THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUEgaseous phase, diffusion in aqueous phases is much slower, by someorders of magnitude. In case of waters rich in carbon dioxide, however,this can accelerate the carbonation.6.7.3. Lime MortarThe iron content of lime mortars is based, in particular, on the admixedsand (up to 4 % Fe 2 O 3 ). Lime mortar is manufactured using onlyburnt lime (CaO), sand, and water and acquires its solidity through thebinding of slaked lime (Ca(OH) 2 ) with atmospheric carbon dioxide tolime (CaCO 3 ). This procedure takes only days or weeks (depending onthe thickness of the particular layers), due to the cruder porous systemwhich facilitates the diffusion of gas. For fresh lime mortar, an extremehigh water content can be damaging, as the carbon dioxide necessaryfor the binding process can no longer penetrate into the wall.The final ph value of this material lies within the neutral range.Since this medium no longer provides sufficient protection for steelreinforcement rods and offers only slight environmental resistance, it isusually used for the plastering of interior walls and for interior brickwalls only, in the latter case often mixed with cement. 400 The specificsurface of lime mortar lies considerably beneath that of cement mortar(up to one order of magnitude). 404 The water content is similar to cementmortar.6.7.4. Effects upon the Formation of Iron BlueThe first step in the formation of Iron Blue in masonry is the absorptionof gaseous hydrogen cyanide. A cool (10°C ) wall in a cellarwith atmospheric humidity near the saturation point, due to its higherwater content (by a factor of at least 10), has an increased ability (by afactor of 10) to absorb hydrogen cyanide compared to warm walls in aheated room built above ground with lower atmospheric humidity(20°C, 50% rel.).The second step in the formation of Iron Blue is the ionic split(disassociation) of the hydrogen cyanide, that is, its conversion intosimple cyanide. 405 This procedure requires an alkaline environment,404 The reason: no formation of very finely crystalline alumosilicate with higher surface area.405 In masonry, this largely corresponds to the neutralization of the hydrogen cyanide by calciumhydroxide Ca(OH) 2 into calcium cyanide Ca(CN) 2 .185