What is cement? | History of modern cement |Definition of Cement| process of cement | Genral notes of cement |Basic Knowledge of Cement | Basic Concepts of cement|| By unique Civil Engineering|

 What is cement? | History of modern cement |Definition of Cement| process of cement | Genral notes of cement |Basic Knowledge of Cement | Basic Concepts of cement|| By  unique Civil Engineering|

सिमेंट म्हणजे काय |बेसिक माहिती सिमेंट | सिमेंट बनवण्याची पद्धत | भारतातील पहिली सिमेंट फॅक्टरी

CONTENTS:- 

• General.

• Early History of Modern Cement.

• Manufacture of Portland Cement.

• Wet Process.

• Dry Process .

• Chemical Composition.

• Hydration of Cement .

• Heat of Hydration.

• Calcium Silicate Hydrates.

• Calcium Hydroxide.

• Calcium Aluminate Hydrates.

• Structure of Hydrated Cement.

• Transition Zone.

LET US LEARN THIS SUBJECT TO BE

CEMENT :-

*GENRAL OF CEMENT:- 

       The history of cementing material is as old as the history of engineering construction. Some kind of cementing materials were used by Egyptians,

Romans and Indians in their ancient constructions. It is believed that the early Egyptians mostly used cementing materials, obtained by burning gypsum. Not much light has been thrown on cementing material, used in the construction of the cities of Harappaand Mohenjadaro.

An analysis of mortar from the Great Pyramid showed that it contained 81.5 per cent calcium sulphate and only 9.5 per cent carbonate. The early Greeks and Romans used cementing materials obtained by burning limestones. The remarkable hardness of the mortar used in early Roman

brickworks, some of which still exist, is presenting sufficient evidence of the perfection which the art of cementing material had attained in ancient times.

The superiority of Roman mortar has been attributed to thoroughness of mixing and long continued ramming.

*Early History of Modern Cement :-

      The investigations of L.J. Vicat led him to prepare an artificial hydraulic lime by calcining an intimate mixture of limestone and clay. This process may be regarded as

the leading knowledge to the manufacture of Portland cement. James Frost also patented a cement of this kind in 1811 and established a factory in London district.

In India, Portland cement was first manufactured in 1904 near Madras, by the South India Industrial Ltd. But this venture failed. Between 1912 and 1913, the Indian Cement Co. Ltd., was established at Porbander (Gujarat) and by 1914 this Company was able to deliver about 1000 tons of Portland cement. By 1918 three factories were established. Together they were able to produce about 85000 tons of cement per year. During the First Five-Year Plan (1951- 1956) cement production in India rose from 2.69 million tons to 4.60 million tons. By 1969 the total production of cement in India was 13.2 million tons and India was then occupying the 9th place in the world, with the USSR producing 89.4 million tonnes and the USA producing 70.5 million tonnes.

*Manufacture of Portland Cement:-

• The raw materials required for manufacture of Portland cement are calcareous materials, such as limestone or chalk, and argillaceous material such as shale or clay.Cement factories are established where these raw materials are available in plenty. 

 

•  Cement factories have come up in many regions in India, eliminating the inconvenience of long distance transportation of raw and finished materials.

• The process of manufacture of cement consists of grinding the raw materials, mixing them intimately in certain proportions depending upon their purity and composition and burning them in a kiln at a temperature of about 1300 to 1500°C, at which temperature, the material sinters and partially fuses to form nodular shaped clinker.

 

•  The clinker is cooled and ground to fine powder with addition of about 3 to 5% of gypsum. 

 

• The product formed by using this procedure is Portland cement.

• There are two processes known as “wet” and “dry” processes depending upon whether the mixing and grinding of raw materials is done in wet or dry conditions.

 

•  With a little change in the above process we have the semi-dry process also where the raw materials are ground dry and then mixed with about 10-14 per cent of water and further burnt to clinkering temperature.

• For many years the wet process remained popular because of the possibility of more accurate control in the mixing of raw materials.

• The techniques of intimate mixing of raw materials in powder form was not available then. Later, the dry process gained momentum with the modern development of the technique of dry mixing of powdered materials using compressed air.

 

•  The dry process requires much less fuel as the materials are already in a drys state whereas in the wet process the slurry contains about 35 to 50 per cent water.

Wet Process

*Dry Process:-

• In the dry and semi-dry process the raw materials are crushed dry and fed in correct proportions into a grinding mill where they are dried and reduced to a very fine powder. 

