Knowing from the past – Ingredients and technology of ancient mortar used in Vadakumnathan temple, Tirussur, Kerala, India

Highlights

• Three group of samples namely wall, gopuram and arch from 1300 years old ancient lime mortar plaster from Vadakumnathan temple, India were studied.

• The binder/aggregate ratio, chemical composition of binders, particle size distribution, role of construction materials used and technology employed for mortar preparation was determined.

• High strength hydraulic lime with hot lime technology preparation was reported.

• This work could be extremely useful during the restoration works for better formulation of appropriate repair mortars.

Abstract

In the present study, lime mortar samples from the restoration site of Vadakumnathan temple, Kerala, India have been analyzed. Samples from three different locations of the temple such as ancient wall, gopuram and arch have been taken. Traditional methods such as chemical analysis, acid loss analysis and organic test were conducted on mortar samples. Modern instrumental techniques such as electronic particle size distribution, X-ray Diffraction (XRD), scanning electron microscopy coupled with energy dispersive X-ray spectroscopy (SEM/EDX), Thermo Gravimetric Analysis (TGA), Differential Thermal Analysis (DTA) and Infrared Spectroscopy (FT-IR) were employed in the study. The binder used in the mortar is calcium high with 30% of clay mineral. A binder to aggregate ratio in the range of 1:1.5–2.5 has been established from acid loss analysis. Particle of the aggregate are mostly silt in nature, hence nominal sand would have been grinded to reduce the particle size and to induce pozzolanic reaction. The presence of carbohydrate, protein and fats are identified by organic test that are in agreement with FT-IR analysis and TGA.

Calcite, aragonite and calcium complexes of silicate and aluminates in form of hydro thermal product namely gyrolite and okenite are present in wall and gopuram samples. The formation of hydrothermal products confirms that the mortar was produced by hot lime technology. In TGA, the decomposition of CaCO₃ to CO₂ between 600 and 770 °C reveals the transformation of calcite from complex forms of CSH (gyrolite and okenite) and CAH. The presence of degraded products such as syngenite and gypsum in arch sample shows that the lime mortar is in complete deterioration where as mortar remained in good condition in gopuram and wall samples.Texture along with elemental analysis (EDX) confirms the results of chemical analysis.

Introduction

The architectural monuments of India, represent the cultural heritage of the nation. The built heritage not only brings out the artistic and technical capabilities of craftsmen but also the aspirations of people around the country. Vadakumnathan temple (Fig. 1) at Tirussur, Kerala is one of the largest Lord Shiva temple, constructed nearly 1300 years back by Parasurama, the sixth incarnation of Lord Vishnu. The temple is famous for its Kerala style architecture and the exceptional work of craftsmanship, on the gopurams, constructed on all four sides of the temple. In India, the ancient texts like Citrasutra of the Vishnudharmottara, Silparatna [1], [2] talks about the construction of walls using lime mortar. As mentioned in the ancient scripts like Silparatna and Vishudharmottara, the temple was constructed with laterite blocks bonded with lime mortar which consists of lime, sand and various organics.

Strength and durability of lime mortar could be enhanced by the addition of vegetable extract. Traditionally, people in India have the practice of adding the natural organic admixtures from the extracts of locally available plants, unrefined sugar and dry seeds.

Ancient buildings were constructed using local and naturally available materials. Stone, lime, clay, timber, palm leaves were some of the materials used for the construction of temples in Kerala. Also, ancient mortars comprises of various binders (or mix of them) and natural or artificial aggregate along with herbal admixtures to enhance the strength and durability of the structures [3].

Regional plants that are rich in carbohydrate, protein and fat are generally added to the lime mortar mix. In central Kerala, different types of herbs, namely Oonjalvalli, Kulamavu, Kadukai, Pananchikai and jaggery (Unrefined sugar) were used as admixture in the temple for restoration work. Kadukai and jaggery were added as carbon source where as other plant extracts are rich in protein and fats. These are added to improve various properties like carbonation, plasticity of the mix and also to enhance the durability of the hardened mortar [4]. These mortars also served as a platform for mural painting.

Although few research reports were published about the role of organic admixtures on mortars, a thorough knowledge of lime mortar used in ancient structures is essential for the preparation of repair mortar. Many researchers have been working on the characterization of ancient mortar, adopting various tools and techniques on the physico-chemical, mechanical and mineralogical aspects. As the ancient mortar contains variety of raw materials along with organics, it is difficult to understand the characteristics, production and application techniques [5]. Even environmental parameters such as climate and humidity could affect the strength and durability of the mortar. All these parameters are interrelated system that leads to complex problems during the analysis of historic mortars.

In view of conservation and preparation of new restoration mortars, it is essential to investigate the technology involved in the traditional method of preparing lime mortars and plasters. It is important that the mineralogical, chemical, micro-structural and elemental composition has to be studied in order to understand the procedure to produce restoration mortar similar to the original one. The present study identifies the raw materials (lime, sand and organics) used for the preparation of restoration mortar based on the results of the characterization of the ancient mortars and its application technology.