The role of smectite composition on the hyperalkaline alteration of bentonite

• Alkaline interaction is studied for testing long-term performance of bentonites.
• Analcime and K-feldspar are synthetic and natural byproducts of alkaline alteration.
• Pure Mg or MgAl octahedral compositions favoured the chemical resistance of smectite.

Cementitious materials will act as a source of alkaline fluids modifying the mineralogy, pH and physical–chemical properties of bentonite barriers implemented for long-term pollutants retention. A series of reactivity experiments in batch reactors at 175 °C were performed for nearly 15 months to evaluate the chemical and mineralogical responses of four different bentonites with potential use as buffer barriers against a hyperalkaline K–Na–OH-type solution. This solution simulates the early stage of an ordinary Portland cement leaching in a clay hosted nuclear waste repository. The tested buffer materials were three montmorillonitic bentonites: MX-80 (Wyoming type, USA), FEBEX (Almería, Spain), and a bentonite from Chile (MMT); and a saponitic bentonite from el “Cerro del Águila” (Toledo, Spain). Partial dissolution of smectite and precipitation of secondary phases such as analcime and K-feldspar (sanidine or orthoclase) were found as reaction products in the dioctahedral smectite-based bentonites whereas the trioctahedral character bentonite showed more resistance to alteration. In addition, mixed-layered illite/smectite (I–Sm) interstratification was observed in the four bentonites with different degrees of order. The most heterogeneous octahedral sheet with significant Fe and Mg occupations (FEBEX and MX-80 bentonites) altered to produce R0 random mixed-layered I–Sm (60–40% I), whereas virtually ideal Al–Mg octahedral sheet (MMT bentonite) altered to produce saponite and R ≥ 1 ordered I–Sm (> 80% I). The physical–chemical properties (CEC and SSA) were also altered differently in the four bentonites. The methodology used in this work is thought to develop a direct comparison of the performance of potential buffer materials in a nuclear or hazardous waste repository. The evaluation of other basic hydro-mechanical properties should be further evaluated to serve as an indicator of long-term stability of bentonites used in different reactive environments.