The manufacture of cement requires strict control on its quality.
Therefore the following tests and specifications have been laid as per I.S.I. 269-1967 and 1976:
1. Chemical composition
2. Fineness
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3. Soundness
4. Consistency test
5. Setting time
6. Compressive strength
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7. Tensile strength
8. Heat of Hydration.
1. Chemical Composition:
The ordinary and rapid hardening Portland cements can be tested in accordance with the methods given in I.S. 4032. The results of tests should comply with the following chemical requirements.
(a) Ratio of percentage of lime to percentage of silica, alumina, and iron oxide when calculated by the following formula-
should not be greater than 1.02 and less than. 0.66.
(b) Ratio of percentage of alumina to iron oxide should not be less than 0.66.
(c) Weight of insoluble residue should not be more than 2%.
(d) Weight of magnesia should not exceed 6%.
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(e) Total sulphur content calculated as sulphuric anhydride (SO3) should not exceed 2.75 or 3.0%, when C3A by mass is 7% or less and greater than 1% respectively. C3A is calculated by the formula-
C3A = 2.65 (Al2O) – 1.69 (Fe2O3)
(f) Total loss on ignition should not be more than 5%.
Test Procedure:
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(a) Loss on Ignition:
Take 1.0 gram sample of cement and weigh it carefully. Heat this cement in weighed and covered platinum crucible for 15 minutes at about 900°C to 1000°C and cool. On cooling, weigh again. Total loss on ignition should not exceed 4%. If the porcelain crucible is used, then heating should be done for about 1 hour.
The loss on ignition shows the extent of carbonation and hydration of free lime and free magnesia due to exposure of cement to the atmosphere. Usually in a freshly ground normal cement, the free lime is less than 1%, but total uncombined free lime should not be allowed greater than 3% as it will cause unsoundness in the cement due to slow hydration of this free lime.
(b) Sulphuric Anhydride (SO3):
Weigh 1.0 gram cement carefully and add 25 ml (c.c.) cold water to it and stirred vigorously. While stirring 5 ml HCl (hydrochloric acid) may be added to the mixture. If necessary the solution may be heated and the cement is ground well with a flattened end of a glass rod. The process of grinding the cement should be continued till the decomposition of cement is complete.
Now dilute the solution to 50 ml and allow it to remain in this condition for 15 minutes. Filter the solution and wash the residue thoroughly with hot water. Leave the filter paper and residue. Dilute the filtrate to 250 ml and heat to boiling. To this solution add drop by drop 10 ml of hot barium chloride solution and continue the boiling till precipitate is well formed and heat it on steam bath atleast for 4 hours.
Now filter the precipitate and wash it thoroughly. Place the filter paper with residue in a crucible and incinerate the paper slowly and then ignite the product at 800°C to 900°C. Now cool it in a desiccator and weigh the barium sulphate. From this weight calculate the weight of SO3. It should not exceed 2.75% as excess of SO3 will cause unsoundness of cement.
(c) Weight of Magnesium:
Magnesium oxide (MgO) reacts with water very slowly that is the hydration of MgO is very slow. On hydration after the cement paste has set, MgO also swells in volume and causes cracks in the structure. Thus excessive MgO causes unsoundness in the cement. Thus as per I.S. 269-1967, the amount of MgO in cement should not be more than 6% by weight. It can be determined as per I.S. 269.
(d) Insoluble Residue:
It is a measure of adulteration of cement largely arising from the impurities in gypsum. As per I.S. 269-1967 it should not exceed more than 2%. The test can be carried out as per I.S. 269.
2. Fineness of Cement:
Regardless of composition, fineness is a vital property of cement, which influences the rate of reaction of cement with water. For a given weight of a finely ground cement, the surface area of the particles is greater than for a coarsely ground cement. For a satisfactorily ground cement, usually there should 12 be 1.1 x 1012 particles per kg of cement.
However if the cement is ground too finely, the extremely fine particles may become pre-hydrated by moisture present as vapour in the grinding mills or during storage etc., and thus no cementing value can be derived from such particles. If the particles are coarser than normal, the rate of hydration of cement will be slower.
The advantages of finer cements can be summarized as follows:
Advantages:
(a) The finer the cement, greater the rate of hydration.
(b) Finer the cement, rapid and greater the strength development.
(c) Coarser the cement, greater the period of hydration and complete hydration is not possible.
