Optimization of Traditional Lime-based Mortars for Cement Replacement Toward Consistent Conservation of Rocky Architecture of the Kandovan Historical Village

Kandovan historical village, located in the vicinity of Osku town, is one of the most important attractive Geoheritage sites in East Azerbaijan province in Iran, thanks to the hewn-cut houses from Sahand Mountain. The rocky architecture of Kandovan, from a geological point of view, is on the surfaces of a thick Ignimbrite layer, which, due to natural weathering and erosion along the main joints and discontinuities of Kandovan valley, have created conical forms and structures. From a conservation point of view, these deterioration factors have resulted in poor living conditions, abandonment and seasonal residence in these rocky houses. Compared to the other Igneous rocks, Tuff (ignimbrite) is a light rock with a high ratio of porosity and proper workability. Therefore, it has been used in the construction of many stone heritage buildings in different countries. This kind of stone, therefore, requires permanent care and maintenance. Over the past half century, in response to the decay and damage of these monumental stone houses, villagers used Portland cement (as an accessible and useful material in construction industry) for repair and maintenance of their rocky houses. Given the differences in terms of porosity, thermal and humidity expansion coefficient between Kandovan volcanic tuffs and cement and also due to other characteristics of cement like continuous generation of destructive salts, it became one of the destructive forces of the Kandovan heritage site.
`The aim of this study is to develop a vernacular mortar to replace sand-cement mortar in order to sustain conservation in Rock-hewn Architecture of Kandovan historical village. For this purpose, the study was carried out with field and laboratory investigations; the fieldwork was conducted from the perspective of pathology and identifying the causes of damage. Different modes of water were identified as the main deterioration factors. In order to protect the surface and using mortars in the outer surfaces exposed to erosion and also filling the cracks and structural joints, 24 type of mortar were prepared. Mortar type specimens were made by mixing (fine-grained sand, wood ash, Pozzolanic and soil), binder (hydrated lime) and water. All of the samples prepared were in the cubical forms.
In order to determine the most resistant mortar type, three accelerated aging tests, including salt crystallization attack with sodium sulphate anhydrous (Na2SO4) as well as wetting and drying and freezing and thawing cycle tests were performed. The results show that Pozzolanic lime mortar (PLM), prepared by mixing crushed Kandovan tuff, wood ash, and lime paste as a binder, exhibits high resistant against the aging process. In the next step, characterization and comparative study between Kandovan tuffs and PLM was carried out via X-ray powder diffraction and the relevant physical–mechanical properties of that were determined by measuring the capillary water absorption, water absorption on atmospheric pressure, P wave ultrasonic velocity, void index dry density, total porosity content, compressive strength and slake durability. The results demonstrated that Kandovan tuff stone has poor durability and high sensitivity to predisposing factors to deterioration, such as wetting and drying and freezing-thawing cycles. Furthermore, our results show that the mechanical properties and resistance of PLM are as twice as and its density is half of that in Kandovan tuffs. Therefore it can be used for conservation and repair and replace cement in Kandovan village.