Compacted bentonite and bentonite–sand mixtures have major importance as buffer materials in deep geological repositories, due to their low hydraulic conductivity and high swelling pressure. The main purpose of the buffer material is to seal the canister to prevent any high-level radioactive waste leakage. For this purpose, bentonite stands out as a good option for sealing due to its high swelling pressure. The swelling pressure of bentonite is affected by temperature changes. Therefore, in the present study various additives were added to bentonite and the effects of high temperature and additives on the swelling pressure of the compacted bentonite mixtures were investigated at room temperature and 80 °C. Boron minerals, namely colemanite and ulexite due to their low thermal expansion properties, ferrochrome slag and carbon fiber due to their high temperature resistance, were selected as additives. Additives were added to bentonite at the rate of 10% by dry weight. Also using modified conventional oedometer systems and uniaxial pressure test systems, a series of swelling pressure tests were performed at both temperatures. The modifications in the conventional systems allowed for a more accurate simulation of the conditions in deep geological repositories. Results indicate that high temperatures and additives significantly affect the swelling pressure, suggesting a potential change in the material's performance in high temperature conditions. This finding is crucial for the design and safety of deep geological repositories.