Portland cement clinker contains four principal chemical compounds, which are normally referred to as the clinker minerals. The composition of the minerals and their normal range of levels in current UK and European Portland cement clinkers are summarized in Table 1.1.

Table 1.1 Ranges of principal minerals in European clinkers

It is the two calcium silicate minerals, C3S and C2S, which are largely responsible for the strength development and the long-term structural and durability properties of Portland cement.

However, the reaction between CaO (lime from limestone) and SiO2 (silica from sand) is very difficult to achieve, even at high firing temperatures.

Chemical combination is greatly facilitated if small quantities of alumina and iron oxide are present (typically

5% Al2O3 and 3% Fe2O3), as these help to form a molten flux through which the lime and silica are able to partially dissolve, and then react to yield C3S and C2S.

The reaction requiring the greatest energy input is the decarbonation of CaCO3, which takes place mainly in the temperature range 700–1000°C. For a typical mix containing 80% limestone the energy input to decarbonate the CaCO3 is approximately 400 kCal/kg of clinker, which is approximately half of the total energy requirement of a modern dry process kiln.

When decarbonation is complete at about 1100°C, the feed temperature rises more rapidly. Lime reacts with silica to form belite (C2S) but the level of unreacted lime remains high until a temperature of ~1250°C is reached. This is the lower limit of thermodynamic stability of alite (C3S).

At ~1300°C partial melting occurs, the liquid phase (or flux) being provided by the alumina and iron oxide present. The level of

unreacted lime reduces as C2S is converted to C3S. The process will be operated to ensure that the level of unreacted lime (free lime) is below 3%.

Normally, C3S formation is effectively complete at a material temperature of about 1450°C, and the level of uncombined lime reduces only slowly with further residence time.

The ease with which the clinker can be combined is strongly influenced by the mineralogy of the raw materials and, in particular, the level of coarse silica (quartz)

present. The higher the level of coarse silica in the raw materials, the finer the raw mix will have to be ground to ensure satisfactory combination at acceptable kiln temperatures.

Coarse silica is also associated with the occurrence of clusters of relatively large belite crystals around the sites of the silica particles. Figures 2 (a) and (b) are photomicrographs of a ‘normal’ clinker containing well-distributed alite and belite and clinker produced from a raw meal containing relatively coarse silica.

The belite present in the clusters is less reactive than small well-distributed belite and this has an adverse influence on cement strengths.

As the clinker passes under the flame it starts to cool and the molten C3A and C4AF, which constitute the flux phase, crystallize.

This crystallization is normally complete by the time the clinker exits the rotary kiln and enters the cooler at a temperature of ~ 1200°C.

Slow cooling should be avoided as this can result in an increase in the belite content at the expense of alite and also the formation of relatively large C3A crystals which can result in unsatisfactory concrete rheology (water demand and stiffening).