Manufacturing of Portland cement
Portland cement is made by the mixing of limestone and clay or shale, then heating the mixture in a rotary kiln at a temperature of 1450 C. the materials supplied from quarries are crushed and blended into a uniform composition and fine size enough to enable the reaction at the rotary kiln. The heating of raw material will produce clinker. The clinker is grounded with gypsum to form the portland cement. Figure 1 showing the stages of manufacturing portland cement. The basic raw materials used to produce cement is limestone, clay, and iron ore.
The chemical composition of cement:
- Major compounds: major compounds of portland cement is listed in table number 1. C2S and C3S occupy 68% to 75% of portland cement. Since the primary constituents of Portland cement are calcium silicates, we can define Portland cement as a material that combines CaO and SiO2 in such a proportion that the resulting calcium silicate will react with water at room temperature and normal pressure.
- Minor compounds: the important minor components includes gypsum and alkali sulfate. Gypsum (2CaSO4 · 2H2O) is added to the clinker at the final stage of cement manufacturing. Gypsum will prevent the flash setting of cement as a result of fast-reacting of C3A. Gypsum will react with C3A and form ettringite, which acts as a barrier that stoping further reaction of C3A. Gypsum added to cement by a percentage of 4-5%. Alkalies (MgO, Na2O, and K2O) increases the ph of concrete up to 13.5, and this increase will be useful for protecting the reinforcement from corrosion, but it may react with aggregate that contains silica. Alkali silica reaction (ASR) will cause a destructive expansion of concrete. The durability of concrete will be reduced as a result of ASR.