GRAVEL LIME ? Research into Danube gravel as the main ingredient of the historic mortars at the castle Prandegg in Upper Austria (original) (raw)

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 was how the necessary amounts of quicklime were produced and transported. As analyses of historic mortar samples at the castle ruin of Prandegg showed constant slight impurities of magnesia/silicium and non-burnable pebbles with glass-like surfaces were found in the cores of the ruin walls, the assumption came up that the source of the historic quicklime could be burned Danube pebbles. Danube gravel is a mixture of limestones and quartz pebbles, which could be able to cause the discovered hydraulic effects. So a complete reconstruction of the historic mortar of Prandegg was done by burning Danube river gravel again in an archeotechnical experiment. During the slakings all burned limestone pebbles turned completely into homogeneous lime putty. The reconstructed " Danube gravel lime " mortar was then laboratory confirmed as it showed aside from the magnesia and silica phases a general similarity to the historic mortar in its chemical and physical structure.