In the present research, characterization of ancient lime mortar from the 300-year-old Renukadevi Temple at Chandragutti, Karnataka, India, was carried out to propose an appropriate repair procedure for fixing a water leakage through the roof slab. The reason for the water seepage was a poor binder with the aggregate, with a ratio of 1:7. When the lime was washed away due to intense rain, the mortar became porous. XRD indicated that the original structure contains hydraulic lime with mineral-rich clay including calcite, aragonite, vaterite and tobermorite. The presence of tobermorite results in good performance and durability of the structure. The presence of organics rich in carbohydrate and protein was indicated by the FT-IR results. The repair mortar was simulated with an Oxal VP IT injection grout with 5 % of fermented kadukkai and jaggery as carbohydrate-rich organic sources. The rheological performance of the organic grout was good during Marsh-cone and stability test. After 28 days, the compressive strength of the 1:3 mortar was 1.13 N/mm2, suggesting it behaved like an artificial pozzolanic mortar with good strength. The organics played the main role in the carbonation of lime mortar and precipitation of tobermorite. A new organic grout compatible with the old mortar of the structure was prepared for the restoration. © 2020