(d) Finer the cement, lesser they bleed.
(e) Finer the cement, better the workability.
Disadvantage:
(a) Finer the cement, higher the cost of grinding.
(b) Finer the cement, more rapidly it deteriorates on exposure to atmosphere.
(c) Finer the cement, stronger the reaction with alkali reactive aggregates.
(d) Finer the cement, greater the shrinkage and the proneness to cracking.
(e) Finer the cement, greater amount of C3A they possess and more amount of gypsum they require for proper retardation of setting time.
(f) For a standard consistency pastes, finer the cement, greater the amount of water needed.
Methods of Determining of Fineness of Cement:
(a) By Sieve Analysis:
Weigh 100 grams of cement accurately and place it in a standard I.S. Sieve No. 9 (90 micron) and break lumps if any by pressing with fingers. In no case it should be rubbed on the sieve. Holding sieve in both hands sieve for 15 minutes continuously with a gentle motion of wrist, so that no cement spills out. After 15 minutes sieving, weight the residue left on the sieve. In case of ordinary Portland cement and rapid hardening cement it should not be more than 10% and 5% respectively.
(b) Specific Surface Area Method:
Fineness of cement can be tested by BLAINE’S air permeability method.
(c) By Lea & Nurse Method.
3. Standard Consistency Test:
Standard consistency of a cement paste can be defined as that consistency which will permit the vicat plunger to penetrate to a point 5 ± 2 mm from the bottom of the vicat mould when the cement paste is tested by this apparatus. The water content of the standard paste is expressed as a percentage by weight of the dry cement. Usually this percentage varies from 26% to 33%.
Vicat Apparatus:
The vicat apparatus consists of a frame bearing a movable rod of weight 300 grams. At the upper end of this rod a cap is provided, while at the lower end there is arrangement to attach either the 1.13 mm needle or plunger of 10 mm diameter.
The needle C is of 1 mm square in section or 1.13 mm dia round needle. There is another needle with a metal attachment at the end of the needle projecting 5 mm from the end of the needle.
Procedure:
Weigh about 300 gram cement carefully and put it in a clean crucible and add about 25% water of the weight of cement to the cement and mix it thoroughly with a standard spatula. Care should be taken that gauging (mixing) time should not be less than 3 min. and not more than 5 min. Gauging shall be counted from the time of adding water to the dry cement upto starting filling the paste in the mould.
The paste should be filled in the mould from bigger end to the smaller end. Now wipe out the excess paste from the bigger end and put it on the glass plate and put the mould with plate on the base of the frame of the apparatus. If paste is in excess on the upper end, then remove it with the help of a trowel. In the entire process of filling paste in the mould, no pressure should be exerted on the paste.
Now fix the 10 mm dia plunger in the moving rod and bring it in touch with the paste and note down the indicator’s reading on the scale. Now release the plunger to penetrate it in the paste and find out its depth of penetration.
Repeat the procedure till the plunger is able to penetrate upto 5 + 2 mm from the bottom of the mould. The temperature of experiment should be 27 ± 2°C (25 – 29°C). Express the amount of water used in the paste as the percentage by weight of the dry cement. Usually this percentage varies from 26% to 33%.
4. Determination of Setting Time:
Broadly speaking setting refers to a change from a fluid to a rigid state. During setting, the paste also acquires some strength, while hardening refers to the gain of strength of a set cement paste. Setting of cement may also be defined as when the cement paste has lost its plasticity and becomes sufficient rigid to with-stand an arbitrarily defined pressure then it is said to have set.
The period of setting is divided into two parts as follows:
(a) Initial setting period
(b) Final setting period
(a) Initial Setting:
Initial setting period is defined as the time from mixing water to the cement till the cement paste is unable to fill the cracks developed in it by itself. Initial setting period of cement is very important as it affects the durability and strength of cement concrete. If the setting period is too short, it will be impossible to transport and place the concrete at the site of work before it becomes too rigid to compact. On the other hand if the setting period is too long, it would tend to slow down the progress of work unduly as adequate strength will not develop at the desired age.
(b) Final Setting Time:
This is the stage of setting of cement paste or cement concrete at which it has gained sufficient strength.
Determination of Setting Time:
Setting time also is determined with the help of vicat’s apparatus.
