Hydraulic reactions characterizing ancient mortars at the archaeological site of Petra

In the sixth century BC Petra was conquered by the Nabateans who built an elaborate water system and turned a desert city into an artificial oasis and a prosperous centre controlling the main commercial routes of the region (100 BC-100 AD). In 2007 it was added to UNESCO's prestigious list of World Heritage Sites, as one of the seven wonders of the modern world. The aim of this research is to analyse in detail the hydraulic components of the mortars, which constituted Petra's complex water system at the basis of the great prosperity of the city. A detailed study of the hydraulic components of Petra mortars will allow us to understand their composition, technique and reactions which makes these structures waterproof and very durable. Samples were collected from Petra in order to study the compounds of hydraulic mortars and the hydraulic reactions they were subjected to. In particular, samples from The Petra Great Temple and The Petra Garden and Pool Complex have been analysed. Each sample comes from a hydraulic structure, specifically from pipe-lines and tanks. A first characterisation was carried out by optical microscopy observations on thin sections for choosing specific reaction areas, which underwent subsequently to a more detailed microchemical and morphological analysis (SEM). The aggregate, analysed by optical microscopy, consists mainly of carbonate rocks fragments, quartz grains, crushed bricks fragments and rare chert. Furthermore the analysis shows that the development of hydraulic components is concentrated near several quartz grains and chert fragments, which show an evident reaction rim, and within fractures and porosity. In the latter case, acicular crystals were observed, these are probably calcite re-crystallizations. Nevertheless, during SEM analysis the presence of calcium silicates and aluminates were detected in these areas. Moreover, the presence of Portlandite was also found. The outcomes of the study, other than having historic significance, will be useful for the research of new restoration materials, so they can be as much as possible compatible with the original ones. Furthermore, it is proved that the study of past techniques is helpful also in the field of modern construction.