Introduction to Cement.

Cement - The product manufactured by burning and crushing to powder an intimate and well proportion mixture of calcareous and argillaceous material is called cement.

1. Dry process :

In this method mainly lime stone and clay are ground separately and then reduced to very fine powder it is called as raw slurry then it is further blended with correct composition and mixed by means of compressed air.

Under this the aerated powder behave almost like liquid and after certain lime uniform mixture is obtained.

This raw material is sieved and are ground to fine powder with the help of granulator by adding water which turn blended meal into pellets.

This process of producing cement is relatively slow and tedious.

Manufacture of Cement

2 . WET PROCCESS :

In this process the lime stone is brought from quarries and then it is taken to ball mill after crushing it to small fragments and is mixed with clay or shale and ground to fine slurry by addition of water.

The cement slurry formed contains 30-50% water as water is added in grinding it is called as wet grinding

The slurry formed is pumped into rotary kiln for burning clinker formed due to burning are cooled and stored.

The calcinations of cement slurry is done in a rotary kiln. When slurry enters the kiln the hot gases and water is removed,

decomposition occurs and formation of aluminates takes place and formation of clinker takes place.

The cooled clinkers collected from lower end and ground in ball mill with the addition of 3-5% of gypsum to prevent flash setting of cement.

Manufacture of Cement

Flow Diagram Wet Process:

CHEMICAL CONSTITUENTS OF OPC

Following are the various test carried on OPC :

1.Finess test             2.Standard consistancy test   3.Initial and final setting time test4.Compressive strength test5.Soundness test.

Test carried on OPC :

Fineness test :   Fineness of cement refers to the size of grains or particles of cement. Lesser the grain size, finer the cement. Due to small grain size, more surface area is available for contact with water and the reaction of hydration becomes faster, gaining of strength is more rapid and rate of evolution of heat increases.There are two methods prescribed in IS codes for testing fineness.

Test carried on OPC :Sieve Test (IS 4031, Part 1 1996) :The procedure is given below :1.Weigh 100 gm of cement correctly and take it on a standard IS sieve number 9, i.e. a sieve of size 90 microns. 2.Break down any air-set lumps in the sample with fingers.3.Continuously sieve the sample giving circular and vertical motion for a period of 15 minutes. Mechanical sieving devices may also be used.4.Weigh the residue left on the sieve.5.This residue shall not exceed by the limits given below : OPC 10 gm Rapid Hardening cement 5 gm

Test carried on OPC :Air Permeability Method (IS 4031, Part 2 1999) :     The procedure of the test is given below :1.Fill the weight of cement as calculated for making a bed of 0.475 porosity into the permeability cell.2.Slowly pass dry air through the cement bed at a constant velocity.3.Read the difference in manometer limbs and note it as h1.4.Read the difference in flow meter limbs and note it as h2. It should be between 30 to 50 cm.5.Repeat the readings till steady state is achieved. i.e. h1/h2 is constant.6.Specific surface is calculated by following formula7.The specific surface should not be less than the limits given below : OPC : Not less than 2250 cm2/gm Rapid hardening cement : Not less than 3250 cm2/gm Low heat cement : Not less than 3200 cm2/gm.

Definition of Standard consistency :Standard consistency is defined as that consistency which will permit a standard Vicat plunger to penetrate a depth of 33 to 35 mm from the top of the mould in a standard Vicat apparatus.

Procedure of standard consistency test is given below:1.Take about 500 gm of cement and prepare a paste with known weight of water say about 20 – 25% of weight of cement.2.Fill the paste in Vicat mould within 3 to 5 minutes. Level the top surface.3.Shake the mould to expel any air bubbles.4.Attach the plunger with the screw provided on the rod of the sliding weight.5.Bring down the weight till plunger just touches the top surface of paste.

6.Release the weight so that the plunger penetrates the paste. Measure the penetration. This can be measured by the pointer attached to the sliding weight and moving on the scale.7.If the penetration is less than 33 to 35 mm from top of mould, increase the water percentage to make a fresh paste. If the penetration is more, decrease the water percentage to make a fresh paste.8.In this manner, by making a fresh trial paste a number of times, find the water percentage by weight, which will give the penetration of the plunger upto 33 to 35 mm depth from top of the mould.9.This water percentage is known as standard consistency of the given cement and is denoted by P.

Initial setting time :Initial setting time is defined as the time elapsed between the moment when water is poured in cement to the moment when the cement paste starts losing its plasticity.

The procedure of the test is given below :1.Take about 500 gm cement and mix it with 0.85 P percent of water where P is the water percentage required for standard consistency to make a smooth paste. Start a stopwatch at the moment when water is added to cement.2.Fill and shake the Vicat mould with the paste within 3 to 5 minutes after adding water.3.Lower the initial setting time needle till it touches the surface of the cement paste in the mould.4.Release the weight so that the needle penetrates the paste. Initially, it will penetrate the complete depth, i.e. 40 mm, of the mould.

