Iron Cement

When ferric oxide replaces aluminium oxide in cement, calcium ferrites are probably formed. Percy combined lime and ferric oxide at white heat. By adding lime-water to a neutral solution of ferric chloride and igniting the resulting precipitate, a compound corresponding to the formula CaO.Fe₂O₃ is obtained. Pelouze obtained 4CaO.Fe₂O₃ by precipitation. According to Dufau, however, the pure compound cannot be isolated because it is unstable.

Hilpert and Kohlmeyer, on studying the behaviour of mixtures of calcium and ferric oxides in the electric furnace, concluded that the following compounds existed: calcium orthoferrite, 3CaO.Fe₂O₃, melting-point 1410° C., disintegrating like calcium orthosilicate on cooling; 3CaO.2Fe₂O₃, melting-point 1450° C.; 5CaO.3Fe₂O₃, formed by reaction in the solid state at 1220° C.; 2CaO.3Fe₂O₃, melting-point 1400° C.; and probably calcium metaferrite, CaO.Fe₂O₃, formed below the eutectic temperature. They found also that mixtures containing calcium oxide of a molecular percentage of 60-70 are hydraulic.

Cooling curve and micrographic investigations seem to indicate the presence of only mono- and di-calcium ferrite.

From a fused mixture of lime, ferric oxide, and alumina, mixed crystals containing both ferric oxide and alumina crystallise out. The cooling curves of fused mixtures of silica, lime, and ferric oxide indicate the formation of ferrous silicate and a double compound, 2Fe₂SiO₄.3Ca₂SiO₄.

More lime can be safely introduced into cements rich in alumina than into those rich in ferric oxide. Ferric oxide, like alumina, acts as a flux.

In the hydration of iron Portland cement, tricalcium ferrite and ferrous silicates are formed. According to Ludwig, ferric oxide contributes nothing to the hardening of the cement, and acts only as a flux by forming a fusible glass with alumina, no combination with lime taking place, and probably none with silica.