Uhlík P., Guspan B., Gread F.A., Pálková H. & Osacký M., 2026: Hydration, dehydration and dehydroxylation of bentonite from Lutila I deposit, Kremnické Vrchy Mts., Western Carpathians. Acta Geologica Slovaca, 18, 1, 1–13.

Hydration, dehydration and dehydroxylation of bentonite from Lutila I deposit, Kremnické Vrchy Mts., Western Carpathians

Peter Uhlík¹, Boris Guspan¹, Faisal A. Gread¹, Helena Pálková² & Marek Osacký¹

¹Department of Mineralogy, Petrology and Economic Geology, Faculty of Natural Sciences, Comenius University Bratislava, Mlynská dolina, Ilkovičova 6, 842 15 Bratislava, Slovakia; peter.uhlik@uniba.sk, guspanb@gmail.com, faisal.gread@yahoo.com, marek.osacky@uniba.sk
²Institute of Inorganic Chemistry, Slovak Academy of Sciences, Dúbravská cesta 9, Bratislava 845 36, Slovakia; helena.palkova@savba.sk

Abstract

Slovakia ranks among the five largest bentonite producers in Europe, with the Lutila I deposit in the Central Slovakia Volcanic Field representing one of its most significant bentonite resources. Despite its economic importance, information on the properties of this bentonite remains limited. Water loss through dehydration and dehydroxylation of two commercial bentonite samples from the Lutila I deposit was investigated at 105, 200, 400, 600, and 800 °C. The effects of temperature and interlayer cation composition on swelling behavior were also evaluated. These properties are important for applications related to supplementary cementitious materials (SCMs) and deep geological disposal of radioactive waste. The RLU sample was a Na-activated bentonite containing 55 wt.% montmorillonite and 25 wt.% opal-C/CT, whereas the ELU sample was a natural Ca–Mg bentonite with 67 wt.% montmorillonite and 9 wt.% opal-C/CT. In both samples, dehydroxylation was characterized by a broad derivative thermogravimetry peak with a maximum near 665 °C, indicating cis-vacant montmorillonite. The ELU sample exhibited nearly twice the molecular water loss during drying (at 105 and 200 °C) compared with RLU, reflecting the presence of hydrated Ca²⁺ and Mg²⁺ interlayer cations associated with two water layers, whereas Na⁺ in RLU was associated with a single water layer. RLU exhibited a substantially higher swell index than ELU. After heating to 400 °C, RLU retained approximately 35% of its original swelling capacity, whereas ELU lost its swelling capacity almost completely. Heating to 800 °C transformed montmorillonite in RLU predominantly into an amorphous phase, potentially enhancing pozzolanic reactivity. In contrast, ELU formed both an amorphous phase and a new crystalline phase, which may reduce the pozzolanic activity of the calcined material. The relative contributions of montmorillonite and opal-C/CT to total water loss were quantified for both samples. The results highlighted the critical role of interlayer cation composition in controlling the thermal behavior, swelling properties, and application potential of bentonites from the Lutila I deposit.

Key words: Lutila I bentonite deposit, calcium-magnesium and sodium montmorillonite, hydration, swelling, dehydration, dehydroxylation

Manuscript received: 2026-06-04

Revised version accepted: 2026-06-18