Cement is a hydraulic binder. The finely ground inorganic material which, when mixed with water, forms a paste which sets and hardens by means of hydration reactions and processes. After hardening, it retains its strength and stability even under water. It is produced in a chemical production process. With more than 10 minerals or chemical compounds, it is a quite complex product.
A pozzolana-lime mixture was used for this purpose already thousands of years ago. In the first half of the 19th century, a substance was patented called Portland Cement. Its name is derived from its similarity to Portland stone, a type of building stone quarried on the Isle of Portland in Dorset, England. This substance was further developed to a similar product as today’s Portland Cement.
Today, Portland types cement are the most general types of cement in use and is a basic ingredient for concrete, mortar, etc. Several types of cement exist and are defined in standards like ASTM C150 or EN 197.
There are high expectations from cement users. The material is required react with water producing hydrated minerals which not only develop strength but also form a dense, hardened product resisting the elements like wind, water, sun, etc. Despite being produced from varying materials, the users expect uniform performance of the product.
Portland cement consists of cement clinker, a hydraulic binder material, which is fine ground together with gypsum as an inhibitor to prevent flash and fast setting.
EN 197-1 specifies 5 main cement types and 27 types of common cement. These are not used all over Europe. For example, climatic conditions allow the use of only some of these 27 common cements in Austria.
The standard EN 197-1 defines main constituents of cement including Portland cement clinker, granulated blast furnace slag, pozzolanic materials, fly ashes, burnt shale, limestone and silica fume. Calcium sulfate (e.g. gypsum) is added to the other constituents to control the setting. Furthermore, minor constituents and additives are defined.
Portland cement (CEM I) is defined as 95-100% Portland cement clinker and 0-5% minor additional constituents plus calcium sulfate and additives.
The standard specifies also some characteristics of the cement like early strength (after 2 and 7 days), standard strength after 28 days), setting time, chemical requirements, etc.
This document is only a short summary about cement and will describe the general principles of cement production and the equipment in use.
Before introducing the production process of cement, it is necessary to provide a short overview on the clinker chemistry:
Portland cement clinker consist of four oxides CaO (lime, simplified notation C), SiO2 (silica, simplified notation S), Al2O3 (alumina, A) and Fe2O3 (iron oxide, F).
The sources for these 4 oxides are different raw materials: limestone, clay, marl, bauxite, iron ore, sandstone, sand, etc. The number of components and the amount thereof added to the raw mix depends on the composition of the single raw material; important is to fulfill the requirements of the clinker.
The four main oxides form the minerals in the clinker: Tricalcium Silicate (Alite, C3S), Dicalcium Silicate (Belite C2S), Tricalcium Aluminate (Aluminate, C3A) and Tetracalcium aluminaferrite (Ferrite, C4AF).
At least two thirds by mass shall be calcium silicates (Alite and Belite). The remainder shall consist of aluminum and iron containing clinker phases and other compounds. The MgO content shall not exceed 5%.
The process of clinker formation can be split in three phases which are named as per the region of kiln in which they mainly occur, i.e. the calcining zone, the burning zone and the cooling zone.
In the calcining zone, the CaCO3 is reduced to CaO, i.e. free lime is formed. This is endothermic reaction which also forms CO2. Nowadays, this process mainly occurs in a separate vessel between preheater and kiln, namely the pre-calciner.
In the burning zone, at temperatures of up to 1450°C, partial melting occurs. Whereas lime and sand do not melt at these temperatures, alumina and iron oxide form a liquid phase. Lime and silica partially dissolve in this liquid and the reactions to form the clinker minerals can take place much faster. In areas of excess lime, C3S will be formed. In areas with silica excess, C2S will be initially formed. The C2S will react with more lime to from C3S.
As the clinker passes under the kiln flame, temperatures go down to 1200°C. In this cooling zone, the C3A and C4AF crystallize. The size of formed crystals depends on the cooling rate: the faster the cooling, the smaller the crystals. After this zone, the clinker enters the clinker cooler for further cooling.
