Reaction kinetics of FEBEX bentonite in hyperalkaline conditions resembling the cement–bentonite interface

Bentonite of the Serrata de Níjar (Almería, Spain) and concrete made with an ordinary portland cement (OPC) are candidate materials to be used as engineering barriers in the high level radioactive waste disposal in argillaceous rock. This experimental alteration study has been conducted in order to determine the kinetics of alteration of a montmorillonitic bentonite under hyperalkaline conditions (NaOH 0.5 to 0.1 M) in the presence of portlandite. The amount of montmorillonite destroyed and the secondary minerals formed have been measured by means of X-Ray Diffraction (XRD) in the solid phase after the performance of batch reactions carried out in airtight cells (3/1 liquid/solid) during 30 to 540 days. Scanning Electron Microscopy (SEM-EDX) and chemical analysis of a < 0.5 μm Ca-homoionized fraction have been used to characterize the reaction by-products, mostly zeolites (analcime and phillipsite type); calcium silicate hydrates (amorphous and 11 Å-tobermorite type) and saponite. ture of the solution chemistry determined in the aqueous phase has allowed us to calculate the chemical speciation and the saturation indices for the observed minerals. The evaluation of the equilibrium state in the system supports the dissolution of montmorillonite from undersaturation conditions as the driving process for the alkaline reaction of bentonite. The rate of montmorillonite dissolution has been calculated from the mineralogical quantification of smectite in the 75–200 °C tests. The global kinetics for the conversion of montmorillonite can be fitted to R (mol s−1) = A(m2) k[OH−]0.5; ln k = (− 20.09 ± 1.37) − (2731 ± 543) ⁎ (1 / T); Ea = 22.7 ± 4.4 kJ/mol. These data are in close agreement with the rate of dissolution of montmorillonite obtained by methods based on solution chemistry.