In a nutshell, cement is a binding material made of lime or clay, mixed with water to form mortar, or mixed with sand, gravel, and water to make concrete.
Cement is the substance that is used extensively in the work of construction to set or harden or adhere materials. The main function of cement is to bind the material together and that is why cement is essentially known as a binding agent.
There are very rare occurrences of cement being used on its own, cement is always used with aggregates, the likes of sand and gravel along with water. Through many surveys, cement came up as the most commonly and extensively used material in this world and it also takes only the second place when it comes to consuming resources, losing the first place to water.
Cement mixed with sand, gravel, and water produces concrete, which is a common material for all most all types of construction work. Cement also produces mortar for masonry works by getting mixed with finer aggregates.
Nearly 4 billion tons of cement are produced every year, globally. If the cement industry is considered as an independent country, it would be the third largest emitter of carbon dioxide, with nearly 2.8 billion tons every year. The initial process of cement production is responsible for nearly 4% of global CO2 emissions.
The history of cementing material is as old as the history of engineering construction. Some kinds of cementing materials were used by Egyptians, Romans, and Indians in their ancient constructions.
Chemically speaking the cement we see today, (a product of lime that works as a primary bonding agent) is far away from the material used in past for cementation. The Assyrians and the Babylonians were found to use bitumen to bind their alabaster or burnt bricks.
Ancient Egyptians on the other hand used mortar, made of half burnt gypsum and sand to cement their blocks of stone. Most of these mortars more than often had calcium carbonate present in them.
In ancient Greece, especially in Crete, lime was used. There is also evidence that supports the use of crushed potsherds as artificial pozzolan, in the place of hydraulic cement.
These pozzolans were found as different tuff in Roman and Greek civilizations. Whereas the Greeks, generally used volcanic tuff that they got from their island of Thera. Meanwhile, the Romans used volcanic ash with lime.
From the middle age, there are no such knowledge found as a literature form, but the military engineer and medieval masons used a mixture of aggregates and lime mortar as a cementing agent to build structures like fortresses, harbors, mansions, and even canals.
Then, in the 16th century, Spaniards introduced Tabby to the Americans. Tabby was a building material where the binding agent was the lime found in the shells of oysters and the whole oyster shell was a part of the concrete.
Then in the 18th and 19th centuries with the advancement of chemical engineering, cement went through many experiments and experiences to finally create the great binding agent, that we see today.
There are various types of cement available in the market today, but the most common type of cement is Portland cement which is used all over the world. We have already discussed ‘types of cement’ in another article, you can read it here – Types of Cement.
The atmospheric moisture is sufficient to make the cement worthless. The cement should be stored in such a way as not to be exposed to the atmosphere. Following are the process to store cement in a proper way:
1. The floor of the cement stored room should be at least 60 cm above the ground.
2. Cement bags are arranged on a wood or bamboo bed. The bed will be at least 15 cm above the floor.
3. Cement bags will be at least 30 cm away from any wall of the stored room.
4. To enter the light and air, there will be some small opening or windows in the room.
5. Cement storage should be avoided in rainy seasons as possible.
6. If cement needs to be stored during the rainy season, the cement stack should be kept completely closed using a waterproofing membrane so that moisture does not enter the cement stack in any way.
7. The cement bags are arranged in such a way that does not have difficulty in taking out or get in other bags, and one-meter wide space should be left to move between two rows of cement bags.
8. Above all, cement is not well to store for a long time.
6 Ways to Check The Good Quality of Cement on Site
To make concrete, there are three main ingredients are used which are cement, aggregate and water. Therefore, to make a good concrete, the quality of the cement should be good. Following are the 6 ways to check the good quality of cement on site:
1. A good cement will be always grey in colour, with a light greenish shade.
2. You will feel smooth when you rub the cement in between the two fingers.
3. If you feel cool, not warm after you insert your hand into the cement bag, so the quality of the cement will be good.
4. If a hand full of cement is thrown in a bucket of water, first it should float for some time on the surface of the water before it sinks.
