Durability assessments of rare green andesites widely used as building stones in Buca (Izmir), Turkey

Miocene volcanism was active in the city area of İzmir, Turkey, where volcanic deposits formed predominantly made of gray and pinkish colored andesite. These rocks have been widely used in the construction of many buildings in the region. In the northern part of Izmir, known as the Buca region, there is an area where andesite presents green color. The occurrence of the green andesite is rather rare, and historically this rock type has been used in the construction of ancient valuable architectonic structures including aqueducts, historical buildings, and mosques. Some of these structures date back to the beginning of the first century AD when the region was under the control of the Roman Empire. Interestingly, the recent survey of these structures showed that after all of these years, the green andesite still appears pretty much well preserved, only showing slight deterioration in the form of limited crumbling. Although these rocks were used in this particular region in the past, their material properties have not been investigated so far. The purpose of this study was therefore to identify mineralogical, chemical, physical, and mechanical properties of these rocks, which eventually determine their durability. The first phase of the research focused on detailed site investigation, which resulted in the definition of the geological extent of the green andesite and the identification of two historical quarries, from one of which representative samples was collected for laboratory investigation, which was the main task of the second phase of the research. The main cause for the green color of the andesite resulted the presence of celadonite, which is also accompanied by cristobalite. The presence of cristobalite in the pores as a secondary mineral appears to have contributed to the increased strength and durability of Buca andesite. Laboratory evaluation of the durability was in general agreement with field observations, although some test gave ambiguous results. This discrepancy revealed some weakness of laboratory tests to precisely assess durability of building stones.