Elevated temperature effects (T > 100 {\deg}C) on the interfacial water and microstructure swelling of Na-montmorillonite

TL;DR

This study uses molecular dynamics simulations to explore how elevated temperatures above 100°C affect the interfacial water and swelling behavior of Na-montmorillonite, revealing changes in interlayer forces and limitations of classical theories.

Contribution

It provides new molecular-level insights into Na-montmorillonite's swelling mechanisms at high temperatures, extending understanding beyond classical electrostatic models.

Findings

Swelling behavior is governed by van der Waals and hydration forces above 100°C.

Swelling pressure decreases due to weakened hydration and electric double layer repulsion.

Classical DLVO theory's applicability diminishes at elevated temperatures, but combined models can predict swelling.

Abstract

Montmorillonite-based barriers are key elements of the engineered barrier systems (EBS) in geological disposal facilities (GDF). Their performance at temperatures above 100 {\deg}C is not sufficiently understood to assess the possibility of raising the temperature limits in GDF designs that could reduce construction costs and CO2 footprint. The present work provides new fundamental insights through molecular dynamics (MD) simulations of Na-montmorillonite's water-clay interactions and swelling pressure at temperatures 298-500 K and basal spacings of 1.5-3.5 nm. At temperatures above 100 {\deg}C, the swelling behaviour is governed by the attractive van der Waals force and the repulsive hydration force instead of the repulsive electrostatic (double layer) force. The swelling pressure reduction with increasing temperature is related to the weakened hydration repulsion and electric double layer repulsion, which result from the deterioration of the interlayer water layer structure and the shrinkage of the electric double layer. The applicability and breakdown of the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory at elevated temperatures are examined. By excluding the osmotic contribution in the DLVO theory, the summation of the van der Waals interaction in DLVO and an additional non-DLVO hydration interaction can predict our MD system's swelling under high temperatures. The findings of this study provide a fundamental understanding of the swelling behaviour and the underlying molecular-level mechanisms of the clay microstructure under extreme conditions.