HYDERABAD: Undeterred by the test of time, Charminar has been standing tall for the last 427 years. The secret to longevity is the unique blend of materials used in the preparation of lime mortar that covers the iconic monument. According to experts, this is the first time they have decoded all the ingredients that could have been used to make the lime mortar used at Charminar.
A study conducted by a civil engineering professor from the Vellore Institute of Technology(VIT), Tamil Nadu, in collaboration with Archaeological Survey of India, has revealed that the key component used was water fermented with plant extracts. While lime in the lime mortar is in the form of Calcium oxide(CaO), exposure to Carbon dioxide(CO2) converts it to its most stable form, Calcium carbonate(CaCO3). The process, known as Carbonation, happens with over years of exposure to the atmosphere.
The fermented plant extract, which is used to mix the lime mortar, contains alcohol which slows down the process of carbonation. Rather, it encourages the formation of an unstable intermediary called Vaterite. To the surprise of researchers, all lime mortar samples collected from Charminar had the presence of Vaterite, meaning it still has a long life.
The lime mortar was also found to have presence of carbohydrates from plant extracts, which acts as a magnificent adhesive controlling breaking of mortar. “Carbohydrates from plant extracts like Kaddukai or Jaggery and fermented water from plant extracts, have been found to be used in lime mortar of other monuments of India. It is very rare to find the presence of vaterite in a more than 400-year-old monument like Charminar. Those who made lime mortar for Charminar knew the perfect amount of fermented water and carbohydrates from plant extracts to be added,” said Dr S Thirumalini, Associate Professor at the Department of Structural and Geotechnical Engineering, VIT.
Dr Thirumalini pointed to another important factor contributing to longevity of lime mortar of Charminar. She said, “Various studies proved that 1:3 ratio of Lime to Sand provides better mechanical properties, reduces pores and boosts carbonization.” Dr Thirumalini pointed out in her study that the limestone available in Deccan region used in making mortar for Charminar, naturally contains 10 percent Magnesium oxide(MgO), which has increased durability to the mortar, unlike lime in other areas.