Unveiling the hidden deterioration of the Alcazaba of Almería (Spain): A comprehensive study on arabic architectural materials

The Alcazaba of Almería (Spain) is one of the most important Arabic fortresses of the Iberian Peninsula archaeological record. It is located in the south of Spain, on the Mediterranean coast, and its construction dates back to the late 10th century. It consists of three enclosures, two of which belong to the Andalusian period, while the last one dates from the 16th century. However, very few materials’ studies have been performed in this complex, despite the incomparable historic relevance. In this study, we present an extensive mineralogical and physical-chemical characterization of the construction materials of this site, divided in mortars, earthen materials and bricks. We provide an unprecedent scientifical-technical study of the building materials of this fortress, so as to understand the building techniques used and the causes of deterioration. All collected samples were analyzed by means of X-ray diffraction, scanning electron microscopy, optical microscopy thermogravimetric analysis and stereo-zoom microscopy. Our results reveal that main phases used for construction materials on this site were Ca and Mg-carbonates, accompanied by Fe-Ca-Mg silicates. Mortars showed a carbonate-binder composition with non-hydraulic nature whereas calcination of raw materials occurred below 800 ºC due to the permanence of dolomite. Furthermore, we demonstrated that the construction materials from the Alcazaba suffered such relevant damages due to the precipitation of salts (i.e., gypsum and halite) caused by coastal environment, key information for future archaeological restoration work in this highly significant location. We show that, in mortars and earthen materials, halite nucleates in fractures and dissolution pits exerting a crystallization pressure ∼ 100 MPa to cause fracture. Furthermore, calcium and magnesium silicate hydrates were observed confirming some pozzolanic properties. Finally, the bricks studied here demonstrated that firing T maximum of 700 ºC due to the presence of carbonates. This work opens new pathways to develop specific consolidation methods of materials and structural elements, as well as to ensure adequate restoration and conservation, of similar archaeological sites in coastal-related environments