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The facing masonry of the Gothic Charles Bridge in Prague (Czech Republic) has been largely altered during nineteenth and twentieth century repairs, due to extensive replacements of weathered sandstone ashlar blocks. Natural stone varieties used during these replacements show different responses to a variety of weathering processes which lead to their rapid decay. The decay of the newer stone has been accelerated by use of Portland cement paste as the binding and fill material, instead of the original hydraulic lime-based and cocciopesto-type mortars. The hardened Portland cement paste makes an almost impermeable barrier, with permeability three to four orders of magnitude lower, compared to the original mortars and natural stone. The low permeability of this new alien material results in the accumulation of water-soluble salts (specifically nitrates, but also sulphates and chlorides) in the facing masonry ashlars, with their crystallisation coming in the form of both sub- and efflorescence, as well as the development of various decay forms (blistering, granular disintegration and/or scaling and flaking). The source of the water-soluble salts can be traced back to previous restoration attempts. These included chemical cleaning (black crust removal) by nitric acid, ammonium hydroxide ('ammonium water'), or ammonium hydrogen carbonate, all which can be linked to high nitrate content in the efflorescence. [ABSTRACT FROM AUTHOR]