The results of the impact of cement type and expanded vermiculite powder (EVP) on the thermal, mechanical characteristics and durability of lightweight mortars at high temperature and RSM modelling are presented in the paper. The lightweight mortars were fabricated by replacing fine aggregate with EVP in certain proportions. In the production of the mixtures, two different cement types namely, Portland cement (PC) and calcium aluminate cement (CAC) were used as binders and fine aggregates were replaced with EVP at the rates of 0%, 15%, 30% and 45%. The microstructure, thermal, mechanical properties and durability of PC and CAC mortars produced in this way were determined after standard curing for 28 days at temperatures ranging from 20 degrees C to 900 degrees C. Mechanical performance of mortar mixtures was evaluated based on compressive, flexural strength and ultrasonic pulse velocity (UPV). The durability of the specimens was also determined using porosity, water absorption and dry unit weight. Thermal conductivity and Scanning electron microscope (SEM) observations of 28-day mortar specimens were also evaluated. The response surface methodology (RSM) was adopted to develop the relation between parameters and response for thermal conductivity. The test results reported that lightweight refractory and insulating mortar can be manufactured by combined use of CAC and EVP when considered strength and durability and thermal performance after high-temperature exposure. Test results also pointed out that the combined use of CAC and EVP gained significant hightemperature resistance performance in terms of compressive strength as compared to PC blended mortar specimens.