Early 20th-Century Mortars from Teatro Nacional de São João (Porto, Portugal): Characterization of Degradation Mechanisms

Teatro Nacional de S. 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 sys- tem based on traditional load-bearing masonry walls with large granite blocks. In con- trast, other architectural elements, such as slabs, staircases, and exterior decorative com- ponents, 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 frame- works 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. Neverthe- less, 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 sam- ples 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 mor- tar composition, while capillary water absorption and mechanical strength tests were con- ducted 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.