Porosity plays a significant role in concrete strength and durability. It also imparts the randomness in paste and aggregate bonding in cement concrete composite. The paper presents the optical evaluation and analysis of pores in cement mortar sample. The analysis has been done using microscopic images of a thin section of the cement mortar obtained from optical microscope. The mortar samples were prepared by varying its water–cement ratio, different batching process and substituting supplemental cementious material. The microscopic images have processed through ImageJ software for pore size evaluation and its distribution the surface of cement mortar sample. The classification of pores has been made based on its size and their formation for different water–cement ratio, batching processes and usage of cementious material. The overall results obtained through optical evaluation have satisfactory variation and significance in order to understand the progress of hydration and its conductive properties through non-destructive ways. © 2021, The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd.