THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUE6.7.1.2. ExcursusThe chemical composition of bricks varies massively due to thedifferent sorts of mar and loam used as initial material. The content ofclay (included in this are 20 to 60% Kaolinite, consisting roughly of47% SiO 2 , 40% Al 2 O 3 , 13% H 2 O) may lie between 20 and 70%, therest being carbonate, finest sand and iron oxides. 390 According to myown analyses, the latter content may vary between 2 and 4%.The porosity values of bricks lie between 20 and 30 vol.%, 391 accordingto other sources up to 50%. 392 According to my own mercurypenetration tests, the pore size of bricks lies heavily concentratedaround 1 µm. 393Due to the decreased specific surface (0.5 to 1 m² per g, BET, 394own tests), the reactivity of the iron oxide is strongly reduced. However,partly dissolved iron at brick surfaces immediately exposed toweathering can be set free for reactions in bigger amounts.The normal free, i.e., not chemically bound water content of bricksin dry rooms (20°C) is in the area of one volume percent, but it can riseup to 4% at a relative humidity of over 90%. 3956.7.2. Cement Mortar and Concrete6.7.2.1. OverviewThe rust content (Fe 2 O 3 ) of Portland cement, of particular interestto us here, the cement most frequently used for concrete and cementmortars, is usually between 1 and 5%. 396 The sand added to the mortarcan also exhibit a high iron content (up to 4%). As mentioned in chapter6.5.3., a large surface area at the solid-liquid phase limit (iron oxide-cyanidesolution) is favorable to the formation of Iron Blue. This isextraordinarily large in cement and concrete mortars (microscopic inte-390 O. Hähnle, Baustoff-Lexikon, Deutsche Verlagsanstalt, Stuttgart 1961, p. 384.391 Landolt-Börnstein, Zahlen und Funktionen aus Physik, Chemie, Astronomie, Technik, volumeIV Technik, part 4b Wärmetechnik, Springer, Berlin 6 1972, pp. 433-452.392 S. Röbert (ed.), Systematische Baustofflehre, volume 1, VEB Verlag für Bauwesen, Berlin4 1983, p. 120.393 These mercury penetration tests were performed at the research institute of the VARTA BatterieAG in Kelkheim, Germany, in late 1991.394 Method to determine the specific surface with nitrogen adsorption following Brunauer, Emmet,Teller.395 K. Wesche, Baustoffe für tragende Bauteile, volume 1, Bauverlag, Wiesbaden 1977, p. 37.396 W.H. Duda, Cement-Data-Book, Bauverlag, Wiesbaden 1976, pp. 4ff., as well as my ownanalysis.181