The work aims to study the effect of substitution of nano-silica (NS) on the behavior of composite cement pastes containing ordinary Portland cement (OPC) and blast-furnace slag (GBFS) exposed to elevated temperature up to 1000 °C. The composite cements are composed of different amount of NS up to 6 mass% as well as 30–60 mass% GBFS. The behavior of hydration kinetics were studied for 1, 3, 7, 28 and 90 days. The fire resistance of composite cement pastes was evaluated for specimens cured for 28 days after firing at 250, 450, 600, 800 and 1000 °C with rate of heating 3 °C/min for 3 h soaking time, then cooled to room temperature in the furnace switched off. The compressive strength of OPC–NS and/or GBFS cement pastes increase with NS content up to 4%. Increases of the contents of NS up to 6% the values of compressive strength of OPC–NS cement pastes decreases. Gel/space ratio of OPC–GBFS–NS containing 4 mass% NS shows an increase with the increase of GBFS up to 40 mass% (mix IV.2). The compressive strength of III.2 and IV.2 show higher values at all thermally treatment temperature up to 1000 °C. The compressive strength of the superplasticized OPC–NS–GBFS composite shows a higher values up to 1000 °C in comparison with those pastes. It can be included that 30–60% GBFS in the presence of 4 mass% NS has a higher resistance to fire than all composite cement pastes.