Introduction
One of the most notable examples of Roman work is the Pantheon. It consists of a concrete dome 43.43m in span. The calcareous cements used by the Romans were either composed of suitable limestones burned in kilns or were mixtures of lime and puzzolanic materials (volcanic ash, tuff) combining into a hard concrete. Vitruvius’s work was followed by the researches made by M. Vicat of France.
Joseph Aspedin of Yorkshire (U.K.) was the first to introduce Portland cement in 1824 formed by heating a mixture of limestone and finely divided clay in a furnace to a temperature high enough to drive off the carbonic acid gas. In 1845.
Joseph Aspedin of Yorkshire (U.K.) was the first to introduce Portland cement in 1824 formed by heating a mixture of limestone and finely divided clay in a furnace to a temperature high enough to drive off the carbonic acid gas. In 1845.
Issac C. Johnson invented the cement by increasing the temperature at which the mixture of limestone and clay were burned to form clinker. This cement was the prototype of the modern Portland cement. From then onwards, a gradual improvement in the properties and qualities of cement has been made possible by researchers in U.S.A., U.K., France and Germany.
Cements in a general sense are adhesive and cohesive materials which are capable of bonding together particles of solid matter into a compact durable mass. For civil engineering works, they are restricted to calcareous cements containing compounds of lime as their chief constituent, its primary function being to bind the fine (sand) and coarse (grits) aggregate particles together.
Cements used in construction industry may be classified as hydraulic and non hydraulic. The latter does not set and harden in water such as non-hydraulic lime or which are unstable in water, e g. Plaster of Paris. The hydraulic cement set and harden in water and give a product which is stable. Portland cement is one such. Cement can be manufactured either from natural cement stones or artificially by using calcareous and argillaceous materials.
The examples of natural cements are Roman cement, Puzzolana cement and Medina cement and those of artificial cement are Portland cement and special cements. Today cement finds extensive use in all types of construction works; in structures where high strength is required e.g. bridge piers, light houses, lofty towers, and large structures such as bridges, silos, chimneys. And also in structures exposed to the action of water, e.g. reservoirs, dams, dock yards etc.
Cement mortar, concrete, reinforced brick work, artificial stones, plastering, pointing and partition walls are routinely used in buildings.
Portland cement
It is a cementing material resembling a natural stone quarried from Portland in U.K. Portland cement may be defined as a product obtained by finely pulverizing clinker produced by calcining to incipient fusion, an intimate and properly proportioned mixture of argillaceous and calcareous materials. Care must be exercised in proportioning the raw materials so that the clinker of proper constitution may be obtained after burning. The ordinary Portland cement has been classified as 33 Grade (IS269:1989),
43 Grade (IS 8112:1989), and
53 Grade (IS 12669-1987).
The physical requirements of all these three types of cement are almost same except for compressive strength and are as follows:
Categor Strength (MPA)
A. 32. 5-37. 5
B. 37. 5-42. 5
C. 42. 5-47. 5D. 47. 5-52. 5
E. 52. 5-57. 5
F. 57. 5-62. 5
Chemical composition of raw materials
The three constituents of hydraulic cements are lime, silica and alumina. In addition, most cements contain small proportions of iron oxide, magnesia, sulphur trioxide and alkalis. There has been a change in the composition of Portland cement over the years, mainly reflected in the increase in lime content and in a slight decrease in silica content.
An increase in lime content beyond a certain value makes it difficult to combine completely with other compounds. Consequently, free lime will exist in the clinker and will result in an unsound cement. An increase in silica content at the expense of alumina and ferric oxide makes the cement difficult to fuse and form clinker. The approximate limits of chemical composition in cement are as follows
1. CaO. 60-65
2. SiO2 17-25
3. Al2O3. 3-8
4. Fe2O3 0. 5-6
5. MgO. 0. 5-4
6. Na2O+K2O . 0. 5-1. 3
7. TiO2. 0. 1-0. 4
8. P2O5 . 0. 1-0. 2
9. So3 . 1-2
Function of these oxides
1. CaO : It control the strength and soundness.
Its deficiency reduce the strength abd setting time.
2. SiO2 : It give strength.
Excess of it cause slow setting
3. Al2O3 :Responsible for quick setting
If it use in excess it lower the strength
4. Fe2O3 :Give colour and shape in fusion of different ingredients
5. MgO :Imports colour and hardness.
If excess it causes crack in mortar and in concrete .
(6. Na2O3 :7. TiO2 8. P2O5 ). These are residue if used in excess it cause efflorescence and cracking
9. So3: Make cement sound
Composition of cement clinkers.
The various constituents combine in burning and form cement clinker. The compounds formed in the burning process have the properties of setting and hardening in the presence of water.They are known as Bogue's compounds after the name of Bogue who identified them. Le-Chatelier and Tornebohm have referred these compounds as Alite (C3S), Belite (C2S), Celite (C3A) and Felite (C4AF).
The following Bogue compounds are formed during clinkering process.
The properties of Portland cement varies markedly with the proportions of the above four compounds, reflecting substantial difference between their individual behaviour.
Composition. Formula. Name Symbol
1. Tricalcium
silicate 3CaO.SiO2. Alite C3S
2. Dicalcium
silicate. 2CaO.SiO2. Belite C2S
3. Tricalcium
aluminate . 3CaO. Al2O3 Celite. C3A
4. Tetracalcium
alumino ferrite. 4CaO. Al2O3. Fe2O3 Felite C3AF
1. Tricalcium silicte
Is supposed to be the best cementing material and is well burnt cement. It is about 25-50% (normally about 40 per cent) of cement. It renders the clinker easier to grind, increases resistance to freezing and thawing, hydrates rapidly generating high heat and develops an early hardness and strength. However, raising of C3S content beyond the specified limits increases the heat of hydration and solubility of cement in water. The hydrolysis of C3S is mainly responsible for 7 day strength and hardness. The rate of hydrolysis of C3S and the character of gel developed are the main causes of the hardness and early strength of cement paste. The heat of hydration is 500 J/g.
2. Dicalcium slilicate.
It iss about 25-40% (normally about 32 per cent) of cement. It hydrates and hardens slowly and takes long time to add to the strength (after a year or more). It imparts resistance to chemical attack. Raising of C2S content renders clinker harder to grind, reduces early strength, decreases resistance to freezing and thawing at early ages and decreases heat of hydration. The hydrolysis of C2S proceeds slowly. At early ages, less than a month, C2S has little influence on strength and hardness. While after one year, its contribution to the strength and hardness is proportionately almost equal to C3S. The heat of hydration is 260 J/g.
3. Tricalcium aluminate.
It is about 5-11% (normally about 10.5 per cent) of cement. It rapidly reacts with water and is responsible for flash set of finely grounded clinker. The rapidity of action is regulated by the addition of 2-3% of gypsum at the time of grinding cement. Tricalcium aluminate is responsible for the initial set, high heat of hydration and has greater tendency to volume changes causing cracking. Raising the C3A content reduces the setting time, weakens
resistance to sulphate attack and lowers the ultimate strength, heat of hydration and contraction during air hardening. The heat of hydration of 865 J/g.
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