We were discussing “Cements and its types” and " Raw materials for manufacturing of cements”
and “pyroprocessing in cement industry” in our previous posts.
Today we are going to start here one very important
topic i.e. chemistry of cement manufacturing. We will understand the various
terminologies and engineering concepts used in cement technology with the help
of this category.
Further we will go for discussion of hydration
of cement, properties of cement compounds, selection of the process, process
technology, burning technology, firing technology, clinker cooling and
grinding, cement packing and dispatch, pollution control and much more facts
about cement technology in our next post in this category of cement technology.
Chemistry of cement
The cement clinker is produced by igniting a mixture
of raw materials, one of which is composed mainly of cal. carbonate and the
other of alumino-silicates and oxides of Al and Fe. The following complex
mineralogical compounds constitute the cement clinker
Tri Calcium Silicate: 3CaO.SiO2 (C3S)
Di Calcium Silicate: 2CaO.SiO2 (C2S)
Tri Calcium Aluminate: 3CaO.Al2O3
(C3A)
Tetra Calcium Alumino Ferrite: 4CaO.Al2O3.Fe2O3
(C4AF)
From the mineralogical composition of the clinker,
it is noted that cement reactions are mole-mole in practice. After drying and calcinations,
sintering of the kiln feed takes place combining various oxides in solid-solid
phases. But it is essential to have certain amount of liquid phase during sintering.
The liquid phase is formed mainly from iron and aluminium oxides. MgO, alkalis
and chlorides of the kiln feed also form liquid phases.
Portland cement clinker
The Portland cement clinker consists substantially
of four crystalline phases in close inter- penetrating association. In
addition, clinker contains voids and usually some free lime; more rarely,
periclase is present.
Alite:
C3S formed during sintering does not
occur pure in the clinker; it always incorporates oxides e.g. MgO, Al2O3,
Fe2O3, TiO2 etc. They modify the properties of
alite. Below 1280 C, C3S decomposes into C2S and CaO if
cooled slowly.With regard to cement properties, particularly strength
development, C3S is the most important constituent
Belite:
It also contains foreign oxides. Its strength
development is slow, but in the long run, it attains strength as well as alite.
The beta-form may change to gamma- (hydraulically inactive form) on slow
cooling accompanied by an increase in volume and causing rapid disintegration
of clinker.
Aluminate phase:
It possesses a high degree of reactivity further
increased by incorporation of alkalies. In order to retard reaction of
aluminate phase at the start of hydration, cement must contain some added
sulphate.
Ferrite phase:
It does not possess a constant chemical composition;
it is a member of a solid solution series extending from C2A to C2F. It is very
slow reacting and is of little importance to the properties of cement.
Do you have any suggestions? Please write in comment
box.
We will see other topic i.e. hydration of cement in
our next post in the category of Steel and cement technology.
I am very thankful to Mr. Subrata Bhaumik,
Independent cement consultant, for providing such beautiful information and
contents about cement technology.
Mr. Subrata Bhaumik has more than 50 (Fifty) Years
(1965 - 2016) of Experience in Cement and other related Industry covering more
than 100 assignments in cement plants with capacities ranging from 100 tpd to
10,000 tpd in India and abroad involving visit to 25 countries overseas in
connection with work.
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