Abstract This article aims to study the physico-mechanical and fire resistance of cement pastes containing Al 2 O 3 nano-particles. The results show that the water of consistency increases as well as the initial and final setting times accelerated with NA content up to 6 mass%, indicating that NA enhances the hydration reaction of OPC cement phases. The compressive strength increases with 0%, 27.22% and 11.07% for unsuperplasticizered NA–OPC pastes, whereas with superplasticizered NA–OPC pastes increase by 10.89%, 30.67% and 16.70% with the increase of NA contents from 0, 1 and 2 mass%. The results show that 1 mass% of NA gives a higher value of compressive strength, bulk density, chemically combined water content and gel/space ratio than OPC paste. 1 mass% NA acts as nano-filler which accelerates the rate of hydration. SEM micrograph reveals the formation of much denser nano-structure with nano-Al 2 O 3 addition. The structure was found to be denser compact matrix. The increase of thermally treated temperature from 25 to 450 °C, the compressive strength increases by 10.89%, 31.03% and 20.33% with the increase of NA contents from 0, 1 and 2 mass% for unsuperplasticizered NA–OPC pastes, whereas with superplasticizered OPC–NA increases by 25.22%, 45.74% and 28.49% with the increase of NA contents from 0, 1 and 2 mass%. It can be concluded that NA–OPC pastes containing 1 mass% NA has an optimum mix composition, which having a good firing resistance than those of other pastes.