Decoding of lime-based binder materials of ancient vernacular Chettinad constructions in Southern India through microanalytical characterization

Abstract

Vernacular Chettinad lime binder is widely popular in Southern India because of its quality in terms of durability over centuries and the fine glossy finishes attainable. However, its manufacturing technique concerning raw materials, the application process, and resultant characteristics are not explored or documented. The present study attempts to explore the mix compositions and application strategies of Chettinad lime plaster and mortar by characterizing samples collected from two ancient structures approximately 100 years old and in intact condition. Like any Chettinad constructions, the plaster in both buildings constitutes a base coat (coarser as observed) and a superior smooth glossy appearance finishing layer. The plaster samples collected from various locations from the two sites were thoroughly characterized for their physicochemical properties, microstructural characteristics, surface properties, mineralogical compositions, porosity, and pore size distribution. Analyses of the plaster samples suggest that the air lime and locally available, river-based siliceous aggregates were utilized as binder and aggregate, respectively, in ratios ranging from 1:1.51 and 1:2.57 in each of the buildings studied and correlated the ratio to their durability performance as well. Shell lime and organics such as curd and egg white were utilized to obtain the glossy plaster finish. The art of manufacturing and application of such a durable indigenous binder system is lost, not continued, and undocumented. The present study would act as (i) documentation that aids the continuation of the technique of the traditional binder production, as well as (ii) decoding the compositional differences of the plaster and mortar at various locations/applications, which would help in properly maintaining and restoring degraded portions of building structures by reproducing similar and compatible material.