The cement-bentonite interface is a highly dynamic region in engineered barrier systems (EBS) designed to isolate radioactive wastes. Two very different chemical environments are held together: a near-neutral clay mineral-calcite equilibrium-dominated system (hydrated bentonite system) and a thermodynamically unstable hyperalkaline one (cement). The interface is, therefore, in permanent disequilibrium due to the conditions of changing pH that are related to the different degradation steps of the cement. The formation of phillipsite-(Na, K), (Si/Al ratio of 1.8–2.4), has been found in a series of closed-system hydrothermal tests at 35–90°C when the resulting pH was within the 11.7–12.6 range. The chemistry of the equilibrium solutions, rather than the crystallization substrata, controls the Si/Al atomic ratio and the type of zeolite that has been formed. This is based on the decrease of Si/Al at higher pHs and the predominance of phillipsite-Na. The formation of phillipsite-Na in these experiments is in agreement with the available thermodynamic data on zeolites.