Abstract A study of the physicochemical interaction of a high free lime (CaO) content cement kiln dust (CKD) with expansive Na-montmorillonite clay is presented. Moist compacted specimens of the CKD-treated clay, the clay alone, the CKD alone, and (for comparison) the clay treated with 7% CaO were each cured for periods up to 90 days and examined by XRD, TGA, and SEM techniques. The results for the CKD-treated clay indicated that calcium hydroxide, derived from the CaO present in the CKD, was extensively adsorbed on the surfaces of the clay flakes, but apparently only limited pozzolanic reaction occurred. Gypsum was rapidly produced from sulfate-bearing components in the CKD, and subsequently ettringite was produced as well, some of the latter apparently incorporating aluminum derived from the clay. Much of the clay was left unreacted, but the morphology of the clay particle assemblage was significantly modified in response to the CKD treatment. Similar morphological changes were also induced by the CaO treatment, suggesting that similar underlying mechanisms were also active here.