Technical Report NTB 90-26

Thermodynamique du gonflement de systèmes eau-bentonite, eau-métabentonite, eau-illite

The water vapour adsorption isotherms and enthalpies of immersion of 15 samples of natural clays (bentonites, metabentonites and illites) in water have been measured at 20°C. The aim of these measurements was firstly to study the influence of ultrasonic treatment of the solids, and secondly to observe the change in water adsorption at different stages in the illitisation process of montmorillonite.

As regards the ultrasonic treatment, we can assert that it has little influence on the different samples, with the exception of the metabentonite "B". This shows an important increase in its BET surface area and micropore volume, and in the swelling pressure at every degree of hydration. The other solids, on the contrary, show a small decrease in both the BET surface area and the swelling pressure at low water contents. At higher degrees of hydration, the swelling pressure increases after ultrasonic treatment, as pointed out by Kahr and Müller-Vonmoos. However, it is not possible to derive information valid for other degrees of hydration from measurements obtained for a specific water content.

We also observed that adsorption depends strongly on the K+ content of the clay. The higher the content, the weaker the adsorption, and a corresponding decrease is observed for the swelling pressure and the heat of immersion in water.

Finally, owing to the difficulty of finding a suitable mathematical fit to describe the enthalpy change with the amount of pre-adsorbed water, the problem of the entropy of the water molecules is only dealt with in a qualitative way.