The content of Fe2O3 in white cement may vary over the range of 0.35 to 0.5 wt.%.

If the limit is exceeded, the iron oxides are responsible for a greenish shade of the clinker which would be undesirable for example for architectural applications of white cement.

The following reducing reactions of coloring ions take place during manufacture of white cement clinker in a reducing atmosphere:

Fe3O4 + CO 3FeO + CO2

Fe2O3 + CO 2FeO + CO2

MnO2 + CO MnO + CO2

2CrO3 + 2CO Cr2O3 + 2CO2 +  O2 [18].

The colouring effect of Fe, Cr, and Mn cations depends on their valency in the crystalline phases in cement. Oxidized forms of free oxides of Fe3+, Mn3+ and Mn4+ absorb more light than reduced forms Fe2+ and Mn2+.

The higher the temperature and/or the concentration of the reducing agent such as CO, the whiter the cement appears to human eye. This is important particularly in the cooling of white cement clinker.

If it is cooled slowly from the initial temperature in the kiln (1200 – 1100 °C or lower) in the presence of oxygen, the coloring oxides of higher valent metals will be reformed.

Contrarily, if the cooling is quick, the high-temperature reduced crystalline forms are “frozen-up” and the light absorption will be lower.

Development of the methods of bleaching white cement to increase the whiteness, the clinker is fired under the conditions mentioned above and the bleaching is affected by rapid quenching with water.

The clinker at 1250 – 1350 C is withdrawn from the kiln through a special outlet into a water bath and hence passes into a drying chamber where it is dried at temperatures not exceeding 300 C. According to Grachyan, Gayzhotov, Zubyechin and others the bleaching by quenching proceeds in order to fix the phase composition of the clinker formed at high temperatures and exhibiting a high reflection degree.

On slow cooling, the iron oxides are bound to calcium aluminoferrites C6A2F, whereas on fast quenching they are fixed to a smaller number of particles of type C6A2F aluminoferrites while separating white calcium aluminates.

The whiteness of cement increases with increasing content of tricalcium silicate (C3S) and aluminate, and decreases with increasing content of dicalcium silicate (C2S).

In 1937, Cherepovskii and Aleshinova submitted a method of bleaching in oxygen-free atmosphere at 1100 – 1200 C, where at a low concentration of CO2, dissociation to Fe3O4 takes place as a result of different partial pressures of the oxides.

The clinker obtained in this way acquires a higher whiteness degree because Fe3O4 suppresses the coloring ability of iron oxides. Implementation of the method requires introduction of special bleaching coolers inhibiting penetration of oxygen from the ambient atmosphere.

The gas employed for bleaching (containing O2 0.2 vol.%, CO2 5 vol.%), prepared by heating producer gas in special kilns, is then passed to the hermetically sealed bleaching cooler.

Clinker from the kiln passes into this bleaching cooler and is cooled down to 200 C while not coming into contact with the ambient atmosphere, i.e. with oxygen.

As demonstrated by Zubyechin´s experiment [5], positive results were obtained by firing the clinker in an oxygen-free atmosphere and subsequently bleaching it in a water bath. Grachyan has devised an efficient two-stage bleaching method.

The clinker discharged from the sintering zone is cooled for 1-2 minutes in a converted gas and then enters a water bath.

The converted gas is obtained by reacting natural gas with water vapor at 900 – 1000oC according to the equation CH4 + H2O CO + 3H2.

In statu nascendi, carbon monoxide and hydrogen exhibit high reactivity and have strong reducing effects on iron and manganese oxides.

The scientists also succeeded in achieving a higher whiteness degree by bleaching in water with an addition of magnesium, as well as in solutions of dilute acids, hydrochloric, sulphuric and others.

It is assumed that in a gaseous environment or during rapid bleaching in a water bath of low-ferric clinker the whiteness is promoted by reducing the valency of iron oxides, by changing the coordination of coloring oxides and as a result of adjusting the proportions of the aluminate and silicate phases.

In 1990 a Chinese study dealt with the effects of additions of chlorides – NaCl, CaCl2, NH4Cl to the raw material mix at temperatures above 650 C.

The chloride reacts with iron oxides in the raw material and forms volatile FeCl3 which is withdrawn with the flue gases.

However, the method is only suitable for the wet clinker-making technology and for raw materials with a higher Fe content. In the case of the dry technology the chlorides vaporize in dependence on the degree of reacting with the iron oxides.