Study of ancient dolomitic mortars of the church of Santa Marı́a de Zamarce in Navarra (Spain): comparison with simulated standards

Ten ancient mortars from the Church of Santa Marı́a de Zamarce (Navarra, Spain) have been studied by means of chemical analysis, X-ray diffraction and simultaneous thermogravimetric and differential thermal analysis (TGA–DTA). All the studied samples have shown to be dolomitic mortars. Sixteen simulated standards of typically present compounds in dolomitic mortars were prepared and studied. Particularly the occurrence of hydrated magnesium carbonate hydroxide (hydromagnesite (HY), Mg5(CO3)4(OH)2·4H2O) as result of the setting of dolomitic mortars is discussed. No HY has been checked in mortars from Santa Marı́a de Zamarce. However, TGA–DTA studies have clearly shown the thermal decomposition of HY in the prepared mixtures, specially the exothermic peak at 500 °C. The experimental conditions in TGA–DTA, with a high CO2 pressure around the sample, seem to play an important role in the occurrence of this phenomenon. Suitable experimental conditions for thermal studies of these materials have been indicated.