Elevated temperature effects (T >100℃) on the interfacial water and microstructure swelling of Na-montmorillonite

Montmorillonite-based barriers are key elements of the engineered barrier systems (EBS) in geological disposal facilities (GDF). Their performance at temperatures above 100 °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 °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

Elevated temperature effects (T > 100 \unicode{x2103}) on the interfacial water and microstructure swelling of Na-montmorillonite

Montmorillonite-based barriers are key elements of the engineered barrier systems (EBS) in geological disposal facilities (GDF). Their performance at temperatures above 100 \unicode{x2103} is not sufficiently understood to assess the possibility of raising the temperature limits in GDF designs that could reduce construction costs and CO$_2$ 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 \unicode{x2103}, the van der Waals interactions instead of the Coulombic interactions govern the swelling behaviour because of the effective screening of the surface charges by dehydrated interlayer counterions. The reduction of the swelling pressure with increasing temperature is related to the weakening hydration repulsion, which can be observed by the deterioration of the layering structure of the interfacial liquid and the shrinkage and weakening of the electric double layer. The applicability and breakdown of the classic Derjaguin-Landau-Verwey-Overbeek (DLVO) theory at elevated temperatures are examined. It is found that additional non-DLVO interactions in the system cause the breakdown of DLVO. The partial recovery of DLVO in basal spacings between 1.7-2.5 nm and at 500 K is explained by the formed bulk-like structure of interlayer liquid 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.Comment: 46 pages, 12 figure