For good quality clinker and satisfactory chemical composition, it is essential that the components of raw mix are finely divided and uniformly blend. Therefore, in the production process, 1 or 2 steps for homogenization of raw material are included. It is also important to consider the fuel in the calculation (e.g. the ash from coal, the Sulphur from oil, etc.). Typical cement clinkers contain 50-60% C3S, 12-26% C2S, 6-13% C3A and 6-12% C4AF. *) The raw mix composition must meet the requirements to reach this composition.
The clinker is ground together with gypsum to form cement. Cement composes 95% of clinker and 5% of gypsum.
*) values from various literature.
Approximately 80% of the raw mix are limestone or other CaCO3 sources. Therefore, typically, cement plants are placed next to the limestone quarries. Other oxides are either available in the main minerals (for example in shale or marl) or clay, sand, iron ore and bauxite are delivered to the plant.
The first step in cement production is to quarry the limestone and other available raw materials. The rock is crushed to a size of about 10 to 15 cm. Depending on the raw materials, a first step of homogenization for limestone is combined with an intermediate storage in the production process with longitudinal or circular storage facilities. Other raw materials are also stored at the plant site.
From these storages, the raw materials are transported to a small storage/ dosing bins in front of the drying and grinding plant. At this battery of bins, the raw materials are combined as per the required ratio to allow proper clinker composition.
Within the drying and grinding plant, the raw materials are ground together and dried. This can be achieved in the dryer, roller press, ball mills or vertical mill. The mills are heated with the hot off-gases from the pyro unit which are sucked from the top of the preheater tower. Large filters are installed to remove the dust from air flow and utilize in the production process.
After the drying and grinding plant, one more combined homogenization and storage facility is usually installed, the homogenization silo. This silo serves as intermediate storage as well as for homogenization of the raw meal. From this silo, the raw meal is fed precisely dosed to the pyro unit: the preheater with the pre-calciner, the kiln and the cooler.
The raw meal is fed to the hot air stream at the top of the preheater tower. This preheater serves as heat exchanger between the hot air from the kiln sucked from bottom to top and the raw meal flowing from top to bottom of the tower. The hottest gases and the hottest raw meal are at the bottom of preheater tower at the kiln inlet. A special vessel installed between the cyclones of the preheater tower and the kiln serves as the “pre-calciner”.
Whereas the already hot gases from the kiln are sucked through the preheater tower towards the raw mill, the heated raw meal is removed from the air stream by cyclones and fed in the next lower stage of the preheater. This is repeated for every stage of preheater tower, maybe up to 6 stages.
The rotary kiln is a steel cylindrical vessel, inclined slightly to the horizontal, which is rotated slowly about its longitudinal axis. The kiln is lined with special firebricks. Kilns can be as much as 6 m in diameter and 200 m in length (wet kilns). A dry process kiln might be only 70m long and 6m in diameter.
In the kiln, the raw mix with already decarbonized lime is further heated up to 1450°C. The reactions to form clinker are taking place.
Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in a clinker cooler. The cooling is achieved by blowing ambient temperature air into the hot clinker. Parts of the heated air are transferred to the preheater tower and serve as the so-called “tertiary air” in order to increase waste heat utilization.
After the clinker is cooled, usually a large storage unit is available, the clinker silo. From this storage, the clinker is transported to a battery of small storage/ dosing bins. In front of cement grinding facility, in this battery of bins, clinker and the other compounds forming the cement are dosed as per requirements. In the simplest case, clinker and gypsum are ground together.
The final product is transported by means of, e.g. air slides to large storage silos and further dispatched either in bulk on trucks or rail, in big bags or small bags.
Quality control of cement production is established by means of a plant laboratory. The raw materials, intermediate products and final products are checked by chemical and physical tests.
Other cement production processes
Alongside today’s dominant dry production process with preheater and pre-calciner, also production processes called “wet” and “semi-dry” exist. All these processes are (or were) in use around the world. Basically, the processes differ in the way the raw materials are processed and dried before final treatment in the kiln.