The Rheingantz complex, dating from the 19th century, is composed of several elements, such as the factory, daycare center, and workers' house, and is located in the Rio Grande in Rio Grande do Sul, Southern Brazil. Currently, part of the complex is in the process of restoration. From this point of view, the great importance of the buildings for the conservation of the memory and cultural identity of the city and state suggests the study, of the historical mortars, in a pavilion, a former warehouse of the complex, lent to the Federal University of Rio Grande-FURG. The present paper aims to characterize the historical mortars of the pavilion's external coating. To identify the primary constituent materials, the mortars were characterized using tests suggested in the bibliography, such as X-ray diffraction (XRD), mechanical compressive strength, and water absorption. The characterization of the mortar samples showed that they are mainly composed of calcite and quartz, in a 1:5 ratio (binder: aggregate), and also have a significant presence of dolomite, which is indicative of a mortar substantially formed by dolomitic lime and sand. The results provided subsidies for the formulation of mortar suitable for restoration with characteristics similar to the original, providing mechanisms to reduce future pathological manifestations.