5.Take readings after every 1 or 2 minutes and when the penetration decreases, take readings after every 20 seconds and then after every 10 seconds, moving the mould to take reading at different place every time.6.Record the time on the stopwatch when the penetration is 33 to 35 mm from the top surface.7.This time is known as the initial setting time

Final setting time :Final setting time is defined as the time elapsed between the moment when water is added to cement and the moment when the paste has completely lost its plasticity. Final setting time test is conducted as a continuation of the initial setting time test and the same mould of cement is used for the testFinal setting time test is conducted as a continuation of the initial setting time test and the same mould of cement is used for the test.

The procedure is given below :1.Remove the initial setting time needle and attach the final setting time needle, which is very similar to the initial setting time needle, only it has a collar with a rim attached to it. The central needle projects 0.5 mm more than the rim. The collar has a hole known as air-vent through which trapped air in the rim can escape so that it does not interfere with the reading.2.Lower the final setting time needle till it gently touches the paste and release it.3.Observe the impression made by the attachment on the paste.4.Initially, both the rim and central needle will make the impression. Turn the mould around and after every few minutes, take the reading on a different place on the surface of the paste.5.When the impression of the rim starts becoming faint, take readings at very short time intervals.

6.Record the time when only central needle makes an impression but the surrounding rim does not make an impression at all. This is shown in Fig.

Fig. Impressions of final setting time

7.The time elapsed between the moment when water is added to the cement and the moment when only central needle makes an impression but the rim fails to make impression is called the final setting time.

Soundness of cement :The soundness is defined as the test ensures that the cement does not show excessive thermal expansion and if it does, it can be rejected.   Le Chatlier test test is performed with the Le Chatelier’s Apparatus. It consists of a brass cylinder, cut along its height with two pointers welded along each side of the cut. Due to thermal expansion, the cement in the cylinder expands and this expansion is measured by the pointers attached to the cylinder.

 The procedure for soundness test is given below : 1.  Take 100 gm of cement. This is mixed with 0.78 P times water, where P is the water required for standard consistency and it is mixed through for about 3 minutes.2.The paste is filled in the split cylinder, which is covered on top and bottom with glass plates and is kept in water at 27 to 32C for 24 hours with a small weight on the top glass plate for stability.3.The distance between the pointers is measured and mould is submerged in boiling water for 3 hours.4. The mould is removed, allowed to cool and again the distance between the pointers is measured again.5.The difference between these two distances represents the expansion of cement. This must not exceed 10 mm for OPC, Rapid hardening and low heat cements. If it exceeds 10 mm, then the cement is said to be unsound.

Le Chatelier’s Apparatus

Adulteration of Cement :Adulteration means mixing very fine stone-dust, fine silt or mixing powdered fly ash or old cement to increase volume of fresh cement.Adulteration of cement causes many problems and very serious consequences of adulterated cement may result even in loss of human lives. Hence adulteration must be detected and prevented. The adulteration of cement is done in the following ways :1.Mixing of very fine stone-dust from quarries.2.Mixing of fine silt.3.Mixing of fine, powdered fly ash.4.Mixing of lime powder or surkhi.5.Mixing of old cement which has entirely lost its strength due to long period of storage.Adulteration of cement can be easily detected by field tests and then laboratory tests can be performed to quantify the adulteration in percentage.

Field Tests on CementFor immediate detection of unsuitability of cement, even before the laboratory testing, field tests are performed on cement when it is received on the site. Following are the commonly performed field tests :1.Open the bag and observe the cement. There should not be any visible lumps. If any lump found it should be powdered by pressing between thumb and fore finger if it does not turn into powder the cement is considered have been spoiled by air setting.2.Thrust your hand into the cement bag, it should feel cool.3.Rub the cement between finger tips by taking a pinch of the cement. It should not feel gritty but should be smooth in feel.4.Throw a handful of cement on the water surface in a bucket. The cement should float in case of cement is of good quality and if it sinks it contains some impurities.

5.Take 100 gm cement and make a stiff paste by adding very little water. Pat a cake out of this paste on a glass plate. The cake should have sharp edges. Submerge the glass plate with the cake at the bottom of a water bucket. After 24 hours, the cake should not lose its shape and should attain some strength.6.The colour of cement should be uniformly greenish grey.7.It should not feel oily when touched.

Field Tests on Cement

Storing of cement :

Storing of cement has great affinity towards water and it absorbs moisture from humid air. Lumps are formed due to this which go on becoming harder and the cement becomes useless. Hence it is important that precautions should be taken while storing cement on site to safe guard it from damage.

Following precautions should be taken while storing cement on site :

1.Cement should be stored in a special water-tight shed with

a dry and damp proof door, waterproof walls and leak-proof

roof.

2.The stacks of cement bags should be placed at a distance

of at least 30 cm from walls.

3.A stack should contain maximum 15 bags.

4.Width of a stack should not exceed 3 m.

5.Bags should be stacked on a raised platform, with

sufficient aisles for movement between rows of stacks.

6.Bags should be stored in such a way that bags received

first should be used first.

When a sealed bag of cement is opened, oxygen in air starts

reacting with the ingredients as well as the humidity in the air

starts reactions with the ingredients.

Hence it is a dvisible to use cement within 3 months at the

maximum. The strength at different time periods as follows :

Cement fresh from factory

100% strength

The effect of storage of cement on properties of cement :

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