Charles Bridge: connection of material knowledge of antiquity and modern times. Fill of the Gothic Charles Bridge in Prague (Czech Republic) presents a carefully designed mortared rubble masonry which gained benefit from the use of hydraulic lime. The hydraulic lime was prepared by burning a raw material mixture composed of low-silica/alumina-free limestone and clayey-calcareous silicite. This binder mixed with clayey-calcareous silicite as an aggregate formed material which properties resemble modern lightweight structural concrete (LSC).