Gypsum and spalling decay mechanism of tuffeau limestone

The general form of degradation of tuffeau is the progressive spalling of the surface of the stone. Over time, thick plates (1- to 3-cm thick) form gradually on the surface of the stone. Once the plate drops, the resulting stone surface turns into powder. At the Castle of Chambord, mineralogical analysis of the degraded stone, throughout its depth, shows the presence of gypsum (CaSO₄·2H₂O) located mainly within a crack network parallel to the surface. The objective of this research is to study different hypotheses of gypsum formation to improve the understanding of the damaging process of spalling. Fresh stones were subjected to artificial ageing in the laboratory through imbibition/drying tests, to study the resulting distribution of gypsum as a function of several parameters: the source of gypsum, initial stone cracking, drying conditions, and analysis after different measurement sequences. Tested samples were analysed at different stages of ageing by using X-ray diffraction and ion chromatography to localise and quantify the amount of gypsum throughout the depth. Results indicate that gaseous SO₂ can lead to a distribution of gypsum very similar to that observed in stones subjected to spalling in situ at the Castle of Chambord.