Evaluation of compatible mortars to repair 19th century natural cement cast stone from the French Rhône-Alpes region

Abstract

In France, natural cements were produced extensively in the middle of the 19th century. In the French Alps, due to their ocher color, these cements were massively used, notably to produce cast stone to simulate natural freestone. A preliminary survey revealed an overall good state of preservation of the buildings of this period. Two kinds of decay mechanisms were, however, identified: erosion affecting the surface of the majority of the buildings, inducing a gradual disappearance of the initial "fake-stone aspect," and a spalling phenomenon often combined with salt crystallization, observed on only a few buildings. Today, due to a lack of appropriate repair materials, the rehabilitation of these buildings mainly consists in the use of gray portland cement-based mortars combined with a paint finishing, which is not satisfactory, considering the conservation deontology, as the original appearance is lost. Therefore, the aim of this project was to develop and test compatible repair materials to restore the cultural heritage of this region. Based on preliminary characterization of a set of representative ancient buildings combined with a literature review, specifications concerning the composition and main properties of repair materials that could assure a compatibility with the ancient concrete of the region were established. Then, three "prompt" cement- and one portland cement-based mortars were selected, two of which were specifically formulated. First, the appearance, workability, and mechanical and physical properties of those mortars were characterized. Second, to evaluate the compatibility of the selected mortars with ancient concrete, prompt cement-based slabs were cast using a 19th-century concrete formula and were artificially eroded. After applying the four mortars on the slabs, visual observations and pull-out tests will be carried out before and after artificial aging. Finally, the repair mortar presenting the best performance will be tested on-site in a monument of Grenoble. [Editor's note: The Portuguese title of this article is: "Avaliação de argamassas compatíveis com a reparação de pedra artificial moldada em cimento natural no século XIX, da região francesa de Rhône-Alpes."]