Approximately 95% of clinker consists of the oxides of CaO, SiO2, Al2O3 and Fe2O3 (but present in combined form as the clinker minerals) and the remainder consists of the so-called minor constituents.

Table 1  indicates the typical UK levels of the most commonly encountered minor constituents and summarizes their impact on the cement manufacturing process.

The inputs of alkali metal oxides (Na2O and K2O), SO3 and chloride have to be closely controlled because they are volatilized in the kiln and can cause severe operational problems associated with their condensation and the formation of build-ups in the kiln ‘back end’ and preheater.

The alkali metals Na2O and K2O have a very strong affinity for SO3 and a liquid phase containing Na+, K+, Ca2+ and SO42– ions is formed which is immiscible with the main clinker liquid (molten C3A and C4AF). On cooling this crystallizes to yield alkali sulfates such as K2SO4, aphthitalite (3K2SO4·Na2SO4) and calcium langbeinite (K2SO4· 2CaSO4).

The crystallization products depend on the relative levels of the two alkali oxides and the level of SO3.

If there is insufficient SO3 to combine with the alkali metal oxides, then these may enter into solid solution in the aluminate and silicate phases.

C2S can be stabilized at temperatures above 1250oC thus impeding the formation of C3S. A similar stabilization of C2S, requiring ‘hard burning’ to lower the free lime level to an acceptable level, can occur if there is a large excess of SO3 over alkalis.

A deficiency of SO3 in the clinker is associated with enhanced C3A activity and difficulties in achieving satisfactory early age concrete rheology.

Fluorine occurs naturally in some limestone deposits, for example in the Pennines in England, and has a beneficial effect on clinker combination.

It acts as both a flux and mineralizer, increasing the quantity of liquid formed at a given temperature and stabilizing C3S below 1250oC. The level in the clinker, however, must be controlled below ~0.25% in order to avoid a marked reduction in the early reactivity of cement.

Minor constituents also have to be controlled on account of their impact on cement properties and also concrete durability. Related to this, the levels of alkalis, SO3, chloride and MgO are also limited by national cement standards or codes of practice.