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Essay on Cement
Essay Contents:
- Essay on the Meaning of Cement
- Essay on the Early History of Cement
- Essay on the Classification of Cements
- Essay on Portland Cement
- Essay on the Elementary and Chemical Composition of Cement
Essay # 1. Meaning of Cement:
Cement is the main constituent of concrete. It can be defined as a material having adhesive and cohesive properties which make it capable of bonding mineral fragments into a compact mass. Actually this definition covers a large variety of cementing materials. For constructional purposes, the meaning of the term cement is restricted to the bonding materials used with stones, bricks, and sand etc.
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The principal constituents of this type of cement are compounds of lime. On adding water to cement a chemical reaction known as hydration of cement takes place and a large quantity of heat-liberates. On hydration of cement, gel is formed which binds the aggregate particles together and provides strength and water tightness to concrete on hardening. Thus cement has the property of setting and hardening under water by virtue of a chemical reaction with it. Such cements are called hydraulic cements. They consist mainly of aluminates and silicates of lime.
Essay # 2. Early History of Cement:
The history of cementing material is as old as that of Engineering Construction. In ancient times Romans, Egyptians and Indians used some kind of cementing materials in their constructions. It is believed that Egyptians used burnt gypsum (CaSO4) as cementing material in their constructions. Not much is known about the cementing material used by Indians in the construction of the cities of Harappa and Mohenjo-Daro.
The analysis of mortar used in the construction of Great Pyramid showed that it contained 81.5 percent of calcium sulphate and about 9.5 percent calcium carbonate only. It is believed that early Romans and Greeks used cementing materials obtained by calcination (burning), of lime stone. The remarkable hardness of the mortars used by early Romans in their constructions obtained from some of the existing works provides sufficient evidence of the perfection of art of preparing cementing materials in ancient times. The superiority of Roman mortar is attributed to the thorough mixing and continued ramming for a long period.
Latter Romans and Greeks learnt that better cementing material may be obtained by mixing certain volcanic ash and tuff with lime stone, sand and water. This mixture produced a cementing material of superior strength and durability. The tuff used in the mix was found near the village Pozzuoli near Mount Vesuvius in Italy. The tuff or ash mostly is siliceous in nature. This type of tuff or ash was given the name of Pozzolana. After wards any material natural or artificial having the same properties as those of tuff or ash found near Pozzuoli was called pozzolana.
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In the absence of natural volcanic ash, Romans used powdered pottery or tiles as Pozzolana. In India surkhi (brick powder) has been used as Pozzolana in the mortar. In India the thorough mixing and long continuous ramming of lime mortar with or without the use of surkhi yielded strong and imperious mortar which confirmed the secret of Roman mortar superiority.
Latter it is learnt that in order to improve the workability of mortar and concrete Romans added milk, blood and lard in the mix. The blood Haemoglobin is a powerful air entraining agent and plasticizer, which increases the workability, resulting in the increase of the durability of their structures.
After 1756 John Smeaton carried out extensive experiments and made enquires to find out the best material to withstand severe action of sea water. On the basis of his experiments he concluded that lime stones containing considerable proportion of clayey matter produced better lime possessing superior hydraulic properties. However the findings of John Smeaton made little advancement and the old practice of a mixture of lime and Pozzolana remained in use for a long time.
In 1796 hydraulic cement was produced by burning the nodules of argillaceous lime stones. Four years later in 1800 this product was given the name as Roman cement. This cement remained in use till 1850. After 1850 this cement was outdated and new product named Portland cement was introduced.
Essay # 3. Classification of Cements:
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Cements mainly can be classified into two groups, viz.:
1. Natural cement
2. Artificial cement or Portland cement.
1. Natural Cement:
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This type of cement can be obtained by burning lime stone containing 20-40% clay and crushing it to powder. It is brown in colour and sets very quickly when mixed with water. It is very akin to eminent hydraulic lime. The only difference between hydraulic lime and natural cement is that lime starts to slake on mixing water with it and natural cement sets immediately after adding water to it.
