The Ahom dynasty has ruled Assam for nearly six hundred years. With the leadership of Sui-ka-pha (1228 - 1268) prince, they made their first capital in Charaideo district of Assam. During their reign, they built massive brick buildings, houses, and temples. Currently, some of them are preserved under the Archeology Survey of India (ASI), Guwahati circle, Assam. Rang Ghar, Talatal Ghar, and Kareng Ghar are some of the most iconic buildings preserved under ASI. These monuments are constructed within the 1744 - 1768 time period. These buildings are some of the most important tourist attraction spots in the Sivasagar and Charaideo districts. With due course of time, it is observed that the construction materials are decaying. To preserve the aesthetic of the structures, modern bricks and mortar compositions are used to bridge the gap. This paper investigates and compares the early construction materials by analyzing the structural composition and patterns of the bricks and mortars of the three monuments. The analysis of collected brick and mortar samples was done using EDX, FT-IR, and PXRD. The EDX result displays the same periodic elements in bricks and mortars with varying weight percentages. The FT-IR spectra and PXRD patterns of bricks of Kareng Ghar and Talatal Ghar are found similar whereas the Rang Ghar displays a slightly different spectrum and pattern. In the case of mortar samples, the FT-IR and PXRD analysis show similar structural patterns. © 2021 Author(s).