• The dry powder called the raw meal is then further blended and corrected for its right composition and mixed by means of compressed air.

•  The aerated powder tends to behave almost like liquid and in about one hour of aeration a uniform mixture is obtained.

• The blended meal is further sieved and fed into a rotating disc called granulator.

• A quantity of water about 12 per cent by wright is added to make the blended meal into pellets.

• This is done to permit air flow for exchange of heat for further chemical reactions and conversion of the same into clinker further in the rotary kiln.

• The equipments used in the dry process kiln is comparatively smaller. 

• The process is quite economical.

•  The total consumption of coal in this method is only about 100 kg when compared to the requirement of about 350 kg for producing a ton of cement in the wet process. During March 1998, in India, there were 173 large plants operating, out of which 49 plants used wet process, 115 plants used dry process and 9 plants used semi-dry process.

• Since the time of partial liberalisation of cement industry in India (1982), there has been an upgradation in the quality of cement.

•  Many cement companies upgraded their plants both in respect of capacity and quality. Many of the recent plants employed the best equipments, such as cross belt analyser manufactured by Gamma-Metrics of USA to find the composition of limestone at the conveyor belts, high pressure twin roller press, six stage preheater, precalciner and vertical roller mill. 

• The latest process includes stacker and reclaimer, on-line X- ray analyser, Fuzzy Logic kiln control system and other modern process control.

•  In one of the recently built cement plant at Reddypalayam near Trichy, by Grasim Indistries, employed Robot for automatic collection of hourly samples from 5 different places on the process line and help analyse the ame, throughout 24 hours, untouched by men, to avoid human errors in quality control. With all the above sophisticated equipments and controls, consistent quality of clinker is produced.

• The methods are commonly employed for direct control of quality of clinker. The first method involves reflected light optical microscopy of polished and etched section of clinker.

*Chemical Composition:-

• The raw materials used for the manufacture of cement consist mainly of lime, silica,alumina and iron oxide. 

• These oxides interact with one another in the kiln at high temperature to form more complex compounds. 

 

• The relative proportions of these oxide compositions are responsible for influencing the various properties of cement; in addition to rate of cooling and fineness of grinding.

*Hydration of Cement:- 

• Anhydrous cement does not bind fine and coarse aggregate. It acquires adhesive property only when mixed with water. The chemical reactions that take place between cement and water is referred as hydration of cement.

• The chemistry of concrete is essentially the chemistry of the reaction between cement and water.

 

• On account of hydration certain products are formed. These products are important because they have cementing or adhesive value. 

• The quality, quantity, continuity, stability and the rate of formation of the hydration products are important.

*Heat of Hydration:- 

The reaction of cement with water is exothermic. The reaction liberates considerable quantity of heat. 

 This liberation of heat is called heat of hydration. This is clearly seen if freshly

mixed cement is put in a vaccum flask and the temperature of the mass is read at intervals.

The study and control of the heat of hydration becomes important in the construction of concrete dams and other mass concrete constructions. It has been observed that the temperature in the interior of large mass concrete is 50°C above the original temperature of

the concrete mass at the time of placing and this high temperature is found to persist for a prolonged period. 

*Structure of Hydrated Cement:-

To understand the behaviour of concrete, it is necessary to acquaint ourselves with the

structure of hydrated hardened cement paste. If the concrete is considered as two phase material, namely, the paste phase and the aggregate phase, the understanding of the paste phase becomes more important as it influences the behaviour of concrete to a much greater extent. It will be discussed later that the strength, the permeability, the durability, the drying shrinkage, the elastic properties, the creep and volume change properties of concrete is greatly influenced by the paste structure. The aggregate phase though important, has lesser influence

on the properties of concrete than the paste phase. Therefore, in our study to understandconcrete, it is important that we have a deep understanding of the structure of the hydrated hardened cement paste at a phenomenological level.

*Transition Zone:-

Concrete is generally considered as two phase material i.e., paste phase and aggregates phase. At macro level it is seen that aggregate particles are dispersed in a matrix of cement paste. At the microscopic level, the complexities of the concrete begin to show up, particularly in the vicinity of large aggregate particles. This area can be considered as a third phase, the transition zone, which represents the interfacial region between the particles of coarse aggregate and hardened cement paste. Transition zone is generally a plane of weakness and, therefore, has far greater influence on the mechanical behaviour of concrete.