2. Artificial Cement:
It can be classified as follows:
Essay # 4. Portland Cement:
Portland cement was invented in 1824 by Joseph Aspdin, a brick layer of Leeds (Yorkshire) England. He obtained the patent of Portland cement on 21st October 1824. This cement was prepared by heating a mixture of finely divided clay and hard lime stone in furnace until calcium dioxide (CO2) had been driven off. This resulted at much lower temperature than that necessary for clinkering. The prototype of modern cement was made by Isaac Johnson, burning a mixture of clay and lime or chalk at clinkering temperature in 1845.
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Originally, the name “Portland Cement” was given due to the fact that the mortar made with this cement resembled in appearance, colour and hardness to the stone which was quarried on the Isle of Portland, U.K. The German standard specifications for Portland cement were drawn in 1877, while British and American standard specifications were, drawn in 1904.
In India, the Portland cement was first manufactured in 1904 near madras, but the venture failed. Between 1912 and 1913, the Indian Cement Company Ltd. was established at Porbander (Gujarat) and by 1914 this company produced about 1000 tons of Portland cement. By 1918 three more cement factories were established in India and all the factories produced about 85000 tons of cement every year.
After independence during 1951-56 cement production rose from 2.69 million tons to 4.6 million tons. By 1969 the production of cement in India was 13.2 million tons occupying the 9th place in the world, USSR being the highest cement producing country with the production of 89.4 million tons. In 2002 the production of cement in India crossed 100 million tons mark and we attained second position after China in the production of cement in the world.
Essay # 5. Elementary and Chemical Composition of Cement:
The results of chemical analysis of cement have shown the percentage of oxides of principal constituent elements. The oxide composition of cement is shown in Table 2.2 though variation range is fairly narrow, but it may result in an appreciable change in the proportion of the components actually present.
The constituents of cement mainly lime, silica, alumina, and iron oxide interact with each other in the kiln and form complex product of definite molecular structure. The principal reactive compounds present in the clinker as shown in Table 2.3 are known as Bogue’s compounds.
The percentage of the compounds noted above depends upon the oxide percentage in the clinker and vary considerably from cement to cement as shown in table 2.4. In a well burnt modern clinker the amount of C3S is about 40% and that of C2S 25%. The sum total of C3S and C2S in most cement varies from 70-74%. Normally C3S is found in largest quantity in any cement. It occurs as small equi-dimensional colourless grains.
However the sum of the contents of C3A and C4AF has decreased slightly in modern cements. On cooling below 1250°C C3S decomposes slowly, but if cooling is not very slow, C3S remains unchanged and relatively is stable at ordinary temperatures. C2S is found to have three or even four forms, depending on temperature, such as αC2S, which exists at high temperatures, αC2S changes to βC2S at about 1450°C, βC2S further undergoes change to γC2S at about 670°C. However at the cooling rate of commercial cements βC2S is preserved in the clinker. It forms round grains. In modern cement the quantity of C3S is increased upto 45% and total of C3S & C2S to 70-80%.
C3A forms rectangular crystals, while C4AF is a solid solution.
Calculation of Percentage of Major Compounds:
The percentage of major compounds in a clinker may be found from the Bogue’s equations given below:
C3S = 4.07 (CaO) – 7.60(SiO2) – 6.72 (Al2O3) – 1.43 (Fe2O3) – 2.85 (SO3)
C2S = 2.87 (SiO2) – 0.754 (3 CaO.SiO2)
C3A = 2.65 (Al2O3) – 1.69 (Fe2O3)
C4AF = 3.04 (Fe2O3)
The oxide shown in brackets represents the percentage of the same in the raw material. The method of calculation is illustrated by an example below.
Example:
From Bogue equations, calculate the different compounds of the three cements, whose oxide composition is given below in table 2.5.
