Lime Repair Mortars for the Conservation of Swiss Molasses: Analysis, Aesthetic Considerations and Swelling Performance

Molasse sandstones have been widely used in historical constructions across Switzerland. They exhibit the usual alteration patterns of sandstone, such as sanding, erosion, and alveolisation, along with less conventional scaling detachment due to swelling clays. In heritage constructions, damaged or missing elements are often repaired with mortars, whose composition must resemble the original stone as much as possible. This paper aims to develop repair mortars, made with Villarlod molasse stone waste and aerial lime used as sand and binder, respectively. The sand fraction was prepared by crushing and sieving stone waste and selecting a suitable grain size interval, whereas aerial lime (Ca(OH)2) was used to substitute the calcitic cement of the stone (CaCO3). Thermogravimetry (TG), optical microscopy (OM), and electron microscopy (SEM) were used to study the evolution and microstructural properties of mortars. Swelling tests, CIE-L*ab and roughness measurements were also conducted to measure hydric expansion and validate aesthetic features. SEM examination revealed the formation of relatively homogeneous calcitic matrices, with few cracks and well-adhered grains. The formation of white patina derived from lime carbonation was an important limitation of the approach. Therefore, several treatments were studied to uncover the underlying surface using soft mechanical methods and weak acid solutions. Citric acid (20% w/v) produced similar roughness and little colour difference with the stone. Finally, the hydric expansion was significantly reduced in repair mortars regardless of the swelling inhibitor dose. The results suggest that the random distribution of clays in crushed stone and their removal from the initial interstices may play a crucial role in swelling.