Visit ShapedbyConcrete.com to learn more about how cement and concrete shape the world around us. Portland cement is the basic ingredient of concrete. Concrete is formed when portland cement creates a paste with water that binds with sand and rock to harden.
- Cement is manufactured through a closely controlled chemical combination of calcium, silicon, aluminum, iron and other ingredients.
- Common materials used to manufacture cement include limestone, shells, and chalk or marl combined with shale, clay, slate, blast furnace slag, silica sand, and iron ore.
These ingredients, when heated at high temperatures form a rock-like substance that is ground into the fine powder that we commonly think of as cement. Bricklayer Joseph Aspdin of Leeds, England first made portland cement early in the 19th century by burning powdered limestone and clay in his kitchen stove.
With this crude method, he laid the foundation for an industry that annually processes literally mountains of limestone, clay, cement rock, and other materials into a powder so fine it will pass through a sieve capable of holding water. Cement plant laboratories check each step in the manufacture of portland cement by frequent chemical and physical tests.
The labs also analyze and test the finished product to ensure that it complies with all industry specifications. The most common way to manufacture portland cement is through a dry method. The first step is to quarry the principal raw materials, mainly limestone, clay, and other materials.
After quarrying the rock is crushed. This involves several stages. The first crushing reduces the rock to a maximum size of about 6 inches. The rock then goes to secondary crushers or hammer mills for reduction to about 3 inches or smaller. The crushed rock is combined with other ingredients such as iron ore or fly ash and ground, mixed, and fed to a cement kiln.
What are the Raw Materials Used at Cement Industry ??? Let’s Know
The cement kiln heats all the ingredients to about 2,700 degrees Fahrenheit in huge cylindrical steel rotary kilns lined with special firebrick. Kilns are frequently as much as 12 feet in diameter—large enough to accommodate an automobile and longer in many instances than the height of a 40-story building.
The large kilns are mounted with the axis inclined slightly from the horizontal. The finely ground raw material or the slurry is fed into the higher end. At the lower end is a roaring blast of flame, produced by precisely controlled burning of powdered coal, oil, alternative fuels, or gas under forced draft.
As the material moves through the kiln, certain elements are driven off in the form of gases. The remaining elements unite to form a new substance called clinker. Clinker comes out of the kiln as grey balls, about the size of marbles. Clinker is discharged red-hot from the lower end of the kiln and generally is brought down to handling temperature in various types of coolers.
The heated air from the coolers is returned to the kilns, a process that saves fuel and increases burning efficiency. After the clinker is cooled, cement plants grind it and mix it with small amounts of gypsum and limestone. Cement is so fine that 1 pound of cement contains 150 billion grains. The cement is now ready for transport to ready-mix concrete companies to be used in a variety of construction projects.
Although the dry process is the most modern and popular way to manufacture cement, some kilns in the United States use a wet process. The two processes are essentially alike except in the wet process, the raw materials are ground with water before being fed into the kiln.
What are the main materials used in cement production?
The main materials used in cement production are minerals containing calcium oxide, silex, alumina and iron oxide. These components are rarely found in one type of raw material; therefore, for the cement production the raw mix is selected for the following components: CARBONATE COMPONENT (rich in calx) is contained in the raw mix in a quantity of 76-80%:
Limestone. Mohs hardness is 1.8-3.0. The older the geological fraction, the firmer. One of the varieties of lime spar is marble. Unlike limestone, it has a looser, earthy structure; therefore, it refers to specifically designed raw materials for wet production. Chalk is soft raw material, it does not require crushing. Marl is a limestone with admixtures of silex and clay substances, as well as iron oxide.It is a transitional stage to clay. The hardness of marl is lower than the hardness of limestone The more clay substances, the lower its hardness. This is an excellent raw material for cement production.
Each of these materials has different grinding coefficient. The higher grinding coefficient, the more grinding of this material. The grinding factors for base raw materials in cement production are given in table below.
|Rotary kiln clinker
|Shaft furnace clinker
|Granulated blast-furnace slag
CLAY COMPONENT (contains little lime) Clay minerals have a fine-grained structure; the grain size does not exceed 2 microns. CORRECTIVE ADDITIVES Corrective additives are added in cases where the chemical composition of the raw mix does not meet the established requirements.
- For example, sand, tripoli are used to increase silica.
- With a lack of iron oxide to reduce the clinker sintering temperature and increase the saturation coefficient, add pyrites cinder, iron ore.
- This results in saving gas during burning.
- Carbonate and clay components are used to provide the necessary content of CaCO3.
They have different carbonate components of CaCO 3 (titer). The carbonate component contains up to 97% CaCO3, clay contains about 10%. It is necessary to achieve, for example, a titer of 80%. Therefore, there are sludge pools with high and low titer content on cement plants.
- The sludge from pools is mixed in certain proportions.
- In this connection, each cement plant has its own, unique composition of raw materials for production of finished goods.
- For example, a mixture of materials used by some cement plants in Ukraine.
- Limestone, clay, slag; – marl, chalk; cinder 2.5%; – limestone, clay + loam, corrective additives; – limestone-coquina; reddish clay; – coal washing, cake, cinder; – marl, loam, pyrite cinder.
The choice of composition of materials mixture with useful components generally is determined by plant’s geographical location and delivery of raw materials. The only common feature in all cement production plants is the difficulty of grinding the source material as the material is hard enough and has high abrasiveness.
Do you know about raw materials of cement?
Raw Materials of cement – Do you know about Raw Materials of Cement, So let’s go and know about what are the raw materials used to make cement? As we know that the main constituents of cements are Lime, Silica and Alumina. The various ingredients of cement, their approximate percentage and range may be described as follows :-
What are the additives used in cement manufacturing?