Solution:
From Bogue equation the C3S, C2S, C3A & C4AF are given as follows:
C3S = 4.07 (CaO) – 7.6(SiO2) – 6.72 (Al2O3) – 1.43 (Fe2O3) – 2.85 (SO3)
C2S = 2.87 (SiO2) – 0.754 (3 CaO.SiO2) i.e. C3S
C3A = 2.65 (Al2O3) – 1.69 (Fe2O3)
C4AF = 3.04 (Fe2O3)
For cement A, C3S = 4.07(68.2) – 7.6(22.4) – 6.72(4.6) – 1.43 (0.3) – 2.85 (2.4)
= 277.574 – 170.24 – 30.96 – 0.429 – 6.84
= 277.574 – 208.469 = 69.1%
C2S = 2.87 (22.4) – 0.754 (69.1)
= 64.288 – 52.305 = 11.983 = 12%
C3A = 2.65 (4.6) – 1.69 (0.3)
= 12.19 – 0.507 = 11.69 ≈ 11.7
C4AF = 3.04 (0.3) = 0.912 ≈ 0.91
For cement B, C3S = 4.07 (61.0) – 7.6 (25.0) – 6.72 (4.0) – 1.43 (3.0) – 2.85 (2.5)
= 248.27 – 190.0 – 26.88 – 4.29 – 7.125
= 248.27 – 228.295 = 19.975 ≈ 20.0%
C2S = 2.87 (25) – 0.754 (20.0)
= 71.75 – 15.08 = 56.67%
C3A = 2.65(4.0) – 1.69(3) = 10.60 – 5.07 = 5.53%
C4AF = 3.04 x 3 = 9.12%
For cement C, C3S = 4.07 (64.2) – 7.6 (20.7) – 6.72 (3.9) – 1.43 (5.3) – 2.85 (2.0)
= 261.294 – 157.32 – 26.608 – 7.579 – 5.70
= 261.294 – 196.808 = 64.486 ≈ 64.5%
C2S = 2.87 (20.7) – 0.754 (64.5)
= 59.41 – 48.63 = 10.78 ≈ 10.8%
C3A = 2.65(3.9) – 1.69(5.3)
= 10.335 – 8.957 = 1.378 ≈ 1.4
C4AF = 3.04 x 5.3 = 16.1%
Influence of Change in Oxide Composition on the Composition of Cement:
It is interesting to note, the large influence of the change in the oxide composition on the compound composition of cement. In general a small increase in the total lime content of the raw materials appreciably increases the percentage of C3S and decreases C2S. An increase in the iron oxide content decreases the percentage of C3A by causing formation of C4AF.W.
Czernin has shown that if lime content is decreased by 3% and other oxides increased as shown in column (2) of table 2.6 in the raw materials of rapid hardening cement, the value of C3S decreases from 65% to 33% and that of C2S increases from 8% to 38%. Similarly if alumina is reduced from 7% to 5.5%, iron-oxide increases from 3.0% to 4.5% (lime and silica contents being kept constant) the value of C3S increases from 65% to 73% and that of C2S decreases from 8% to 2%, while C3A and C4AF are also affected greatly.
This data has been presented in table 2.6. Column (1) of table 2.6 shows the typical data of rapid hardening cement, column (2) shows the change in other oxides when 3% lime is decreased. Column (3) shows a change of 1.5% in alumina and iron contents compared with the content of column (1). Thus the variation in composition depends largely on the ratio of CaO to SiO2 in the raw materials.
Thus the pattern of formation and hydration of cement can be shown schematically as follow:
Both Le Chateliar and Tornebohm observed four different kinds of crystals in thin sections of cement clinkers. Tornebohm named them as Alite, Belite, Celite and Felite. This description of minerals in cement was found similar to that of Bogue’s description of compounds C3S, C2S, C3A and C4AF. Hence Bogue’s compounds some times are also called as Alite, Belite, Celite and Felite.