The importance of soil mineralogy in erosion of adobe in the historical city of Belqis

Belqis is a city in north-east Iran made entirely of adobe. It is a major example of well-known architectural and archaeological adobe structures. The current study relates to the deterioration of these structures caused by humidity, intensely affected by the local soil quality and the mineralogical/chemical characteristics of clayey materials applied in the Belqis citadel. Examination proved that many factors play a key role in the durability of adobe. The samples collected were analysed by physical examination to determine various factors, such as soil gradation and the liquid limit of soil, to identify the soil texture and index. X-ray diffraction (XRD) and X-ray fluorescence (XRF) were used to define the chemical composition and crystalline phases produced during the deterioration of the adobe. Choosing the right soil was an essential step for preparing the adobe’s raw materials followed by optimal soil chemistry and phase characterisation. The soil is mostly sand and silt with a small fraction of clay. The clay fractions contained muscovite and calcite, contributing to the deterioration process in adobe. It was found that improving soil with zeolites of low to medium plasticity can be well suited for reconstructing adobe structures in the Belqis cultural heritage site.