Influence of ageing conditions on the mineralogical micro-character of natural hydraulic lime mortars

The investigation of the chemical composition and physical and mechanical properties of old building materials is relevant for planning restoration interventions and designing compatible restoration materials. The quality of the binders used in mortars determines their durability, and consequently, influences the durability of the building structures in which they are applied. Lime binders continuously undergo chemical alterations as they age. Moreover, weathering agents cumulatively affect their properties, which adds to the difficulty in identifying their original composition. The mineralogy and microstructure of the binders of three historic mortar specimens collected from medieval bridges in the Czech Republic and a mortar designed in the laboratory to mimic the composition of the historic specimens were characterised using thermogravimetry analysis with mass spectrometry, x-ray powder diffraction and scanning electron microscopy coupled with energy dispersive spectroscopy. Two specimens showed binder decalcification and the presence of metastable CaCO3 vaterite and aragonite polymorphs. A mortar of comparable age, which was not exposed to elevated moisture, contained no CaCO3 modification other than calcite. (C) 2020 Elsevier Ltd. All rights reserved.