The origin of Mg sulphate and other salts formed on pure calcium carbonate substrate – Tufa stone blocks built into the Gradac Monastery, Serbia

• Magnesium-sulphate salts are formed on pure calcium carbonate substrate.
• Natural and anthropogenic factors are responsible for salts efflorescence.
• The main sources of magnesium are original mortars, repair lime cement mortar mix and robber core.
• Mechanism of the salt formation is partly confirmed by laboratory simulation.

Epsomite and hexahydrate occur in the form of efflorescence on the church walls made of pure calcium carbonate – tufa stone. A few sources of magnesium are responsible for the precipitation of Mg-salts at the block surfaces: lime mortar with Mg-rich aggregates, repair lime cement mortar, the dolostone of the rubber core, and the monastery basement composed of Mg-rich rocks. Possible sources of sulphate are: repair lime cement mortar and soil solution. The permanent presence of different solution types in the tufa pore system enabled the precipitation of salts in the following order: calcite/dolomite, gypsum, Mg-sulphate phases, syngenite and blodeite.

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