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White Portland cement
White Portland cement is similar to ordinary, gray Portland cement in all respects except for its high degree of whiteness. Obtaining this color requires substantial modification to the method of manufacture, and because of this, it is somewhat more expensive than the gray product.
Additional recommended knowledge
White Portland cement is used in combination with white aggregates to produce white concrete for prestige construction projects and decorative work. White concrete usually takes the form of pre-cast cladding panels, since it is uneconomic to use white cement for structural purposes. White Portland cement is also used in combination with inorganic pigments to produce brightly colored concretes and mortars. Ordinary cement, when used with pigments, produces colors that may be attractive, but are somewhat dull. With white cement, bright reds, yellows and greens can be readily produced. Blue concrete can also be made, at some expense. The pigments may be added at the concrete mixer. Alternatively, in order to guarantee repeatable color, some manufacturers supply ready-blended colored cements, using white cement as a base.
The characteristic greenish-gray to brown color of ordinary Portland cement derives from a number of transitional elements in its chemical composition. These are, in descending order of coloring effect, chromium, manganese, iron, copper, vanadium, nickel and titanium. The amount of these in white cement is minimized as far as possible. Cr2O3 is kept below 0.003%, Mn2O3 is kept below 0.03%, and Fe2O3 is kept below 0.35% in the clinker. The other elements are usually not a significant problem. Portland cement is usually made from cheap, quarried raw materials, and these usually contain substantial amounts of Cr, Mn and Fe. For example, limestones used in cement manufacture usually contain 0.3-1% Fe2O3, whereas levels below 0.1% are sought in limestones for white manufacture. Typical clays used in gray cement rawmix may contain 5-15% Fe2O3. Levels below 0.5% are desirable, and conventional clays are usually replaced with kaolin. Kaolin is fairly low in SiO2, and so a large amount of sand is usually also included in the mix. Iron and manganese usually occur together in nature, so that selection of low-iron materials usually ensures that manganese content is also low, but chromium can arise from other sources, notably from the wear of chrome steel grinding equipment during the production of rawmix. See rawmill. This wear is exacerbated by the high sand-content of the mix, which makes it extremely abrasive. Furthermore, to make a combinable rawmix, the sand must be ground to below 45 μm particle diameter. Often this is achieved by grinding the sand separately, using ceramic grinding media to reduce contamination.
In general, the rotary kilns used to chemically combine the raw materials are operated at a higher peak temperature (1450-1500°C) than that required for gray clinker manufacture (1400-1450°C). This requires a higher fuel consumption (typically 20-50% more), and results in lower kiln output (typically 20-50% less) for a given sized kiln. The reason for this is the relatively small amount of liquid produced during sintering, because of the low iron-content of the mix. The final reaction in the kiln, conversion of belite to alite, requires the melt liquid as a solvent, and is slower if the amount of melt is low. This can be partially compensated by adding to the rawmix a combination of calcium sulfate and fluoride in the form of calcium fluoride or waste cryolite. This combination reduces the reaction temperature. In cases where the clinker Fe2O3 content is above 0.2% (which is almost always the case), the unique processes of "bleaching" and "quenching" are also employed. "Bleaching" involves directing a second flame (apart from that used to heat the kiln) onto the bed of clinker close to the kiln exit, in order to reduce Fe(III) to Fe(II). This reduction is rigorously avoided in gray cement production, because of the deleterious effect it can have on clinker quality. But in white clinker production, where the iron content is low, this is not an issue. Subsequently, in order to prevent the re-oxidation of the iron, "quenching" is performed. This consists of rapidly lowering the clinker temperature from 1200°C to below 600°C in a few seconds, as it leaves the kiln. This usually involves dropping it into cold water. This contributes to the relatively poor energy efficiency of the process, since the sensible heat of the clinker is not recycled as in normal clinker manufacture.
Clinker grinding and handling
The clinker is next ground to cement (perhaps after a drying stage). Here calcium sulfate is added to control set, in the form of a high-purity grade of gypsum or anhydrite. In some specifications (not ASTM), a small amount of titanium dioxide may be added to improve reflectance. At all stages, great care is needed to avoid contamination with colored materials.
White Portland cement differs physically from gray cement only in terms of its color. Its setting behavior and strength development are essentially the same as that expected in gray cement, and it meets standard specifications such as ASTM C 150 and EN 197. In practice, because much white cement is used in pre-cast concrete products, it is commonly made to a high-early strength specification such as ASTM C 150 Type III. This aids concrete manufacturers' production rate. Higher potential strength also helps to counteract the strength-diminishing effects of pigment addition. In addition to the usual specifications, manufacturers guarantee the whiteness of the product, typically in terms of a reflectance measurement, such as L*a*b L-value, or tristimulus. In the latter case, because off-color white cement tends to be greenish, the Tri-Y (green) value is used. Because the color so much depends upon the "bleaching" and "quenching" operations, merely specifying a low iron content does not guarantee good whiteness.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "White_Portland_cement". A list of authors is available in Wikipedia.|