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Cement is a mixture of calcium silicates and smaller amounts of Tetra-calcium alumino-ferrite & calcium aluminates that react with water and cause the cement to set The requirement for calcium is met by using high calcium limestone (or its equivalent calcareous raw material) and clay as the source of most of the silica, alumina & Iron. Finished cement is produced by finely grinding together around 95% cement clinker with 5% gypsum (or anhydrite) which helps to retard the setting time of the cement.
The quality of cement clinker is directly related to the chemistry of the raw materials used. Around 80–90% of raw material for the kiln feed is limestone. Clayey raw material accounts between 10–15%, although the precise amounts will vary and Iron ore raw material account between 1-2 %. Magnesium carbonate, which may be present in limestone, is the main undesirable impurity. The level of magnesia (MgO) in the clinker should not exceed 5% and many producers favor a maximum of 3%; this rules out dolomite or dolomitic limestones for cement manufacture. Other deleterious materials include excessive alkalis (sodium oxide, Na2O or soda and potassium oxide, K2O) which would be unacceptable because of durability problems with the concrete (due to the reaction of alkalis with some siliceous aggregates to form a swelling gel; a jelly-like substance that increases in volume with adverse effects on cement durability and strength.(1)
The raw materials mixture is called Raw-mix or Raw-Meal or Kiln feed, which is characterized:
According to raw materials chemical composition raw-mix is designed to achieve sufficient clinker minerals’ content and smooth kiln operation. Mostly the following modules are implemented:
1- Alumina Ratio (AR) (2)
This determines the potential relative proportions of aluminate and ferrite phase in the clinker. An increase in clinker AR means there will be proportionally more aluminate and less ferrite in the clinker, in addition to characterizes the composition of the melt and its viscosity in the clinker; low AR <1.5 increases the Fe2O3 content, decrease the viscosity and increase infiltration of the liquid, deep into the bricks(refractory). In ordinary Portland cement clinker, the AR is usually between 1 and 4.
Effect of high AR
AR determines the flux in raw-mix; if < 1.23 Al2O3 act as flux, if > 1.23 Fe2O3 act as flux.
If AR is too low and raw-mix is without free silica clinker sticking & balling is high.
2- Silica Ratio (SR) (3)
A high silica ratio means that more calcium silicates are present in the clinker and less aluminate and ferrite in addition to characterizes the ratio solid/liquid the amount of liquid phase in the clinker. SR is typically between 2.0 and 3.0.
Effects of high SR:
Effect of low SR:
3- Lime Saturation Factor (LSF):
The LSF is a ratio of CaO to the other three main oxides, a clinker with a higher LSF will have a higher proportion of Alite to Belite than will a clinker with a low LSF. There are several formulas to calculate LSF as per the following:
The impacts of high LSF (> 0.95) are:
The impacts of low LSF (< 0.95)
And the clinker minerals can be calculated using the following BOGUE equations:(4)
4- Alite;(Tri-Calcium silicate)
Responsible for initial strength
5- Belite;(Di-calcium silicate)
Responsible for final Strength
6- Celite;(Tri-Calcium Aluminate):
Responsible for setting time
7- Felite;(tetra-calcium aluminoferrite):
Responsible for setting properties & Color
8- Percent Liquid @ 1450°C (Liquid Phase) [25 – 27.5%](5)
(*MgO should not exceed 2%)
If AR > 1.38
If AR < 1.38
If %LP= 30%, Dens and firm coating
%LP= 25%, Good coating conditions
%LP= 20%, Loose and porous coating
Al2O3,K2O & Na2O increasing the viscosity of liquid phase where Fe2O3 & SO3 decrease it;SO3, is not included in the equations for calculation of the amount of liquid phase present at different temperatures in a cement kiln! Alkali sulphates certainly melt contributing the flux and affecting its properties.(6)
9- Potential Free Lime:
The presence of SrO and BaO in the raw material increases F.CaO
As SrO & BaO react with silica earlier than CaO!
10- Coating Tendency/index [27 – 32]
If AW< 28 indicate light coating
If AW> 30 indicate heavy unstable coating, rings & snowmen (7)
11- Burnability Index: [2.8 – 3.5]
12- Burnability Factor [100 – 110]
Parameters influencing burnability
13- Minimum burning zone temperature:
14- Alkali Sulfate Ratio (A.S.R.):
This ratio indicate whether there is enough alkali to completely combine sulfur
Optimum A.S.R. value = 1.0 and range from 0.6 to 1.4, this apply to raw material
Where Sulphate Modulus apply to hot meal
Optimum M.SO3 value = 0.9 and range from 0.8 to 1.0
15- Kiln feed uniformity factor index [KFUI](8)
In general the steps to design the mix are:
Mainly the raw mix consist of three or four raw materials named respectively
Calculate the Raw-Mix online; using the following table:-
Get raw-mix calculator for "Four Components System"
[(1)BRITISH GEOLOGICAL SURVEY- MINERAL PROFILE CEMENT RAW MATERIALS-November 2005]
[(2)Cement-Data-Book – Duda – 2nd edition]
[(3)Cement-Data-Book – Duda – 2nd edition]
[(4)Handbook of Designing Cement Plants, S P Deolalkar]
[(5)Cement plant operations hand book for dry process plants – Philip alsop & otheres special edition may 2003]
[(7)cement plant operations hand book for dry process plants – Philip alsop & otheres special edition may 2003]