Soil–bentonite (SB) and slag–cement–bentonite (slag-CB) slurry trench cut-off walls are widely used low-permeability subsurface barriers against groundwater or contaminants. The bentonite in SB walls is instrumental to the performance of the barrier. However, when mixed with Portland cement and slag in slag-CB walls, bentonite participates in ‘secondary’ chemical reactions throughout cement hydration. Using mineralogical methods, the goals of this paper are to examine the fate of bentonite in slag-CB mixtures and to discuss the implications of the associated chemical reactions for the slag-CB wall performance. Laboratory-prepared mixtures of dry and hydrated slag-CB were analysed using X-ray diffraction over 3 months to investigate the evolution of mixtures during curing. Results confirm that bentonite clay minerals dissolve in slag-CB slurries and that bentonite is virtually absent in the hardened slag-CB material. Furthermore, carbonation and pozzolanic chemical reactions also take place on slag-CB hydration and curing, which form secondary minerals over time. Hence, unlike SB mixtures, the bentonite in slag-CB mixtures no longer retains its original crystalline structure, as it participates chemically in hydration processes, although it becomes a precursor of pozzolanic products. Nonetheless, bentonite is instrumental to the workability and initial set of the mixture. It is concluded that the concepts of compatibility traditionally accepted for SB walls are inappropriate for slag-CB walls. Possible implications include that due to the onset of carbonation, the barrier could degrade if the contaminants are acidic; also, the formation of pozzolanic hydrates may improve the contaminant sorption properties of the barrier.