DRAFT 12th International Congress on the Deterioration and Conservation of Stone Columbia University, New York, 2012 1 ROLE OF SWELLING CLAY MINERALS IN THE SPALLING DECAY MECHANISM OF THE “PIERRE DU MIDI” LIMESTONE (SOUTH-EAST OF FRANCE) Jérémie Berthonneau 1,2,3 , Olivier Grauby 2 , Philippe Bromblet 3 , Jean-Marc Vallet 3 , David Dessandier 1 and Alain Baronnet 2 1 BRGM, 3 Av. Claude-Guillemin B.P. 36009, 45060 Orléans Cedex 2, France. 2 CINaM-CNRS & Aix-Marseille Université, France. 3 CICRP Belle-de-Mai, 21 rue Guibal, 13003 Marseille, France. Abstract Widely used into Provençal building heritage, the "Pierre du Midi" shows a heterogeneous durability in situ. While some historical buildings made up with this Burdigalian (Miocene) limestone do not display any superficial modification, others show specific stone deterioration pattern called spalling after comparable time of exposure. With variable extent, this decay pattern sometimes includes the loss of rock fragments. The affected stones are located on the upper façade parts, subjected to the stronger climatic variations, but free from salt weathering, mechanical charges, biologic colonization or atmospheric pollution. In order to understand the decay process triggered by the climatic variations, the present study aims to define the internal properties involved in the macroscopical mechanical behavior leading to spalling. Eight stone types, representative of the material heterogeneity, were collected from both active and given up quarries spread inside four geological sub-basins located in the South-East of France. The extraction sites have first been replaced inside the sedimentological profile of Miocene deposits. Then, comparison of common hydro-mechanical characteristics (including mineralogy, water transfer properties, hygric and hydric dilatation) with the decay extent has shown the occurrence of mixed-layer minerals containing smectitic layers in the most subjected samples to spalling. As these smectitic layers are known to undergo intracristalline swelling upon water content variation, a specific attention has been given on their quantification by combining mineralogical and chemical methods : oriented X-ray diffractogram fitting procedure, and TEM-EDS analysis of the infra 4μm fraction. The relationships between swelling clay minerals proportion, their location and the dilatometric behavior measured are finally discussed into the frame of this study. Keywords: spalling, clay minerals, hydric and hygric expansion, intracristalline swelling. 1. Introduction A large part of the building heritage dated between the 1 st century and the XVII th century in the PACA region (South East of France) have been erected using a bioclastic limestone commonly named “Pierre du Midi”. On the basis of their extraction site, different limestone types have been distinguished. And, once exposed to environmental conditions a heterogeneous durability can be observed. On one hand, among others deterioration patterns, spalling (Vergès-Belmin et al. 2008) is observed on façades of historical buildings Figure 1. After a field campaign, it has been pointed out that the