Portland cement is the most common type of cement and consists of lime, silica and alumina. All these ingredients are readily available in abundance in nature in the form of chalk and clay. Cement is made at high temperature (about 1500°c) by either a wet process or a dry process. It basically consists of mixing the ground components and feeding into a rotary kiln against a cross flow of hot air. As the mixture come down, it undergoes a series of reactions as the temperature increase. The water is evaporated. Then, as it becomes hotter the chalk is decomposed to give quicklime. At 1500°c, the final fusion takes place and all the constituents melt and react together to form the cement clinker. The clinker is cooled and ground to form cement powder. Some gypsum is added to the powder and the Portland cement is ready for use.
To form concrete, water is added to cement in controlled amount. The cement reacts to the water (hydration) to form a thick paste which has some cohesion but still mouldable and even pourable. At this stage heat is evolved and this stage is called setting. Setting is the stage when the concrete remains workable. After that, hardening starts.