A significant attention is drawn by researchers in recent years towards cements with partial replacements of supplementary cementitious materials [SCM], to reduce the adverse effect of global warming. The research presented in this paper involves the microstructure study of composite cement [CC] which falls into the category of ternary blended cements, consisting of ordinary Portland cement [OPC] and SCMs: Fly Ash [FA] and Granulated Blast Furnace Slag [GBFS], as partial replacement of OPC. The proportion of OPC, FA, and GBFS considered are 45%, 22%, and 33% respectively which is in accordance with IS 16415: 2015. The efficacy of using the CC effectively in construction practice needs the understanding of the behaviour and phase transformations occurring during the process of hydration, which affects the strength and performance of mortars. This study examines the phase changes at different curing periods viz. 3,7,14,21,28,56 and 90 days, along with the strength of OPC and CC mortars as well as the microstructure investigation. As it was expected, the initial rate of strength gain of CC is lower, compared to OPC, due to the slow development of pozzolanic activity. The strength gain of OPC has practically obtained its class at 28 days but in CC it has continued till 90 days. The ultimate strengths of both the cements are quite comparable at the end of 90 days.