Compatibility of Repair Mortars with Nineteenth Century Natural Cement Cast Stone from the French Rhône-Alpes Region

In the French Alps, near Grenoble, in the middle of the nineteenth century, natural cements were massively used to produce “cast stone” (concrete block) and to simulate natural yellowish to reddish cut stone. In a first project, several historic concrete buildings were studied, and a major decay mechanism was identified: erosion, leading to the loss of the original fake stone appearance. Today, due to a lack of appropriate repair materials, grey Portland cement-based mortars and paintings are used, leading to a complete loss of the original aspect. Therefore, the aim of this second study was to develop and test compatible repair materials to conserve the cultural heritage of this region. Based on the results of the first project, specifications concerning the composition and main properties of compatible repair materials were established. Then, four mortars were selected, two of them being specifically formulated. In a second step, the intrinsic properties of those mortars were characterised, and finally, their mortar/concrete compatibility was assessed.