STUDIES OF SOME PHYSICO-CHEMICAL & BIOLOGICAL ASPECTS OF BUILDING STONE DETERIORATION & PRESERVATION

Abstract

Stone is considered to be the most durable building material but its durability is not absolute. Stone deterio ration is attributed to a number of physical, chemical and biological factors along with environmental factors. The unfavourable environmental conditions result in the compli cated process of chemical deterioration and physical dis integration of stone. The biological factors are closely interrelated with physico-chemical factors. The present study embodies the physical, chemical and biological factors responsible for the decay of building stones. The samples of stones, used in the construction of historical monuments of India, namely Makrana marble, red sandstone, white spotted sandstone, basalt and scoria, were collected from monuments as well as from the respective quarries for a detailed investigation of their decay and durability characteristics under different environmental conditions. The biological studies were made to identify the micro organisms resposible for the deterioration of building stones. Microbial samples collected from a variety of monuments of Delhi region showed the wide occurence of algal and bryophyte growth over the rough and disintegrated stone surfaces. Among algae, the species of Cyanophyceae and among bryophytes, Riccia and Funaria were found abundantly. Altogether seventeen species of Cyanophyceae algae could be identified. The deve(

lopment of microbial population over the stone surfaces can cause morphological, mechanical and chemical changes in the stones which further enhance the decay of stones. The appli cation of preservatives have been found effective to retard the growth of microbial population over the stone surfaces. The physico-chemical studies of different stones have been carried out by durability test, weathering test, seawater test and acid test. The durability studies have clearly revealed that the decaying behaviour of e<ich variety of stone has direct relation with the physical characteristics e.g. water absorption and porosity. The general trend found in most cases is the progressive increase of these characteri stics with the decay of stone. Further, the stone starts to disintegrate when it attains certain fixed values of water absorption and porosity depending upon the inherent charac teristics and nature of the stone. Based on the above rela tionship established between the physical properties and decaying behaviour of each variety of stone, one can easily assess the remaining life of the particular stone used in the monuments and suggest the need for the preservation. The pattern of disintegration of stone has been found to depend on its texture and structure. In Makrana marble the disintegration starts with the surface roughness and corner disintegration followed by development of irregular micro-cracks, while in sandstones, the disintegration starts

with surface roughness and development of cracks along the bedding plane which become more prominent with the decay of stones resulting into separation along the bedding plane. In basalt the leaching of weathered secondary minerals from the amygdals promotes the development of grooves and pits over the surface-rich in brownish alteration material leading to irregular cracks over the surface. The scoria samples showed the decaying pattern in the form of high degree of surface erosion causing damage to corners and resulting into heavy loss in weight. The durability and weathering characteristics of un treated scoria stone was found to be different from other stones which may be due to highly porous structure of the stone. Among the various preservatives, e.g. barium hydroxide, polymethylmethacrylate, styrene and silicone resin applied on the stones and studied for their decay and durability charac teristics, the silicone resin has been found most effective preservative for the conservation of these stones. It forms a strong adhesion bond with the stone material and also preserves the original colour, structure and texture of the stones.