pores, together with the swelling of bentonite, contribute to the mechanical performance and impermeability of the backfills.