Clinkering zone (1300˚C – 1550˚C): This is the hottest zone where the formation of the most important cement mineral, C3S (alite), occurs. The zone begins as soon as the intermediate calcium aluminate and ferrite phases melt. The presence of the melt phase causes the mix to agglomerate into relatively large nodules about the size of marbles consisting of many small solid particles bound together by a thin layer of liquid (see Figure 3-3). Inside the liquid phase, C3S forms by reaction between C2S crystals and CaO. Crystals of solid C3S grow within the liquid, while crystals of belite formed earlier decrease in number but grow in size. The clinkering process is complete when all of silica is in the C3S and C2S crystals and the amount of free lime (CaO) is reduced to a minimal level (