The study investigated how simultaneous heating and fluid percolation influence the source pressure and permeability of three fine‑grained materials relevant to deep geological disposal: a compacted Bavarian bentonite (B25), an intact Friedlandton clay, and an intact clayey facies of the Opalinus Clay. In a miniature oedometer cell, air‑dried samples were first heated to a target temperature while being bathed in a pore solution derived from Opalinus Clay. After reaching the desired temperature, the samples were saturated under a two‑sided solution pressure of 7 MPa, and permeability was measured at the same temperature. The experiments were complemented by mercury porosimetry and carbon‑content analyses to monitor pore‑size distributions and mineral dissolution.

For the Bavarian bentonite, the temperature dependence of oversaturation was non‑monotonic, attributed to the increasing density of pore water outside the smectite interlayers. Across all three clays, source pressure and permeability displayed a similar non‑monotonic temperature trend. At temperatures up to 60 °C for the bentonite and Opalinus Clay, and up to 100 °C for Friedlandton, the source pressure decreased, likely because the hydration pressure diminished. During these ranges the permeability remained essentially unchanged, indicating that the pore‑structure was not altered. When the temperature exceeded these thresholds, the source pressure rose monotonically, a behaviour interpreted as the result of increasing osmotic pressure linked to mineral dissolution. For the bentonite and Friedlandton, this rise in source pressure was accompanied by a monotonic increase in permeability, reflecting larger pore‑entrance diameters and an increased macro‑porosity fraction. In contrast, the Opalinus Clay showed a pronounced drop in permeability—by more than a factor of two—between 60 °C and 100 °C, attributed to carbonate dissolution and a presumed rearrangement of clay mineral particles. Beyond 100 °C the permeability of the Opalinus Clay stabilized again.

The experimental findings were fed into a coupled thermal‑hydraulic‑mechanical (THM) model for the Bentonite B25 and into geochemical models for both Bentonite B25 and the Opalinus Clay. These simulations helped to interpret the observed temperature effects and highlighted open questions regarding the implementation of the modelling approaches, particularly concerning the treatment of temperature‑dependent solid‑phase deformations and the representation of mineral dissolution kinetics.

The project was funded by the Federal Association for Long‑Term Storage (Bundesgesellschaft für Endlagerung mbH, BGE), which also provided the overarching research question and financial support. The experimental work was carried out by a multidisciplinary team from the Gesellschaft für Anlagen‑ und Reaktorsicherheit (GRS) and collaborators. Key contributors included Michael Kröhn, Julia Gose, Veronika Prause, Viktor Gillich, Nadine Zilling, and Dr. Tina Scharge, who coordinated laboratory operations and test‑stand development. The mineral‑composition data and mercury‑porosimetry measurements were supplied by Dr. Lan Nguyen‑Thanh (Institute for Applied Geosciences, TU Darmstadt) and performed by Dr. Neven Ukrainczyk and Yvette Schales (Institute for Materials in Construction, TU Darmstadt). GL Test Systems GmbH built and finalized the test stand, while Dr. Stephan Kaufhold (BGR) provided the Bentonite B25, Dr. Ben Laurich (BGR) and Dr. David Jaeggi (Swisstopo) supplied the Opalinus Clay, and Dr. Jörn Kasbohm (Consulting) supplied the Friedlandton. Dr. Christian Ostertag‑Henning (BGR) advised on the passivation of the oedometer cells. All experimental data are publicly available through the DOI 10.17632/zmzs2b6xnh.1, enabling further analysis and model validation by the scientific community.