Different autoclaved specimens were prepared using ordinary Portland cement (OPC) and ordinary Portland cement– ground granulated blast-furnace slag (OPC-GGBFS) with the presence and absence of sand. All autoclaved specimens were prepared by the hydrothermal curing at a pressure of 8 atm. of saturated steam for different autoclaving ages of 0.5, 2, 6, 12 and 24 hours. The Kinetics of hydration process were studied via determination of chemically combined water and free lime contents at the different autoclaved ages up to 24 hours. The specimens made of OPC and OPC-GGBFS-GS blends were found to have higher strength values at almost all autoclaving ages. This attributed to the formation calcium of silicate hydrates (CSH) and calcium hydroxide (CH) hydrates, as the main hydration products, which possess a stronger hydraulic character with higher compressive strength. The hydrothermally hardened OPC-GGBFS pastes were found to have weaker strength, which could be due to the formation of calcium aluminosilicate hydrates (C3ASH4).