White Portland cement is also used in combination with inorganic pigments to produce brightly colored concretes and mortars.
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 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 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.
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).
The reason for this is the relatively small amount of molten liquid produced during sintering, because of the low iron-content of the mix.
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.
Here calcium sulfate is added to control set, in the form of a high-purity grade of gypsum or anhydrite.
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.
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.
Because the color so much depends upon the "bleaching" and "quenching" operations, merely specifying a low iron content does not guarantee good whiteness.