role of iron and alumina in liquid phase?

It is possible to make C3S by mixing very finely ground silica and calcium oxide and firing in a laboratory furnace at high temperature, then removing the mix, regrinding very fine and burning again, and again and again if you want all the ingredients to mix.  However, this is a very uneconomical and impractical way tomake C3S.  In the cement kiln the alumina and iron form a melt phase with calcium oxide and into this melt more CaO is dissolved.  When dissolved it is transported to the silica and C2S which has already formed and the combination is possible in one pass.

The ratio of alumina to iron controls the temperature at which the melt phase forms.  Because the outside of the kiln is cooler than the inside it also controls where coating forms due to the relative stickiness of the melt.  By changing the ratio of alumina to iron oxide you also change the temperature at which coating will form.  A sudden influx of iron oxide in a typical clinker will make the liquid form earlier in the kiln and will probably make the coating strip away. Iron oxide is the easiest to use to control the ratio because it is relatively pure and can be added or removed in very small quantities to have an effect.  If there is a pure source of alumina this could be used, but these are expensive unless a suitable waste can be used.

Hello Raju,

In addition to all of the reasons that Arthur mentioned, the ratio of alumina to iron also controls the ratio of C3A to C4AF in the clinker, as well as the viscosity of the liquid phase. (Liquid phase viscosity generally decreases with increasing iron content.)

For a given burning zone temperature, clinkers with higher liquid phase viscosity tend to nodulise better than those with made with low viscosity liquid. Higher viscosity liquid phase is also less damaging to the refractory lining.

Regards,

Ted.

thanks