PRODUCTION OF BLENDED AND SPECIAL CEMENT IN CHINA

摘要

In 2003 about 750 million tons of cement were produced in China, which is approximately 40% of the world production. Thereby about 640 million tons of clinker have been produced. The balance are cement additives such as limestone, gypsum and natural pozzolans as well as ground granulated blast furnace slag (GGBFS) and hard coal fly ash (FA), which are byproducts from the steel and power plant industries. The production of 640 million tons of clinker requires about 960 million tons of raw materials, typically 75% of that is limestone, while the rest are clay and shale or limestone substitutes such as marl and dolomite. Accordingly, the clinker production of 640 million tons per year requires about 720 million tons of limestone. On the contrary, the base limestone reserve for China is given with 29 billion tons and the part, that could economically be extracted is only 22.4 billion tons. This means, that the limestone reserves for clinker production will only last for another 31 years, resp. 40 years, if the actual clinker production rate is kept. To overcome the situation and to improve the availability of the limestone reserves in China at a cement production of about 750 million tons, only the clinker factor in the cement can be lowered. This can be achieved with a higher use of blended and special cements. The basic clinker substitutes in cement are GGBFS and FA, which are already used in China, but to a relatively low extend, when compared with the available resources. Other substitutes are natural pozzolans and silica fume, which have only very limited resources. The clinker substitutes can improve the cement characteristics, including strength, workability, hydration heat and resistance to sulphate attack, when compared with Ordinary Portland Cement (OPC). In China high quality cements have to refer to a number of different national standards, including GB 18736-2001, which regulates the properties of GGBFS in concrete, and GB1596 for fly ash used in cement and concrete. In the presentation an overview about prerequisites for the production of blended and special cements is given and how this could effect the resource situation. Instead of a less economical combined grinding of different cement raw materials, a wide variety of composite Portland cements can be produced by mixing (blending) the individual components, which have been processed separately. Depending on the requirements, a continuous or batch type mixer has to be used. The differences and advantages of the one or the other type are clearly presented.