Monuments built by the Mughals and the Lodhis from the 13th to the 16th century extensively follow a Persian style of architecture with plasterwork decorations.
Now, researchers from the Indian Institute of Technology (IIT) Madras, which hosts the National Centre for Safety of Heritage Structures, have looked at five different historic buildings in New Delhi and have uncovered the microstructure of the binding material used in such plasterwork.
The researchers studied the mineral composition and porosity of the material and noted the different transformations that had occurred. They investigated if any secondary materials had formed over the years and whether they had altered the internal structure.
Organic fibres
The results showed that the plasterwork was mostly made up of lime binder and silica sand. Interestingly, organic fibres were also found in the plaster. Divya Rani, a Ph.D. scholar at the institute and first author of the study published in Construction and Building Materials, explains that the organic fibres could be anything from jute or straw to animal hair. More studies are needed to point out which exactly was used.
Plaster is prone to shrinking and fibres prevent the formation of cracks. Fibres are also used in modern construction to prevent cracks in concrete.
Dr. Manu Santhanam (left) corresponding author of the work
When asked if these insights can be used in modern construction, Dr. Manu Santhanam pointed out that the lime which was the principal binding material in the past was fundamentally different from cement. Dr. Santhanam is from the Institute’s Department of Civil Engineering and a corresponding author of the work. Lime-based binders need exposure to air (carbon dioxide) for a significant period before they can harden, while cementitious binders are capable of rapid hardening even in wet conditions.
As a result, lime-based mortars are still used for repair and restoration of historic monuments, since cement mortars are not compatible with historic materials. In modern construction, architects like the use of lime for interior wall plastering as it is known to reduce carbon dioxide and also has better thermal properties compared to cement plasters.
“This study hopes to get insights into the materials and methods used in past buildings in order to restore our historic monuments,” adds Dr. Santhanam.
Explaining why buildings of the past have a better lifespan, Dr. Santhanam says: “Old structures used stones, bricks, and lime-based binders, which weather slowly, unless subjected to an environment rich in salts. But modern construction mainly depends on reinforced concrete. The life of the structure is governed by how long it takes for the steel to corrode. But reinforced concrete is also the best option available.”
Steel has high tensile strength and ductility, and makes it possible to design optimally and economically, unlike the materials used in the past. “Modern buildings can also have an extended life if we pay attention to the blend of materials used for concrete, and invest in good quality construction practices,” Dr. Santhanam says.
