THE RUDOLF REPORT

GERMAR RUDOLF · THE RUDOLF REPORTsive for the reactivity towards gases. If the main pore volume is formedby micropores, then the gas diffusion into the material is more inhibitedthan if the main pore volume is formed by larger pores. Graph 7shows the accumulated pore volume distribution of concrete and onewall mortar (exact composition unknown, since taken from an old wall,but according to its brittle consistency probably a lime mortar).Having a similar total pore volume like the wall mortar (here only14% due to the test method), the concrete’s largest portion of pore volumelies between a pore radius of 0.01 and 0.1 µm, whereas the wallmortar’s largest portion lies between 0.1 und 10 µm. Hence, if comparedwith the wall mortar, the gas diffusion into the concrete will bedisadvantaged. In general, the average pore size of cement buildingmaterials changes to larger values when increasing the content of sandand lime.Fresh concrete is relatively strongly alkaline, caused by the highcontent of calcium hydroxide, which, however, gets bound as calciumalumosilicates rather quickly. However, depending on the type of cement,a certain amount of it is released as time goes by. The pH valueof non-carbonated concrete is around 12.5. It later falls, however, dueto the binding of carbon dioxide from the air.The speed of carbonation into the depth of the concrete dependsstrongly on the consistency and porosity of the material and follows asquare root relation: 403D = c · t (5)d = depth of carbonationc = constantt = timeIn water tight concretes, it takes many years for the limit of carbonationto advance only a few centimeters due to the inhibition ofdiffusion in this highly compact material.In the area of carbonation, the pH value decreases to roughly 7,the equilibrium value of saturated calcium carbonate solutions. But ifthe wall is wet, this results in a proton exchange and therefore no sharppH border is formed. If a large portion of the air pores (size in the orderof a tenth of a millimeter) flooded with water poor in carbon dioxide,the carbonation advances more slowly, because compared to the403 W. Czernin, op. cit. (note 397); Verein Deutscher Zementwerke, op. cit. (note 401); N.V.Waubke, op. cit. (note 398).184