White Portland cement or white ordinary Portland cement (WOPC) is similar to ordinary, gray Portland cement in all aspects except for its high degree of whiteness. Obtaining this color requires substantial modifications to the method of manufacturing. It requires a much lower content in colored impurities in the raw materials (essentially limestone and clay) used to produce clinker: low levels of Cr2O3, Mn2O3, and Fe2O3), but above all, a higher temperature is needed for the final sintering step in the cement kiln (1600 to 1700 °C in place of 1450 °C for ordinary Portland cement) because of the higher melting point of the mix depleted in iron oxides (serving as flux in Portland cement). Because of this, the process is more energy demanding and the white cement is somewhat more expensive than the gray product.
Uses 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 not economical 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, to guarantee repeatable color, some manufacturers supply ready-blended colored cements, using white cement as a base. To be "white", the powdered material must have a reflectance value ("L value") in excess of 85%. A particular success in the use of WOPC and added pigments is monocouche renders. In some countries an off-white clinker, which gives a reflectance value over 70 when ground, is produced at a cost only a little over normal gray clinker. When this is blended with ground blast furnace slag (up to 60% depending on use and early strength) a cement with reflectance over 80 can be produced. The blended cement can have a production cost less than General Purpose Portland cement (gray), but normally attracts a margin as it is sold to compete with white Portland cement.
Manufacture
Raw mix formulation The characteristic greenish-gray to brown color of ordinary Portland cement derives from a number of transition elements in its chemical composition. These are, in decreasing 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 limit the chromium contamination. With off-white clinker the calculated Fe2O3 level in clinker is higher (0.6-0.8%) Coal can be used (if the ash has little Fe2O3 or other trace elements). The ash in the coal is helpful in the reaction because it is finer than the ground raw materials and it reaches higher temperatures and is molten in the flame. Kaolin is sometimes found in association with coal deposits. It may be possible to use coal washery waste, oil shale and spent oil shale ash. Off-white clinker has a calculated C3A (tricalcium aluminate) of 7-9%. When blended with ground granulated blast furnace slag it can meet requirements for sulfate resistance and low heat.
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