# An Insight into the Chemistry of Cement--A Review
## 2 The Manufacture of OPC  
OPC is a fine grey/white powder consisting of a mixture of calcium silicates, aluminates, and aluminiofferites. These raw materials can be classified into four distinct groups: calcareous, siliceous, argillaceous, and ferriferous. Such raw materials are thermally treated via pyrolysis and mechanically processed in order to obtain the desired product with specific compositions and defined mechanical properties. Cement manufacturing implies four different steps: quarrying, raw material preparation, clinkering, and cement preparation [42]. Figure 1 schematically represents the production line of an OPC [43]. The entire production process begins with the extraction of the previously described raw materials that are rock mixes mainly constituted by ca. 80% of limestone (i.e., primarily calcium carbonate mineral CaCO\({}_{3}\)) and 20% of either clays or shale (i.e., a source of silica SiO\({}_{2}\), alumina Al\({}_{2}\)O\({}_{3}\), and hematite Fe\({}_{2}\)O\({}_{3}\)) [44]. Once raw materials are extracted, they are pre-crushed inside a quarry. Subsequently, crushed raw material is prepared by applying a variety of blending and sizing operations to confer appropriate chemical and physical properties to the feed. These processing steps can be divided into two different approaches, namely, either dry or wet. Concerning the dry route, both limestone and clays are independently crushed, and then fed together inside a mill. The wet route, instead, requires clays to be mixed forming a paste inside a wash mill (i.e., a tank where clays are grinded in the presence of water), and crushed limestone is added only at the end. Finely ground materials are dried, thermally treated inside a kil, and then cooled down. This series of steps is defined as clinkering and it confers the main relevant features of the OPC manufacturing. In fact, clinkering reshapes the starting raw materials mixture into clinkers, which are grey, spherically shaped nodules whose diameter is 5-25 mm in size [45]. The chemical reactions occurring inside the kil, kept heated by the spontaneous ignition of the pulverized coal dusts introduced inside the system due to the high temperatures, is still not completely understood. These chemical reactions are influenced by several factors, such as the large variety in terms of starting chemical compositions, the operating parameters of processing, and the difficulties in performing an efficient in situ sampling under the processing conditions (i.e., high temperatures). However, the chemical reactions and physical processes involved in this step can be rationalized as follow:  
1. Evaporation of the physically sorbed water molecules from the raw mix (20-100 \({}^{\circ}\)C). 2. Dehydration (100-430 \({}^{\circ}\)C) with the production of oxides, such as silica, alumina, and hematite. 3. Calcination (800-1100 \({}^{\circ}\)C) with the development of calcium oxide, according to the carbonate decomposition reaction: CaCO\({}_{3}\)\(\rightarrow\) CaO + CO\({}_{2}\)
4. Exothermic reactions (1100-1300 \({}^{\circ}\)C) with the formation of secondary silicate phases: 2 CaO + SiO\({}_{2}\)\(\rightarrow\) 2 CaO \(\cdot\) SiO\({}_{2}\)
5. Sintering and reactions occurring inside the melt (1300-1450 \({}^{\circ}\)C) with conversion of secondary silicate phases into both ternary silicates and tetracalcium aluminioerrites: 2 CaO \(\cdot\) SiO\({}_{2}\)+ CaO \(\rightarrow\) 3 CaO \(\cdot\) SiO\({}_{2}\) 3 CaO \(\cdot\) Al\({}_{2}\)O\({}_{3}\) + CaO + Fe\({}_{2}\)O\({}_{3}\)\(\rightarrow\) 4 CaO \(\cdot\) Al\({}_{2}\)O\({}_{3}\)\(\cdot\) Fe\({}_{2}\)O\({}_{3}\)
6. Cooling of the system, and the crystallization of the other mineral phases. Finally, the resulting clinker is further united with gypsum and then ground. From the energetic viewpoint, the grinding process consumes ca. 60% of the total electrical energy involved in a standard cement plant (high cost). In a conventional cement making process, the electrical energy consumed is approx. 110 kWh/tons, with ca. 30% used for raw material preparation and ca.