Raw Materials. Minerals of natural origin as well as industrial products/by-products can be used for cement production as long as the main components of cement (Cao, SiO2,Al2O3,Fe2O3)are present in a required proportion on mixing and the impurities or undesirable components like alkalies, sulfur, chlorides, Mgo etc are below the allowable levels to ensure cement quality and operational stability.Cement mixes vary from ‘natural cement rock’, a single component which, as mined, contains appropriate proportions of all the required minerals, to 2- or 5- component mixes comprising one or two grades of limestone, shale or clay/silcastone, and one or more additives to augment SiO2, Al203 or Fe2O3 levels.
- Raw meal typically contains 78-80% CaCO3 so that lime- stone can only fall close to this level to the extent that it also contains the other ingredients.
- It is essential to have sufficient flux/liquid (Al, Fe, Mg, F) to promote fusion in the kiln, but MgO should not exceed 4-5% or the cement may be expansive.
Excess alkalies (K, Na) affect both kiln operation (build-ups) and product quality (alkali-aggregate reactivity). Excess sulfur causes kiln build-ups and limits the addition of gypsum which may result in setting problems. The stoichiometric ratio of alkalies to sulphur is normally kept between 0.8-1.2.
Excess Cl causes serious build-up problems for preheater operation. Apart from chemistry, grindability is also a factor in selecting raw materials. In particular, silica additives containing large-grain quartz are very difficult to grind and can result in hard burning and high fuel consumption. If quartz silica is employed it should, preferably, have a natural grain size of less than 50% µ.
Generally, cement plants are located on limestone deposits and shale or clay is sufficiently abundant for most plants to mine this locally. Additives are usually brought in small quantities. An approximate analysis for raw mix on ignited basis, or for clinker, is:
Lime Component Limestone: Common forms of calcium carbonate used as raw material for cement manufacturing are limestone and chalk.Limestone is of predominantly fine grained crystalline structure, its hardness is between 1.8 to 3.0 of the Mohs scale of hardness and specific gravity 2.
To 2.8. Limestone usually contains admixtures of clay substance or iron compounds, which influences its color. Only the purest varieties of limestone are white. Chalk: Unlike limestone chalk is characterized by a soft earthy texture. It is a sedimentary rock which was formed during the cretaceous period in geological time, it is relatively young geologically.
Blasting is not required for quarrying of chalk, and the crushing process can also be omitted which will reduce considerably cement production cost. Marls: Limestone with admixtures of silica, clay substance and iron oxide are called marls. Marls form the transition element to the clay.
Because of the wide distribution of marls, they are frequently used as raw material foe cement production Clay Component The second important raw material for cement production is clay.The main component of clay is formed by hydrous aluminium silicates.The chemical composition of clay may vary from those close to the pure clay, to that containing a significant amount of admixtures as iron hydroxide, iron sulfide, sand, calcium carbonate, etc.
Clay is used as an argillaceous component of raw mix. Corrective Ingredients If the primary components needed in the cement raw mix are not present in the required percentage, the corrective materials are used as additives. For example to augment SiO2 component, sand, high silica clay, etc., is used as additive.
Similarly to augment Alumina content in raw mix, bauxite or alumina rich clay is used and to augment for iron component, Iron Ore, pyrite cinders, etc are used as additives. Auxiliary Components Some of the most important auxiliary components whose quantities in the cement are curtailed either by standard specifications or by manufacturing experience are discussed here.
Magnesium Oxide (MgO) 0-5%: MgO combines up to 2% by weight with clinker phase and beyond that it appears as free MgO (periclase). Crystalline matured periclase reacts with water to form Mg(OH)2, but this reaction is proceeds slowly, while the other hardening reactions are already concluded.
Since the Mg(OH)2 occupies a large volume than the MgO and is formed on the same spot where the periclase particle is located, it can split apart the binding of the hardened cement paste, resulting in expansion cracks. Fast cooling of clinker (quenching) in cooler helps to freeze MgO in a glassy form which remains as it is and does not get hydrated to give expansion problem in cement paste.
Alkalies: Some of the Na2O (Sodium oxide) and K2O (Potassium oxide) is built into the clinker menials C3A, C4AF and C4AF.Most of the remaining will remain water soluble. An increased percentage of alkalies, particularly water soluble alkalies influences adversely the strength (28 Day).
Low alkali cements should have Na-equivalent below 0.6% by weight. If alkalies are not balanced by sulfates, they will remain highly volatile and can accumulate in the circulation between kiln and preheater increasing troubles of kiln inlet, kiln riser coatings. Main sources of alkalies are raw materials and coal.
Sulfur (SO3): Sulphates may be present in clinker up to about 3%. Sulphur in raw materials increases SOx emission and cause build-up in preheater. Sulphate can form a stable compound with Potassium (K2SO4) and to lesser extent Sodium (Na2SO4). The sulphates in the clinker comes from raw materials and fuel.
Sulphates must be balanced with alkalies otherwise an excess or deficiency of sulphates with respect to alkalies will increase volatile circulation phenomena. Cl Chlorides: Chlorides form stable compounds with the alkalies and are more volatile than sulphates. About 1% in hot meal is considered maximum for smooth operation.
Clinker can contain about 0.012 to 0.023% Cl. A bypass generally known as alkali bypass may be required to vent out chlorides from kiln system. : Raw Materials.
Calcareous raw material – Raw materials containing calcium carbonate as the main component are all calcareous raw materials. It can be divided into natural calcareous raw materials and artificial calcareous raw materials. Commonly used in cement production is natural ore containing calcium carbonate (CaCO3).