5. A thin paste of cement with water should feel sticky when rubbed between the fingers.
6. A good quality cement always should be free from any types of lumps, which are mostly formed due to the absorption of moisture from the atmosphere.
Necessity of Adding Gypsum (CaSO4) in Cement Manufacture
Portland cement is obtained from grinding clinkers and usually sets and hardens immediately after the addition of water. In order to slow down the setting time of cement, gypsum @ 3% to 4% is added during the process of clinker grinding.
If the quantity of gypsum(CaSO4) is more, the cement becomes very slow setting. It also hardens slowly which results in delay the removal of formwork. Gypsum combines with tricalcium aluminate and prevents flash setting.
But, if more gypsum is added with cement, the excess amount after combining with tricalcium aluminate remains free in cement. It expands and makes the cement unsound.
The quality of concrete depends on the quality of the cement. There are several types of cement available in the market. Therefore, it is very important to check the quality of cement before using it. Various tests on cement are conducted to ensure its quality. Cement tests Can be classified as follows:
The Purity and quality of cement can be judged by applying the following rough and ready field test of cement:
⇰ The color of the cement should normally be greenish-grey.
⇰ When the hand is inserted into a bag of cement, It should feel cool and not warm.
⇰ Any lump found in the cement bag should be powdered by pressing between the thumb and the fore finger. If it does not turn into powder form, the cement is considered to be spoiled by air setting.
⇰ It should give a smooth feeling when rubbed in between fingers.
⇰ A handful of cement thrown into a bucket of water should float for some time.
The following 8 Types of cement tests are usually conducted in the laboratory, this is also known as a physical test of cement:
1. Fineness Test. 2. Normal consistency test. 3. Initial and final setting time test. 4. Soundness test. 5. Compressive Strength Test. 6. Chemical composition test. 7. The heat of hydration test. 8. Tensile Strenght Test
1. Fineness Test
The object of this cement test is to check the proper grinding of cement. The fineness of cement is tested in two ways:
i) By Sieving. ii) By Determining the specific surface.
i) Sieving
First of all, weight correctly 100 grams of cement and take it on a standard IS sieve no 9 (90 microns). Break down the air set lumps in the sample with fingers.
Continuously sieve the sample manually or mechanically for 15 minutes, Weigh the residue left on the sieve. This weigh shall not exceed 10% for ordinary cement & 5% for rapid hardening or low heat cement.
2. Normal Consistency Test
Normal Consistency test of cement is conducted to determine the quantity of water required to produce a cement paste of standard or normal consistency for use in other tests.
This test is performed with the help of Vicat’s Apparatus, Standard or normal consistency of a cement paste is that consistency which will permit the Vicat plunger (10 mm in diameter & 40-50 mm in length) to penetrate to a point 5 to 7 mm from the bottom of the Vicat mould.
To perform this test about 400 gm sieved cement is taken & 100 gm of water is added and mixed thoroughly for about 3 minutes. The paste is then filled into the Vicat mould, Making it level with the top of the mould.
The filled up mould is placed centrally below the movable rod fitted with a plunger. The bottom surface of the plunger is brought in contact with the surface of the cement paste and the reading of the scale is taken. The rod is then quickly released and the penetration is noted.
If the rod penetrates 5 to 7 mm from the bottom, the paste is said to be of normal consistency. Otherwise, the trial paste should be made with a varying quantity of water and the test is repeated as above till the desired penetration is obtained.
Let, W = Weight of cement taken. W1 = Weight of water for desired penetration.
Then, the percentage of water for normal consistency P = (W1 / W)× 100
When cement is mixed with water is stiff and sticky paste is formed. This cement paste remains plastic for a short period. As the time lapses, the plasticity gradually disappears and the paste changes into a solid mass.
The phenomenon by virtue of which the cement paste changes from a plastic state to a solid-state is known as the setting of cement. The time to reach this stage is known as setting time.
The time is reckoned from the instant when water is added to the cement. The setting time is divided into two parts namely initial setting time and final setting time.
