Re: egg yolk
Brown cores are quite commonly seen within clinker nodules when they are broken. Clinker which appears to be normal, with a dark grey colour, is found to have brown cores when the nodules are broken. This phenomena is brought about by high temperature reduction of Fe3+ to Fe2+ at the high temperatures in the burning zone. This happens after the C3S has been formed and is therefore not detrimental for strength development of cement (although colour may be affected). All clinker will be reduced to some extent by the high temperatures in the burning zone of a cement kiln. After the clinker passes under the flame the secondary air rising from the cooler re-oxidises the Fe2+ to Fe3+, but large clinker nodules may not be completely re-oxidised through to the core and remain brown in the centre. Porosity, as well as size, is a factor determining the re-oxidation of the cores, low SM clinker may be less porous, due to lower flux content, and therefore more prone to brown cores.