This zone slightly overlaps, and is sometimes included with, the calcination zone. As the temperature continues to increase above ~ 900˚C there is still no melting, but solid-state reactions begin to occur. CaO and reactive silica combine to form small crystals of C2S (dicalcium silicate), one of the four main cement minerals. In addition, intermediate calcium aluminates and calcium ferrite compounds form. These play an important role in the clinkering process as fluxing agents, in that they melt at a relatively low temperature of ~ 1300˚C, allowing a significant increase in the rate of reaction. Without these fluxing agents, the formation of the calcium silicate cement minerals would be slow and difficult. In fact, the formation of fluxing agents is the primary reason that portland (calcium silicate) cements contain aluminum and iron at all. The final aluminum- and iron-containing cement minerals (C3A and C4AF) in a portland cement contribute little to the final properties. As the mix passes through solid-state reaction zone it becomes “sticky” due to the tendency for adjacent particles to fuse together.