THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORTwhich, in lime mortars, lasts only for a few days or weeks, but whichare present for months or years in cement mortar and concrete.The next step is the formation of iron(III)-cyanide, a process thathardly occurs in a strongly alkaline environment and which occursslowly in slightly alkaline environments. In the neutral range, this reactionis once again slowed down because the cyanide converts into nonreactive,volatile hydrogen cyanide by the humidity in the wall. Theenvironment around the carbonation limit of concrete and mortar(which is slightly alkaline), can therefore be addressed as the area inwhich iron(III)-cyanide can form easily. In a strongly alkaline area ofthe masonry, it only arrives at this prior stage of Iron Blue formationthrough the slow detour of the reduction of slight traces of iron(III)-cyanide to iron(II)-cyanide. A large surface area, as found in cementmortars and concrete, is especially favorable to the solid-liquid interfacereaction between solid rust and cyanide in a liquid solution. Thesegenerally have the advantage of retaining an alkaline medium forlonger periods of time, so that the cyanide accumulated in the masonryis not lost and has enough time to react with rust. Once again, a highwater content, which broadens the range of moderately alkaline acidvalues, is advantageous. 406 The reduction of a part of the iron(III)-ionsto iron(II)-ions finally, the next to last step in Iron Blue formation, requiresa moderately alkaline acid value, but also occurs in the stronglyalkaline range. A distinction can be made between three areas of differentreactivity in masonry:1. Larger quantities of cyanide ions can accumulate in the noncarbonatedportion, due to the alkaline medium, further favored bythe increased absorption of hydrogen cyanide by the still-humidmaterial. The cyanide is only bound as iron(III)-cyanide to a slightextent. This is converted quite rapidly into the more stableiron(II)-cyanide due to its strong oxidation behavior in the alkalinemedium. An accumulation of iron(III)-cyanide will therefore takeplace over a longer time period.2. In the zone of carbonation, the tendency to accumulate cyanide isreduced, since the disassociation equilibrium lies increasingly onthe side of the hydrogen cyanide. The oxidation strength of theiron(III)-cyanide is also diminished. On the other hand, the pig-406 Very humid mortars and concretes, due to proton diffusion, exhibit no sharp carbonation, i.e.,pH limit.186