The time at which the cement paste loses its plasticity is termed the initial setting time. The time is taken to reach the stage when the paste becomes a hard mass is known as the final setting time.
The initial and final setting time test on cement is performed with the help of Vicat apparatus. The initial setting time of cement shall be the time from the period elapsing between the time when the water is added to the cement and the time at which the needle (1 mm square or 1.13 mm in dia and 40 to 50 mm in length) penetrate to a point 5 mm from the bottom of the Vicat mould.
To perform this test about 400 gm sieved cement is taken and water is added to it @ 0.85 P by weight of cement. Where P is the percentage of water required for normal consistency paste.
At the instant of adding water, the stopwatch is started. Water is mixed thoroughly for about 3 minutes. The paste is then filled into the Vicat mould, making it level with the top of the mould.
The filled up mould is placed centrally below the movable rod fitted with a needle. The bottom surface of the needle is brought in contact with the surface of the cement paste and the reading of the scale is taken. The rod is then quickly released and the penetration is noted.
The procedure is repeated until the needle fails to penetrate the flock for about 5 mm measured from the bottom of the mould. The time from the stopwatch is recorded which gives the initial setting time.
The cement shall be considered it finally set while applying final setting time needle gently, only an impression is marked on the top surface.
4. Soundness Test Of Cement
Soundness test of cement is performed to identify the presence of excess free lime and magnesia in the cement. This test is performed with the help of the Le-Chatelier apparatus.
Statement Of Problem: Two samples of cement are given, find out:
Which cement is unsound? and
Which cement is sound?
Theory
It is very important that the cement concrete is not so much as the change in its volume when it will be hardened. So it is necessary to control or reduce the quantity of the presence of free lime (CaO) and magnesia (MgO) in cement. So, the soundness test of cement is done to determine the quantity presence in cement.
During the manufacture of cement, free lime is produced. Free lime reacts with water and increases in volume considerably. Magnesia also has the same effect but its rate of reaction is slow.
A larger percentage of free lime and magnesia, if present, therefore, tends to increase the volume of the hardened concrete, thus causing disintegration. The cement is, therefore, said to be unsound when the percentage of free lime and magnesia is more than that specified by ISI (Table I). Unsoundness is measured with the help of the Le-Chatelier mould as explained in the procedure.
It must be specially mentioned here that in the event of the cement failing to comply with the above requirement, a further test should be made by the “Le-Chatelier Method” from another portion of the sample after aeration by spreading it out to a depth of 75 mm at relative humidity.
Test Procedure
Following are the procedure for Soundness Test of Cement: a) Weigh accurately 100 gm of cement and place it on a standard IS Sieve 90 microns.
b) Break down any air set lumps in the sample with fingers, but do not rub on the sieve.
c) Continuously sieve the sample by holding the sieve in both hands and giving a gentle wrist motion or automatic sieve shaker may be used for this purpose. The sieving should be continuous for 15 minutes.
d) Weigh the residue left after 15 minutes sieving. This residue shall not exceed the specified limits.
e) For recording observations, use plate 4, in Part IV.
Precautions
a) The cleaning of the sieve should be done very gently with the help of a brush i.e. 25 mm or 40 mm bristle brush with 25 cm handle.
b) After sieving the cement must be removed from the bottom surface of the sieve gently.
c) Weighing machine should be checked before use.
d) Sieving must be carried out continuously.
5. Compressive Strength Test
To perform this test 200 gm of cement and 600 gm of standard sand are taken and mixed thoroughly. To this, water is added @ P/4 + 3% when standard sand is used and @ P/4 + 3.5 % when ordinary sand is used. Where ‘P’ is the percentage of water required for a paste of normal consistency.
It is mixed thoroughly to an even colour. The cube mould of size 7.06 cm is placed on a non-porous base plate and is oiled inside. The above mortar is put into the cube mould and is compacted for two minutes by the vibration machine. The top surface is smoothened off by a trowel.
