White Portland cement

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.

In general, the rotary kilns used to chemically combine the raw materials are operated at a higher peak temperature (1600°C) than that required for gray clinker manufacture (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 molten 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 and fluoride in the form of calcium fluoride or waste cryolite. This combination lowers the melting 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 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, 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 water and removing it quickly with a screw or passing it through a curtain of water sprays. 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 manufacturer.

The temperature in the kiln is not necessarily higher than gray clinker production but it appears higher due to the reflectance especially in gas fired kilns (which do have a higher flame temperature). With off-white clinker coal firing can be used which has many advantages for costs and output. It is important to finely grind the coal and have a burner which gives a very short bright flame (high velocity low primary air) so that the excess oxygen can be kept low close to zero with maybe 0.1% CO. There is no need for a second flame if combustion in the coal flame is rapid and controlled. The Off-white clinker has a light greenish colour due to Fe(II). Rotary kilns have limitations on output based on the surface area required for the heat transfer. Few old kiln process can reach the surface area limits due to limits in auxiliary equipment such as fuel input (e.g. lack of precalcination), grinding of coal and raw materials, exhaust fan capacity, chemical control of raw mix, burner quality, kiln control etc. With well operated off-white clinker production kiln output can be only a few percent lower than gray clinker production.

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.