Cement is the most used construction material in the world. When cement is mixed with water, the resulting chemical reaction transforms this powder into solid rock that the builder can shape and mold. Due to this and other physical and chemical characteristics, this fine powder is often called the “magic powder.”
Joseph Aspdin, an English bricklayer and stonemason, invented Portland Cement in 1824. The word Portland is neither a brand name nor trademark, it is a generic term used to describe the hydraulic property of cement. Aspdin was the first to formulate and get a patent for this cement. He called it “Portland” because the concrete made with this product resembled the color of the stone quarried on the Isle of Portland, a peninsula in the English Channel west of the Isle of Wight.
Clinker (95%) is cooled by air and subsequently ground with gypsum (5%) to form Ordinary Portland Cement (“OPC”). Other forms of cement require increased blending with other raw materials. Blending of clinker with other materials helps impart key characteristics to cement, which eventually govern its end use. Limestone is crushed to a uniform and usable size, blended with certain additives (such as iron ore and bauxite) and discharged on a vertical roller mill, where the raw materials are ground to fine powder. An electrostatic precipitator dedusts the raw mill gases and collects the raw meal for a series of further stages of blending. The homogenized raw meal thus extracted is pumped to the top of a preheater by airlift pumps. In the preheaters the material is heated to 750°C. Subsequently, the raw meal undergoes a process of calcination in a precalcinator (in which the carbonates present are reduced to oxides) and is then fed to the kiln. The remaining calcination and clinkerization reactions are completed in the kiln where the temperature is raised to between 1,450°C and 1,500°C. The clinker formed is cooled and conveyed to the clinker silo from which it is extracted and transported to the cement mills for producing cement. For producing OPC, clinker and gypsum are used and for producing Portland (Pozzolana) Cement (“PPC”), clinker, gypsum and fly ash are used. In the production of Portland Blast Furnace Stag Cement (“PSC”), granulated blast furnace slag from steel plants is added to clinker.
Fly ash is a fine grey powder consisting mostly of spherical glassy particles that are produced as a by-product in coal fired power stations. Fly ash has pozzalonic properties, meaning that it reacts with lime to form cementitious compounds. It is commonly known as a supplementary cementitious material. Fly ash significantly improves concrete performance in various ways and also provides many benefits in cement and non-cement applications. Thermal generating utilities produce coal fly ash. A light, airborne particulate that results from the combustion of pulverized coal, fly ash accounts for 70% to 85% of all coal ash produced. At present 88% of fly ash produced in Europe is recycled. (1999 ECOBA).
Kaolin is a soft white mineral that has an array of uses. It is most commonly found in the form of kaolin clay, a fine clay which was originally produced in China, which is why this clay is sometimes referred to as “China clay.” Among the many uses for this mineral are applications in the paper industry, medications, skincare products, porcelain and cosmetics. Sources of this mineral can be found all over the world, including the United States, China, Brazil, Australia, and parts of Eastern Europe. Mixed with cement it becomes natural kaolin.
Coal is used as an energy source in the cement industry. Large amounts of energy are required to produce cement. Kilns usually burn coal in the form of powder and consume around 450g of coal for about 900g of cement produced. By-products generated from burning coal in coal-fired power plants such as fly ash, bottom ash, boiler slag and flue gas desulphurization gypsum are also used in concrete production (source WCA). Fly ash can be used to replace cement in concrete.
Petcoke is a carbonaceous solid derived from oil refinery coker units or other cracking processes. Its high heat and low ash content make it a good fuel for power generation in coal-fired boilers, but petroleum coke is high in sulphur and low in volatile content, which poses some environmental and technical problems in its combustion. Calcined petroleum coke is the product of calcining petcoke. This coke is the product of the coker unit in a crude oil refinery. The calcined petroleum coke is also used to make anodes for the aluminium, steel and titanium smelting industry.