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CHEMISTRY OF CEMENT MANUFACTURING

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.

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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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