Lime mortar of the Renaissance bridge in Brandýs nad Labem (Czechia): an electron backscatter diffraction study

Lime-based materials have been largely employed as traditional binders in mortars. Their characterization is aimed at gaining knowledge on the traditional production processes, and, ultimately, helps in choosing the correct replacement material for the conservation of cultural heritage. Analysis of mortars is a complex task that is usually accomplished adopting a multi-analytical approach. In this context, it is here reported an example of the level of information that can be retrieved from the electron backscatter diffraction technique (EBSD), which can be implemented in a conventional scanning electron microscope (SEM). The studied sample was obtained from the mortar employed in the construction of the medieval bridge of Brandýs nad Labem (Czechia). The inspection of the sample revealed the presence of large (up to 5 mm) white grains, which have been identified as lime lumps. According to their nature, these binder-related particles can provide information on the nature and the provenance of the raw materials and on the technology adopted for lime production and mortar preparation [1]. The mortar was observed in thin section under petrographic microscope (OM), whereas, polished sections of two lime lumps, removed from the matrix, were investigated with EBSD. Rietveld quantitative phase analysis (QPA) of both crystalline and amorphous fractions of the binder was performed by X-ray powder diffraction. In thin section the sample exhibits the typical texture of a lime mortar. The binder, with the high interference colors of calcite, homogeneously fills the space between the aggregate grains. The QPA evidenced the presence of all three CaCO3 polymorphs with the predominance of calcite (15 wt%) followed by minor amounts of aragonite (3.2 wt%) and vaterite (1.2 wt%). Notably, the amorphous fraction sums up to more than 50 wt%, indicative of silicate-hydrates, therefore, of a significant degree of hydraulicity of the mortar. Under SEM and OM, the lime inclusions appear porous, with occasional rims of hydration, without traces of overburning and relics of the original carbonate rock. They are prevalently composed of calcite (recarbonated) with diffuse presence of fine grained minerals, which have been identified with EBSD. The EBSD analysis allowed for the identification of calcium silicates (C2S, akermanite-gehlenite, wollastonite), brownmillerite, spinel and perovskite (Fig. 1). The following conclusions can be drawn: The analysis of the aggregate fraction and of the local geology, indicate that their provenance can reasonably be ascribed to the local fluvial deposits. The mortar employed in the construction of the Brandýs bridge is a lime mortar with significant hydraulicity. This is consistent with the burning of local limestone, as documented for other medieval artifacts in the region [2]. According to the mineralogical composition of the lime lumps, the burning temperature is in the range 850-1150 ºC [3], although the association brownmillerite-wollastonite-perovskite would suggest values above 950 ºC. The fact that a high number of nearly intact lime particles has been preserved during mortar processing, indicates that the hot mixing technique was adopted [1].