THE RUDOLF REPORT

6. FORMATION AND STABILITY OF IRON BLUETable 6: Composition of Portland cement 402Al 2 O 3 : 5 to 10 % K 2 O: 0.2 to 0.6 %SiO 2 : 20 % Na 2 O: 0.5 to 3 %CaO : 60 % Fe 2 O 3 : < 5 %The specific surface of the cement powder is in the order of 3,000cm 2 per g. Concrete and cement mortar get their stability by hydrationof the cement compounds calcium oxide CaO (burnt lime), siliciumdioxide SiO 2 (Quartz), iron and aluminium oxide Fe 2 O 3 /Al 2 O 3 tomixed, microfibrous calcium alumosilicate-hydrates with a chemicallybound water content of some 25 mass %. 401 It then has a specific surfaceof up to 200 m 2 per g when measured with water adsorption,which is an extremely high value. Other methods (e.g. BET-measuringwith nitrogen) yield a value of only 1 / 3 of this or less. 397 The porosity ofmortar and concrete heavily depends on the amount of water addedduring preparation and lies at a minimum of 27% according to the literature,400 in which case the volume of the microcapillary pores betweenthe silicate fibers is included as well, which cannot be determinedwith mercury penetration measurings.Aside from the absolute porosity, the pore size distribution is deci-accumulated relative porevolume [%]2520lime mortar1510concrete500.001 0.01 0.1 1 10 100pore radius [µm]Graph 7: Accumulated pore volume distribution of concrete,according to “Forschungs- und Materialprüfungsanstalt, Abteilung1: Baustoffe” (Research and Material Testing Agency, Department1: Building Materials), Stuttgart, and of wall mortar, own analysis.In each case determined by Hg penetration.401 Verein Deutscher Zementwerke, Zement Taschenbuch 1972/73, Bauverlag, Wiesbaden 1972,pp. 19ff.402 W.H. Duda, op. cit. (note 396).183