Walls of the Castle of Alcácer do Sal – Contribution to the Study of Mortars in Military Architecture

The strategic location of Alcácer do Sal, on the banks of the Sado River and in a region rich in natural resources, explains the various occupations of this territory throughout history. However, it was the influence of the Muslim occupation (715–1217) that stood out the most, still visible today through archaeological remains, the town’s name, and, in this case study, the castle walls (11th–12th centuries), which have undergone several interventions over time. A visual inspection of the walls was conducted to assess their current condition. This revealed that the main causes of deterioration are related to water infiltration due to insufficient maintenance, lack of proper drainage, and the absence of protective elements such as coping. These issues have led to the loss of render adhesion, biological colonization, and material disintegration in several areas. The mortar samples were characterized using a multi-analytical approach, including Optical Microscopy (stereo zoom and petrographic microscope), X-ray Diffraction, Scanning Electron Microscopy with Energy Dispersive X-ray Spectroscopy, Thermogravimetric Analysis, acid attack, and granulometric analysis. The results indicate that the mortars have similar compositions in terms of raw materials and likely provenance. An intervention strategy was proposed to address the identified pathologies and support future conservation efforts.