Physico-chemical properties and microstructure of bentonite in highly alkaline environments

Cementitious materials and their alkaline pore fluids can change the structure of bentonite used as a raw material for road embankments or concrete storage of garbage cans. This study investigated the alteration of montmorillonite-rich bentonite from northeast Morocco (Trebia deposit, Nador) in alkaline media rich in Ca 2+ , Mg 2+ , Na + , or K + . Specimens based on raw bentonite mixed with variable proportions of oxides (CaO, MgO) or hydroxides (NaOH, KOH) and water were prepared and aged for 28 days. Mineralogical composition by X-ray diffraction (XRD) was determined on raw bentonite and specimens to follow phase changes. Chemical composition and thermal characteristics were determined for raw bentonite and specimens by Fourier-transform infrared spectroscopy (FT-IR) and thermogravimetric/differential thermal analysis (TGA/DTA). Microstructural evolution and alteration of the external surface of bentonite were evaluated using scanning electron microscopy coupled with energy dispersive X-ray (SEM/EDX) analysis. XRD results of bentonite-CaO mixture demonstrated the formation of gels (e.g. C-S-H) and calcite. When the amount of CaO added increased, excess portlandite and the precipitation of calcite in the outer surface of bentonite occurred, stopping pozzolanic reaction and consequently decreasing the compressive strength of specimens. On the other hand, the addition of MgO allowed the formation of brucite. Sodalite and cancrinite were neoformed with the addition of 32 wt.% NaOH after 28 days of hydration. The addition of hydroxides (NaOH or KOH) to bentonite did not reveal any setting due to the absence of the formation of cementitious phases.