Changing colour

Published 30 November 2021

Brown or yellow colours in the clinker core are indicative of unfavourable burning conditions such as reducing conditions in the kiln, according to specialists. At Tehran Cement Co, researchers have carried out a laboratory study into the effect of colour changes in the clinker core on the technical characteristics of cement. By Ali Akbar Kafash Bazari, Tehran Cement R&D Center, Iran, and Mohsen Tadayon, Iran Concrete Association, Iran.

Figure 1: microscopic image of clinker with reducing conditions

According to cement industry experts, the brown to yellow colour in the clinker core indicates unfavourable burning conditions. Numerous studies have shown that the brown colour in the clinker core (BCCC) indicates the occurrence of reducing conditions in the kiln, high temperatures in the burning zone and coarseness of raw materials.

Reducing conditions in a kiln occur when CO is formed from the decomposition of lime (CaCO3) instead of CO2 due to a lack of oxygen or a large amount of fuel (incomplete combustion). Therefore, oxygen is supplied from other sources such as Fe2O3 in the clinker and converted to FeO or Fe. This change in iron capacity reduces the blackening effect of iron oxide.

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