Gravel Lime? Research into Danube gravel as the main ingredient of the historic mortars at Castle Prandeggin Upper Austria

Abstract eng:
Lime was, aside from clay, the main historic mortar material for castles in the northern part of Upper Austria. As there are no geological sources of limestone in the region, the question posed was - how were the necessary amounts of quicklime produced and transported? The assumption that the source of the historic quicklime could be burned Danube pebbles arose as results of analyses on the historic mortar samples from the castle ruin of Prandegg showed constant slight impurities of magnesia/silica and non-burnable pebbles with glass-like surfaces in the cores of the ruin walls. The hydraulic properties of the mortars could be traced to the mixture of limestone and quartz pebbles common in Danube gravel. A complete reconstruction of the historic mortar of Prandegg was performed by burning Danube river gravel as part of an archaeological experiment. During the slaking all burned limestone pebbles completely turned into a homogeneous lime putty. The reconstructed “Danube gravel” lime mortar was then confirmed as the historical source as it showed, aside from the magnesia and silica phases, a general similarity to the historic mortar in its chemical and physical structure.

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