Compatibility of Roman cement mortars with gypsum stones and anhydrite mortars: The example of Valère Castle (Sion, Switzerland)
Gosselin C., Girardet F., Feldman S.B.
12th International Conference on the Deterioration and Conservation of Stone, Columbia University, New York, 22-26 October 2012
Durable and reversible restoration of stones and historical mortars is a major concern to those interested in conservation of historical structures and contemporary practices of restoration are continuously revisited with the help of the technical and scientific researches. However, the historical feedback on repairing techniques recently showed that Roman cements (RC), developed and widely used through the XIXth Century, were particularly well-adapted to repair historical masonries.
The current article presents a case study of RC mortars applied on gypsum stones and historical anhydrite mortars, both soluble and known to be sensitive to the chemical compatibility with hydraulic binders. Mineralogical analysis of samples from the basilica Notre Dame de Valère (Sion, Switzerland) shows that late XIXth C. RC joints and renders have perfectly lasted in contact with the structural gypsum stones and anhydrite mortars from the XIIIth C.
Results from XRD and SEM work suggest that the present RC was produced at a temperature high enough to form significant amounts of C2S and C2AS, remaining unreacted after very long term hydration. The extent of C2S hydration is notably reduced due the precipitation of silica gel, a carbonation product, at the boundary of the cement grains. The high capillary porosity developed during hydration is homogeneously distributed, enhancing the transport properties.
These conclusions were supported by complementary observations. First, elemental mapping through the strong RC /anhydrite mortars interface does not indicate any accumulation of sulfate salts at the boundary. Additionally, in contrast to the RC mortars, the rapid expansion and degradation of grey Portland cement mortars was observed, confirming the limitations of the latter applied on gypsum stones.