The basic raw materials for cement are water, limestone, and clay. Cement, like related building materials such as mortar and concrete, however, can also contain other essential ingredients, such as sand, shale, and iron ore, which can contain traces of aluminum as well. Where the raw materials for cement are being mixed in an environment that frequently drops below the freezing point of water at 32° Fahrenheit (0.0° Celsius), it is common to add salt or sodium chloride and possibly calcium chloride to the mixture to lower the freezing point of the water. Though cement will set even below the freezing point of water, it can take longer to do so and this can contribute to structural weaknesses in the final product. As of 2011, industrial byproducts are also being added to the cement mixtures.
The types of raw materials for cement can also vary in the sense that some mixtures as of 2011 contain industrial byproducts that would otherwise be considered waste, and which can add beneficial qualities to the cement mixture. Cement itself is a raw material for concrete, which consists of cement and both course and fine aggregate mixtures of rock such as gravel, shale, or pebbles with water and air mixed in. Industrial byproduct substitutions for the raw materials for cement and concrete include fly ash, silica fume, and blast furnace slag.
Fly ash is a type of waste product produced during many types of industrial combustion processes. It consists of fine particulates of airborne ash along with some solid fuel particles that can be recovered from smokestack scrubbers. It is considered a useful reinforcing compound in manufactured rock and brick materials like concrete and can replace cement in the mixture at levels from 25% on up to 70%.
Another of the key industrial byproducts used in making cement is silica fume which is used as a 10% replacement compound for cement. It consists primarily of a powdery mixture of silicon dioxide particles and is relatively new to the building industry. Though concrete and cement production have been a practice since at least 2,500 BC, the use of silica fume has only been widely incorporated into the raw materials for cement since the early 1970s. This is because its a uniquely-generated material that is decidedly different from fused silica, and is a byproduct of industrial activity in electric arc furnaces. The passage of increasingly strict environmental legislation in the US and Europe during the 1970s led to manufacturers recovering silica fume as a waste byproduct, and Norway was the first nation to find a use for it aside from dumping it in landfills.
Blast furnace slag is also another important ingredient found when buying the raw materials for cement. It is often referred to as ground-granulated blast furnace slag (GGBFS) and replaces cement in concrete at the same levels for which fly ash is used, usually in mixtures of 25%, 50%, or 70%. GGBFS is a byproduct of the steel and iron furnace industry, and, when used, is known to extend the life of buildings by anywhere from between 50 to 100 years. The incorporation of GGBFS as of 2011 into cement has seen wide application in Asian nations such as Japan and Singapore, as well as in Europe, with more limited use for the product in US concrete manufacturing.