In the ancient times, lime mortars have been modified with regionally available natural polymers to enhance the durability of the structures. Mucilage extracted from Cactus, a bio-polymer has been proposed as a water repellent admixture in lime mortar. The compressive strength of mortar is increased with increasing concentration of the extract. Lime produces calcium hydroxide during hydration which interacts with polysaccharides and proteins of cactus, and forms complexes. It showed a hydrophobic behavior. The lime mortar of low bulk density and lesser water absorption has been produced by using this admixture. This could enhance the capabilities of lime mortars in restoration works and also used to preserve the ancient structures. © Research India Publications.