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:
|Si O2||20- 23%|
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
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.
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.
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.
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.
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.
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.