White Portland cement

Uses

White Portland cement is used in combination with 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.

Manufacture

Rawmix formulation

The characteristic greenish-gray to brown color of ordinary Portland cement derives from a number of 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.

Kiln operation

In general, 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 titanium dioxide may be added to improve reflectance. At all stages, great care is needed to avoid contamination with colored materials.

Specifications

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.