Portland cement high iron

In the hydration of high-iron Portland cement the calcium alrrminate ferrite phase reacts with the calcium sirlfate present to yield the AFt (ettringite) phase. As the... 

In contact with MgSO the reaction of the srrlfate ions with the constituents of the cement paste is essentially iderrtical. Parallel to it, however, a decalcification of the C-S-H phase occurs, resirlting in a distinct strength loss of the material. Bracite [Mg(OH)2] is precipitated in the surface zone, in addition to gypsum. Because of these additiortal reactions, high-iron Portland cement does not exhibit any improved resistance to MgSO attack, contrary to the action of other soluble sirlfates. 

The strength development and most other properties of high-iron Portland cement differ only slightly from those of ordinary Portland cement. Its color, however, is distinctly darker. 

High-iron Portland cements tend to fulfill the existing requirements for cement with increased resistance to sulfates. They are used mainly in applications in which contact of the resulting concrete with water or soils having an elevated sulfate corrterrt must be expected. High-iron clinkers are also constituents of some oil well cements. 

Thus it is apparent that the CaCOj content in the raw meal, and hence the energy consumption, may be reduced by increasing in the clinker the amount of phases with lower CaO contents at the expense of those in which the CaO content is higher. Consequently, high-C2S Portland cement (see section 2.4) and high-iron Portland cement (see section 2.8) qualify as low-energy cements, whereas a cement high in C3S does not. 

Special purpose and blended Portland cements are manufactured essentially by the same processes as ordinary Portland cements but have specific compositional and process differences. White cements are made from raw materials of very low iron content. This type is often difficult to bum because almost the entire Hquid phase must be furnished by calcium aluminates. As a consequence of the generally lower total Hquid-phase content, high burning-zone temperatures may be necessary. Past cooling and occasionally oil sprays are needed to maintain both quaHty and color. 

Little information is available about the corrosion of metals in concrete, although it seems likely that all Portland cements, slag cement and high-alumina cement behave similarly Concrete provides an alkaline environment and, under damp conditions, the metals behave generally as would be expected e.g. zinc, aluminium and lead will react, copper is unaffected, while iron is passivated by concrete. 

The rust content (Fc203) of Portland cement, of particular interest to us here, the cement most frequently used for concrete and cement mortars, is usually between 1 and 5%.396 The sand added to the mortar can also exhibit a high iron content (up to 4%). As mentioned in chapter 6.5. 3., a large surface area at the solid-liquid phase limit (iron oxide-cyanide solution) is favorable to the formation of Iron Blue. This is extraordinarily large in cement and concrete mortars (microscopic inte-... 

Sulphate-resisting Portland cements have relatively high ratios of iron to aluminium, and the ferrite phase cannot have the composition given above if it contains most of the iron. Tables 1.2 and 1.3 include a tentative composition and atomic ratios corresponding to it, based on scanty data for the interstitial material as a whole (G3,G4) and the requirement of reasonable site occupancies. 

Portland cement is the most common hydraulic cement. It is formed by clinkering a mixture of powders of limestone, sand, iron oxide and other additives at a very high temperature ( 1500°C). It is mixed with water to form hydrated bonding phases... 

Portland cement is a finely ground, powdered mixture of compounds produced by the high-temperature reaction of lime, silica, alumina, and iron oxide. The lime (CaO) may come from limestone or chalk deposits, and the silica (Si02) and alumina (AI2O3) are often obtained in clays or slags. The blast furnaces of steel mills are a common source of slag, which is a byproduct of the smelting of iron ore. 

Portland cement (see section 0.00) is hardly suitable for use in acid environments. No significant differences have been found between the performances of ordinary and high-iron, I0W-C3A Portland cements, but cements with a higher C3S content, which liberate more calcium hydroxide in the course of hydration, seem to be more susceptible to acids (Matthew, 1992). Partial replacement of the clinker by pulverized fly ash does not... 

In terms of chemical composition. Class G and Class H oil well cements are low-CgA Portland cements, and the specifications of both are typically met by high-iron, sulfate-resistant Portland cements (see also section 2.8). The free lime content in these cements is not limited by the API specifications, but it should be below 0.5 wt% to avoid difficulties with cement slurty rheology and retarder response. In producing Class G and Class H oil well cements, iron oxide (in the form of hematite or pyrites residues) must usually be added to the raw mix to produce more ferrite phase at the expense of tricalcium aluminate. The required amount of Fe203 is greater when a cement of HSR rather than MSR grade is produced. 

Corrosion is an electrolytic process that requires an anode, a cathode, and an electrolyte. In reinforced concrete the reinforcing steel becomes covered in a passivating layer of y-iron oxide formed in the high-pH environment ( pH 13) provided by the Portland cement. Unfortunately, the high pH can be neutralized by acidic gases such as carbon dioxide in the atmosphere. Once the pH in the concrete drops below 10, generahzed corrosion of the steel will take place in the low-pH area. 

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