This study investigates the effects of various market-accessible admixtures on concrete made from a blend of cement CEM I 42.5N and commercially available slag from Sweden, under elevated temperatures of 400 °C and 800 °C. The reference concrete mix, consisting of cement and slag (50/50%), was modified by adding an air-entraining agent, a plasticizer, and an accelerator in two dosages. The mechanical properties were evaluated through compressive strength tests. The results demonstrated that the addition of these admixtures noticeably impacted the mechanical performance of the concrete at high temperatures. A significant reduction in compressive strength was observed, with the most severe degradation occurring at 800 °C. These findings suggest that the use of some types of admixtures can interfere with concrete properties, and may lead to reduced durability and strength, when exposed to elevated temperatures.