Like this mould, six moulds are filled. The prepared cubes are kept at a temperature of 27° ± 2° C for 24 hours. After this period, mortar cubes are taken out of the moulds and submerged in clean and fresh water for curing. Compressive strength test of cement is performed at the period mentioned below.
For ordinary cement, after 3 days and 7 days. For rapid hardening cement, after 1 day and 3 days. For low heat cement, after 3, 7 and 28 days.
The cube is tested by placing it under the Jaws of the compressive testing machine. The load is steadily and uniformly applied. The load at which the cube is fractured is noted in each case. The compressive strength is calculated by dividing this load by the cross-sectional area of the cube.
6. Chemical Composition Test.
As per IS: 269 – 1975 the chemical requirements of ordinary cement should be as follows
1. The ratio of the percentage of alumina to that of iron oxide should not be less than 0.66
2. The ratio of the percentage of lime to those of alumina, iron oxide and silica calculated with the following formula should not be less than 0.66 and it should not be greater than 1.02.
➤ The loss on ignition should not exceed 4 %. ➤ Total sulphur content as SO3 should not exceed 2.75 %. ➤ Weight of insoluble residue should not exceed 1.5 %. ➤ Weight of magnesia should not exceed 5 %.
7. The Heat of Hydration Test
The heat of hydration is defined as the chemical reaction between cement and water. during hydration of cement, sufficient heat is generated. The process of heat generation is quite rapid in the initial stage of the setting but its rate diminishes with the passage of time.
For mass concrete low heat cement should be used. As per IS: 4031 – 1968, the heat of hydration for low heat cement should be as follows:
After 7 days not more than 65 calories per gram and after 28 days not more than 75 calories per gram of cement.
8. Tensile Strenght Test
For evaluate the tensile strength of the cement, at first, six numbers of standard briquettes are made from cement mortar. All these briquettes are broken after 1 day, 3 days and 7 days of curing. The average strength of briquettes after 1, 3 and 7 days should be as follows:
Following are the four compounds which are known as bogues compounds and their formula & abbreviated formula are described below:
Name of Bogues Compound
Abbreviated Formula
Formula
Tri-calcium silicate
C3S
3 Cao.Sio2
Di-calcium silicate
C2S
2 Cao.Sio2
Tri-calcium aluminate
C3A
3 Cao.Al2o3
Tetra-calcium alumino-ferrite
C4AF
4 Cao.Al2o3.Fe2o3
Properties and Function of Bogue’s Compounds
The Bogue’s compounds (basic compounds) of Ordinary Portland Cement and their function are given below:
Bogue’s Compounds
% by mass of cement
Properties & Function of bogue’s compounds
Tri-calcium silicate (C3S)
25-50
It hydrates at a faster rate and produces higher heat of hydration. It is responsible for rapid hardening with an early gain in strength & has less resistance to chemical attack.
Di-calcium silicate(C2S)
20-45
It hydrates & hardens slowly and produces less heat of hydration. It provides much of the ultimate strength & has greater resistance to chemical attack.
Tri-calcium aluminate(C3A)
5-12
It is the first compound which starts hydrating. It produces the highest heat of hydration & responsible for the setting of cement.
Tetra-calcium alumino-ferrite (C4AF)
6-12
It hydrates rapidly but its individual contribution to the overall strength of cement is insignificant.
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 a temperature to form more complex compounds.
Approximate Oxide Composition Limits of Ordinary Portland Cement
Following are the 19 types of cement classification:
Different Types of Cement
1. Ordinary Portland Cement
It is obtained by burning (at about 1450 degrees centigrade) calcareous, siliceous, and argillaceous raw materials mixed in definite proportion and crushing the resulting clinker to a fine powder.
Ordinary Portland Cement 33 Grade– IS 269: 1989
Ordinary Portland Cement 43 Grade– IS 8112: 1989
Ordinary Portland Cement 53 Grade– IS 12269: 1987
2. Portland Pozzolana Cement
In the case of manufacturing of portland pozzolana cement, about 30 per cent of pozzolanic material is added to the ordinary cement clinker and the mix is thoroughly grounded. The pozzolanic material used are calcined clay or fly ash.
