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Montmorillonite-based barriers are key elements of the engineered barriersystems (EBS) in geological disposal facilities (GDF). Their performance attemperatures above 100 $\unicode{x2103}$ is not sufficiently understood toassess the possibility of raising the temperature limits in GDF designs thatcould reduce construction costs and CO$_2$ footprint. The present work providesnew fundamental insights through molecular dynamics (MD) simulations ofNa-montmorillonite's water-clay interactions and swelling pressure attemperatures 298-500 K and basal spacings of 1.5-3.5 nm. At temperatures above100 $\unicode{x2103}$, the van der Waals interactions instead of the Coulombicinteractions govern the swelling behaviour because of the effective screeningof the surface charges by dehydrated interlayer counterions. The reduction ofthe swelling pressure with increasing temperature is related to the weakeninghydration repulsion, which can be observed by the deterioration of the layeringstructure of the interfacial liquid and the shrinkage and weakening of theelectric double layer. The applicability and breakdown of the classicDerjaguin-Landau-Verwey-Overbeek (DLVO) theory at elevated temperatures areexamined. It is found that additional non-DLVO interactions in the system causethe breakdown of DLVO. The partial recovery of DLVO in basal spacings between1.7-2.5 nm and at 500 K is explained by the formed bulk-like structure ofinterlayer liquid under high temperatures. The findings of this study provide afundamental understanding of the swelling behaviour and the underlyingmolecular-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