Damage from Moisture Cycling in Clay-Bearing Sandstones: Swelling Inhibition Mechanism in Villarlod Molasse

Many sandstones in buildings and monuments of cultural importance, such as Villarlod molasse, contain swelling clay minerals which can cause damaging stresses through wetting and drying cycles. Understanding the mechanism by which these clay minerals swell is key to understanding how to mitigate that swelling and therefore the damaging stresses. Previous work has demonstrated that for Villarlod molasse, similar to other sandstones with swelling clays, the swelling takes place predominantly via crystalline swelling caused by hydration of interlayer cations, and thus can be mitigated with diaminoalkane swelling inhibitors that substitute the native interlayer cations and restrict subsequent swelling. Furthermore, the expansion of the clay interlayer has been linked quantitatively to the swelling strain by combining X-ray diffraction (XRD) with swelling experiments, meaning that through a scaling factor one can predict the interlayer spacing through a simple swelling experiment. This facilitates interpretation of the mechanism through which the diaminoalkane swelling inhibitor works, and is the focus of this study. Here it is shown that the diaminoalkane inhibits through substitution and restricted rehydration, similar to other stones, and that there is also a d-spacing of about 11 Å for treated clays in the dry stone, similar to that predicted for other stones, and indicative of a conformation of the diaminoalkane not seen in XRD experiments of clays in the loose state. This is hypothesized to be due to the external pressure the stone matrix exerts on the clay interlayers. Implications of this for the treatment of Villarlod molasse is discussed.