Early 20th-Century Mortars from National Theatre of São João (Porto, Portugal): Characterization of Degradation Mechanisms

National Theatre of São João (TNSJ), the main theatre in the city of Porto, was built in 1918. Its architectural design, by architect Marques da Silva, is characterized by a structural system based on traditional load-bearing masonry walls with large granite blocks. In contrast, other architectural elements, such as slabs, staircases, and exterior decorative components, incorporated a then “new” binding material—cement. The richly ornamented façades include a wide range of decorative features, from simple geometric motifs to floral elements and large-scale masks. These elements were constructed around steel frameworks and executed using mortars with a probable cementitious binder. Due to their high level of exposure, the façades of TNSJ were subject to repeated interventions. Despite this, significant degradation was observed over time, including the detachment and fall of mortar from decorative elements, which highlighted the need for further conservation actions. In this context, the façades were systematically inspected, and representative samples were collected to characterize material constituents, establish mix proportions, and identify the extent and causes of deterioration. Analytical techniques—including X-ray diffraction, acid dissolution, and optical microscopy—were employed to determine mortar composition, while capillary water absorption and mechanical strength tests were conducted to assess mortar properties. The results indicated that mortar carbonation and the presence of salts, likely associated with marine aerosol exposure, promoted the oxidation of embedded metallic elements, leading to the progressive degradation of the surrounding mortars.