Investigations on alternate lime-pozzolana based mortars for repair of heritage structures

Abstract

Lime was one of the predominantly used construction materials during ancient times world over. However, its prominence was repressed with the invention of cement. Though cement was found to be useful in many applications in construction industry, it was ascertained detrimental as a repair material particularly for heritage structures. The primary reason is the incompatibility of cement with the substrate (ancient lime) and masonry units in terms of mechanical and durability characteristics. From the previous investigations on ancient mortars performed at UNESCO heritage listed Ramappa temple and two other temples in Warangal fort, the binder was found to be lime and the fine aggregate used was siliceous sand. Also, the binder in the ancient mortars was in between weak and moderate hydraulicity. In view of this and some specific interests, it was thought that lime can be a better repair material. Since, lime characteristics depend on its source and location, in the present study, physical, mechanical and durability characteristics of two aerial limes (Lime-I and Lime-II) based mortars with replacements of fly ash/Ground Granulated Blast Furnace Slag (GGBS) from 0 to 75% were focused. In the present work, 1:3 and 1:1 mix proportions of lime-based mortars were studied for mechanical and durability studies. Among the binders, Lime-fly ash and Lime-GGBS binders required lower water content than pure aerial lime binders (Lime-I and Lime-II) for attaining similar consistency. Lime-GGBS binders exhibited faster setting characteristics than lime-fly ash binders and pure aerial lime binders (Lime-I and Lime-II). Among the mortars considered, Lime-II based 1:1 proportion mortar with 66% replacement level of lime with GGBS showed highest mechanical strength properties with compressive and transverse strengths of 9.02 MPa and 2.46 MPa respectively. It showed least porosity, water absorption and rate of strength loss to alkali (NaOH) immersion with 31.23%, 18.52% and 9.42% respectively. However, lower capillary absorption of 0.39 Kg/(m2.min0.5) was attained for a Lime-II based 1:1 proportion mortar with 75% replacement level of lime with GGBS. All the lime-based mortars exhibited good thermal resistance. Salt crystallization resistance was better exhibited by 1:1 mix proportion lime-GGBS based mortars. However, least drying shrinkage of 1.02 mm/m after 70 days was obtained for Lime-II based 1:3 proportion mortar with 75% replacement of lime with fly ash. Lime-GGBS based mortars specifically with 66% replacement of lime with GGBS exhibited better mechanical and durability characteristics. Also, these mortars showed close resemblance with the physical properties of ancient mortars like porosity and were made of similar materials like lime and siliceous sand, thereby establishing their potential as repair material for heritage structures.