Clinker is a nodular
material produced during the furnace phase during the production of cement and
is used as a binder in many cement products. Clinker nodes or nodules are
usually 3–25 mm in diameter and dark brown. It is produced by heating limestone
and clay at a point of liquefaction in a rotary kiln at about 1400 ° C – 1500 °
C. The clinker, when combined with gypsum (to control the setting properties of
cement and ensure compressive strength) and finely ground, produces cement. The
clinker can be stored for a larger period of time in a dry condition without
quality degradation, so it is traded internationally and used by cement
manufacturers when raw materials are found to be unavailable.
Composition
of clinker:
The structure of the clinker is tested in two different ways
Mineralogical analysis, using petrographic microscopy and / or X-ray diffraction analysis method.
Chemical analysis, most accurately by X-ray fluorescence spectrometry method
The
clinker has four main components:
Alite:
approximately tri- calcium silicate (usually about 65% of the total)
Belite:
Approximately di-cillium silicate (usually about 15% of the total)
Aluminate:
A very high amount of tetra-calcium aluminate(usually about 7% of the total)
Ferrite:
very nearly tetra-calcium aluminoferrite (usually about 8% of the total)
Other
substances may be present in small amounts:
Salt phases - various
combinations of sodium, potassium and calcium with sulfate and chloride ions,
such as:
Arcanite - K2SO4
Calcium Langbeinite -
K2Ca2 (SO4) 3
Aphthitalite - K3Na
(SO4) 2
Sylvite – KCl
Low-temperature phases
- various intermediate chemical species that have survived further thermal
processing, such as:
Spurrite : Ca5 (SiO4)2
(CO3)
Ternesite: Ca5 (SiO4)2
(SO4)
Ellestadite: Ca10
(SiO4)3 (SO4)3 (OH)2
Ye’elimite: Ca4 (AlO 2)6
(SO4)
Chemical analysis of the clinker is usually given in the form of oxide, as follows (oxide weight in%):
Free lime = 1.0% CaO
The balance is made by
small amounts of oxides of alkali sulfate and minor impurities, such as
titanium, manganese, phosphorus, and chromium.The amount of different
components varies depending on the desired properties of the clinker produced.
Thermochemistry
of clinker
The raw material
entered into the kiln is transported to room temperature. Inside the kiln, the
temperature continues to rise and when it reaches its peak, the production of
clinker cools rapidly. Although the reaction stages often overlap, they can be
expressed in the following acutely-defined sequence:
1. 65–125 ° C: Free water evaporates: latent heat needs to be
supplied. Pure heat input: 2145 kJ / kg clinker.
2. 400–650 ° C: Clays decompose endothermically, and alkalis react
with kiln atmosphere to form liquid sulfates. Net heat input: 42.2 kJ / kg
clinker.
3. 500–650 ° C: Dolomite decomposes endothermically. Net energy input
rate: 19.7 kJ.
4. 650–900 ° C: Calcium carbonate combines endothermically with silica
to form an "incipient belite". Net heating input: 722.5 kJ
5. 700–900 ° C: Calcium carbonate reacts reactively with alumina, and iron oxide to form incipient alumina and
ferrite. Pure heat input rate: 207.2 kJ
6. 900–1050 ° C: When all the available silica, alumina and iron
oxide have reacted, the remaining calcium carbonate decomposes endothermically
to calcium oxide. Heat input requirement: 601.9 kJ / kg clinker.
7. 1300–1425 ° C: Aluminate, ferrite, and benite is melted
endothermically, and the belite react with calcium oxide to form alite.
8. 1425–1300 ° C: After the peak temperature passes, the melt begins
to re-freeze exothermically to aluminate,ferrite and belite.
Types
of clinker
The most common type of
clinker production is done for Portland cement and its blends. The types of
clinker usually varies depending on the type of cement for which the clinker is
produced. Apart from Portland cement blends, some special types of cement
clinker are liqsted below:
1. Sulfate resistant
clinker
2. Low Heat Clinker
3. White Clinker
4. Low Alkali Clinker
5. Belite Calcium sulfoaluminate
Ternesite (BCT)
1. Sulfate resistant
clinker
It contains 76% Alite,
5% Belite, 2% Tricalcium aluminate, 16% Tetracalcium Alumino-ferrite and 1%
Free Calcium Oxide. Its production has decreased in recent years as sulfate
resistance can be easily achieved by using granular blast furnace slag in
cement production.
2. Low heat clinker
It contains 29% Alite,
54% Belite, 2% Tricalcium aluminate and 15% Tetracalcium Alumino-ferrite with
very little free lime. It is no longer produced because cement produced from
ordinary clinker and ground granular blast furnace slag has excellent low heat
properties.
3. White clinker
It contains 76% Alite,
15% Belite, 7% Tricalcium aluminate, no Tetracalcium Alumino-ferrite, and 2%
free lime, but the composition can vary widely. The white clinker produces
white cement that is used for aesthetic purposes in construction. The majority
of white cement goes into factory-manufactured pre-cast concrete applications.
4. Low-alkali clinker
The reduction of the
alkali content in the clinker is done either by replacing the raw-mix alumina
source with another component (thus obtaining a more expensive material from a
more distant source), or "bleeding from alkali" is established, Which
includes some kiln systems with high temperature gases (which contain alkali as
fume), resulting in some heat dissipation.
5.
This concept is used to
produce a type of clinker with 30% less carbon dioxide emissions. Energy
efficiency improves and electricity costs are reduced by about 15% for the
manufacturing process.
Use
of clinker: conversion to cement
Clinker,added with
additives and grounded in the form of fine powder that is used as a binder in
cement products. The produced cement combines various substances to obtain
specific properties. Gypsum is added
with clinker regulates the setting time and gives the most important property
of cement, compressive strength. It also prevents the agglomeration and coating
of powder on the surface of balls and mill wall. Some organic materials, such
as triethanolamine (used at 0.1 wt.%), are added as a grinding aid to avoid
powder agglomeration.Other additives sometimes used are ethylene glycol, oleic
acid and dodecyl-benzene sulphonate. The most notable type of cement produced
is Portland cement, but some special chemical ingredients can be added to the
clinker to produce some other types of cement, such as:
• Ground granulated
blast furnace slag cement
• Pozzolana Cement
• Silica Fume Cement
Clinker is mainly used
to produce cement. Since it can be stored in dry condition for several months
without noticeable deterioration, it is traded internationally in large
quantities. Cement manufacturers purchase clinkers for their cement plants in
areas where raw material for cement is scarce or unavailable.
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