Potential effects of the shape of gypsum aggregates on the early sulfation of marble and travertine

The damages caused by growth of gypsum crusts on calcareous substrates are well known. Sulfation of stones such as marble and travertine – used in constructions and Cultural Heritage – implies a structural and aesthetical weathering that leads, in time, to flaking, spalling and disruption. The effects of the initial gypsum growth and development were studied by simulating acid attacks on marble and travertine test specimens, evaluating the potential mechanical actions exerted by the shape of the newly grown aggregates. By monitoring the incipient sulfation with optical microscopy and SEM-EDS – backed up by XRPD, a peculiar growth sequence was confirmed: i) at first, tabular ‘Montmartre’ twins grow close and roughly parallel to the calcareous substrate; ii) later, a second generation of prismatic crystals grows on top of the former twins, in aggregates perpendicular to the calcareous matrix. When confined at the grain boundaries or within the narrow pores/fissures typical of marble and travertine textures, these gypsum overgrowths (basal platy twins topped by prismatic crystals) may exert strong crystallization pressures on the host carbonatic matrix, coupled to disjoining pressures granted by persistence of superficial liquid films at the host/guest interface. These phenomena cause the prismatic crystals, shielded by the older twins, to push away the facing calcareous matrix with a repulsive strength that, once the tensile strength is exceeded, brings to flaking, powdering, disruption and spalling. Strictly crystallographic aspects might thus concur to the early deterioration of calcareous substrates after sulfation, favouring mechanical actions that contribute to the damaging effects of gypsum crusts on the surface of buildings and Cultural Heritage in calcareous stones.