Compacted bentonites are used in deep geological repositories and energy geo-structures as a buffer material. In deep geological nuclear waste repository sites, the high temperatures generated by radioactive waste and the heat transfer around energy geo-structures increase the temperature of bentonite buffer. Especially the buffer material surrounding the canister is exposed to high temperatures. This temperature elevation causes a decrease in the bentonite’s water content, leading to the material’s shrinkage. The present study used additive-free bentonite and bentonite mixtures with two different additives, carbon fiber and ferrochrome slag. Carbon fiber was used at 1 and 1.5% contents by dry weight, while ferrochrome slag was used at the ratio of 10 and 30% by dry weight. The shrinkage tests were conducted at ambient temperature, 50 °C, and 80 °C to examine the effect of high temperature. Carbon fiber additives decreased the shrinkage level independent of additive content and temperature. On the other hand, ferrochrome slag had different impacts on the shrinkage behavior depending on its content and temperature.