Cement mortar is used a lot in the construction industry every year, and demand is expected to rise soon. One of the most widely used building materials is cement mortar; Cement is the main component of cement mortar. Cement mortar is increasingly in demand, as a construction material. On the other hand, cement production and use reduces the availability of raw materials (limestone) and pollutes the environment. Every year, portland cement is increasing around the world. As Portland cement production currently contributes to greenhouse gas emissions, additional cementitious and pozzolanic materials are required for cement mortar. Consequently, there is a growing interest, in using industrial waste and byproducts to cut down on the amount of Portland cement which is used. The thermal behavior of partially replacing lightweight cement mortar with GGBS (Ground granulated blast furnace slag) is analyzed by this paper. A building's thermal analysis of lightweight cement mortar can be used as an energy storage system, lowering the demand on the building's auxiliary heating and cooling systems. By adding GGBS (Ground granulated blast furnace slag), which have a high conductivity and high latent heat storage capacity, lightweight cement mortar's heat storage capacity can be improved.