3. Super Sulphated Cement(IS 6909: 1990)
Super sulphate cement is made from well-granulated blast furnace slag of about 80 to 85 %, calcium sulphate of around 10 to 15 %, and Portland cement of about 1 to 2 %. The mixture of all these components is then grounded finer than the Portland cement.
4. Rapid Hardening Cement – IS 8041: 1990
As the name suggests, It hardens by gaining its strength faster than the ordinary Portland cement, because it has a higher percentage of C3S content, with finer grinding. The fineness value of rapid hardening cement is 325 kg/cm2 which is more than of OPC fineness value.
5. Extra Rapid Hardening Cement
6. Sulphate Resisting Cement – IS 12330: 1988
7. Portland Slag Cement
Portland slag cement is obtained by mixing Portland cement clinker, gypsum and granulated blast furnace slag in suitable proportions and grinding the mixture to get a thorough and intimate mixture between the constituents.
Characteristics of slag cement
➤ The fineness value of P.S.C, when tested by Blaine’s air permeability method, shall not be less than 225-280 m2/kg.
➤ The expansion value (i.e. soundness) of Un aerated cement when tested by Le-Chaelier method shall not be more than 10 mm and when tested by the autoclave method shall not be more than 0.8%.
⇰ In cold weather conditions, the low heat of hydration of slag cement coupled with moderately low rate of strength development can lead to frost damage
⇰ Producing pre-cast and pre-stressed concrete with slag cement.
⇰ Slag cement is used in concrete pavements.
8. Quick Setting Cement
9. Low Heat Cement – IS 12600: 1989
10. Air Entraining Cement
11. Coloured Cement: White Cement – IS 8042: 1989
White cement is just a variety of ordinary portland cement, it is white in colour. The whiteness is obtained by reducing the iron oxide content to a minimum. The properties of white cement are the same as ordinary portland cement.
What are the uses of white cement?
It is generally used for decorative work, such as terrazzo flooring, face plasters, marking etc.
12. Hydrophobic Cement – IS 8043: 1991
13. Masonry Cement – IS 3466: 1988
14. Expansive Cement
15. Oil Well Cement – IS 8229: 1986
16. Rediset Cement
In some cases, time is a very important factor. Such as rapid repair of the road, repair of pavement, etc. In such cases, cement should be such that it could achieve high strength in a matter of hours. There is no doubt that High aluminium cement is one that fulfils all the above requirements, but it has also some limitations. It cannot be used in hot weather and in humid regions. So a new type of cement was needed to fill this gap.
For that reason, In the PCA laboratories of the USA began their investigation to develop a new type of cement. And, finally, they developed a cement called Regset Cement. After that, in India, Associated Cement Company (ACC) company developed a similar type of cement which is called “Rediset Cement“.
Rediset cement is able to gain high strength in a matter of hours, without showing any retrogression.
17. Concrete Sleeper grade Cement
18. High Alumina Cement – IS 6452: 1989
This cement is also known as aluminous cement. It contains as high as 35 to 45 % of aluminates. This cement is manufactured from bauxite and chalk or limestone. This cement is very dark in colour. In this cement, setting time is controlled by the rate of cooling of the fused product and not by adding gypsum.
Characteristics of high alumina cement
Following are some important properties or characteristics of high alumina cement:
➤ Its initial setting time is 3 to 6 hours and the final setting time is 8 to 10 hours.
➤ This cement gains strength very quickly, due to the presence of a high percentage of aluminates. In 24 hours it gains as much strength as an ordinary cement acquires in 24 days.
➤ This cement is very much resistant to sea water attacks as well as chemical attacks.
➤ Its compressive strength is almost double that of ordinary cement.
➤ There is no trace of free lime and hence, this cement is more sound than ordinary cement.
What are the uses of high alumina cement?
⇰ It is used for constructions near sea shore and also underwater.
⇰ It is also used for making hard and heat-resistant concrete.
⇰ It is used for sealing rocks or concreting against internal water flow.
