Introduction of slag cement
Blast-furnace slag (BFS) is formed from a liquid at 1350-1550 °C during the manufacture of iron. When reaching the bottom of the furnace, the liquid slag forms a layer on top of the molten iron due to its lower density. Being separated from the molten iron, the liquid slag is cooled in the air or with water. Depending on the way of cool process, three main categories of BFS could be produced, which are namely ground granulated BFS, pelletized slag and air-cooled slag.
Ground granulated blast-furnace slag (GGBFS), which normally contains up to 95% of glass, is formed by cooling the liquid slag with large amount of water and subsequently ground to fine powder.
If the liquid slag is partially cooled with water and then flung to air, pelletized slag could be thus produced. Comparing with GGBFS, pelletized slag contains much low glass content, e.g. 50%, therefore it is normally used as concrete aggregate or as raw materials to produce cement clinkers.
When cooled and solidified in the air, air-cooled slag is produced. During the production process, a water spray procedure is sometimes applied to accelerate the cooling process. Because of the hard and dense property, air-cooled slag is utilized in road bases, asphalt paving or as concrete aggregate, etc.
Among these three kinds of slag, GGBFS is the most valuable one in cement industry. Due to its cementitious properties mixing with lime, alkalis or Portland cement, GGBFS is often used to make blast-furnace slag cement. The history of the production of slag cement in Germany, France, Luxembourg and Belgium has been more than one century.
The usage of slag cement, which based on partial replacement of Portland cement by slag, has mainly three advantages. First, it offer cost reduction because blast-furnace slag is a byproduct of steel industry, which makes it cheaper than Portland cement on the one hand, though the increasing demand increases the market price more or less; on the other hand, utilization of byproducts from steel industry also protects environment. Second, the manufacture of slag cement requires 75% less energy than that needed for the production of Portland cement. The less energy demand brings environmental benefit, cost save, and less emission of carbon dioxide that causes the planet warmer. Third, the use of slag cement can arguably improve the properties of cement products, e.g. better concrete workability, lower permeability, improved resistance to aggressive chemicals, higher compressive and flexural strengths, etc.