The effects of high temperatures up to 800 degrees C (1472 degrees F) on portland-cement-based reactive powder concrete (CRPC) and alkali-activated-slag-based reactive powder concrete (ARPC) were investigated within the scope of this study. The changes in the mechanical properties and microstructure of RPCs with elevated temperatures were evaluated. The test results indicated that CRPC has a superior high temperature resistance up to 300 degrees C (572 degrees F) without any strength loss. Beyond this temperature, however, CRPC samples suffered from explosive smiling. Nevertheless, ARPC did not exhibit explosive spalling up to 800 degrees C (1472 degrees F) due to its nano-sized pored microstructure. These pores permit the quick escape of water vapor from the ARPC matrix, resulting in lower internal pore pressures. In conclusion, it can be said that the ARPC has a better high temperature resistance compared to the conventional RPC.