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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