Procedure:
Weigh 400 grams cement carefully and prepare neat cement paste by adding 0.85 times water by weight of cement required to give a paste of standard consistency. Fill this paste into the mould of the apparatus and attach the 1.13 mm diameter needle in the moving rod of the apparatus.
Now place the mould resting on the non-porous plate under the needle on the base plate of the apparatus and lower the needle gently in contact with the surface of the paste and release, allowing it to penetrate into the paste. In the beginning the needle may penetrate completely into the paste.
Repeat this procedure till the needle fails to penetrate into the paste for about 5 mm above the bottom of the mould. Each time the mould should be rotated so that needle may not penetrate at one point only all the time. The period elapsing between the time when water added to the cement and the time at which the needle fails to pierce the paste by 5 mm from the bottom is known as initial setting time.
Final Setting Time:
Now replace the 1.13 mm diameter needle of the vicat apparatus by the needle with an annular attachment. The cement shall be considered as finally set when applying gently 1.13 mm diameter needle to the surface of the paste may make an impression on it, whereas the attachment fails to do so.
The temperature of the test should be 27 ± 2°C and relative humidity as 90%.
As per I.S. 4031-1968 the setting times of different cements should be as follows:
5. Determination of Compressive Strength:
For determining the compressive strength of cement, cubes 50 cm2 in surface area having a length of 7.06 cms are prepared of cement and standard sand and compacted by means of a standard vibration machine.
Standard Sand:
As per I.S. 650-1966, standard sand should have the following composition:
Thus this sand when sieved through 2 mm I.S. sieve, it passes through it 100% and when sieved on 90 micron sieve, 100% will be retained on it. Further it should be free from organic impurities.
Preparation of Mix:
For each cube of 7.06 cm length, 185 gram cement is needed. The ratio of cement and standard sand should be as 1:3 by weight. Weigh the materials separately for each cube and place them on a non-porous plate. Now mix the cement and sand dry for one minute with trowel.
Now add water given by the following relation, and mix it till the colour is uniform. After adding water, mix should be mixed for not less than 3 minutes. If it takes more than 4 minutes to obtain a uniform colour, the mix should be rejected and the operation should be repeated with a fresh quantity of cement, sand and water.
For each cube, material required shall be as follows when the cement is tested with single graded sand:
Where Pn is percentage of water required to produce a paste of standard consistency.
When graded sand is to be used, the material for each cube should be as follows:
Cement = 200 gram
Standard sand = 600 gram
Moulding of Specimens:
Clean the dust of the mould and apply a thin film of oil or grease on inside of it. Now place the mould in a vibrating machine, having a vibrating frequency of 12000 ± 400 per minute and pour mixture of cement, sand and water prepared as above in the mould and vibrate at least for 2 minutes. Wipe out the excess mortar from the mould and make the surface smooth with the help of trowel.
Curing:
The cubes should be cured after vibration for 24 hours at a temperature of 27 ± 2°C and at a relative humidity of 90%. After 24 hours, remove the mould and immerse the specimens in fresh water and keep them there till testing. Before testing the specimen should be taken out of water and wipe out excess water with jute or cloth.
Test:
For compressive strength, cement cube can be tested at the age of, 7 and 28 days and the average of at least three cubes should be taken at each age. The difference between the average of three cube’s strength and individual strength should not be more than 15%. The specimen should be put in the centre of the testing machine on the face other than the face cast. The rate of loading should not be more than 140 kg/cm2 per minute.
As per I.S. 269-1967, the compressive strength of cement should be as follows:
6. Determination of Tensile Strength:
Cement is very weak in tensile strength. Its tensile strength is determined with the help of briquettes shown in Fig. 2.15. As shown in Fig. 2.15 the thickness of the briquettes is 25.4 mm and the minimum area of its cross section is 645 mm2. For preparing briquettes cement sand mortar of 1:3 ratios is used. For each briquette 250 gram cement and 750 gram sand is used.
The amount of water to be used in the mortar is given by the relation:
Preparation of Specimen:
The sand cement mixture is prepared as for cubes with the proper quantities of cement, sand, and water as explained above. The mould is cleaned and oiled. It is now put on a steel plate and the mixture of sand and cement is filled in it and compacted with the help of a standard spatula and finished plane.
Now it is put on upside down and finished as explained above. Now the mould is kept for 24 hours at 27 ± 2°C and 90% relative humidity. After 24 hours, the mould is removed and the specimen is put in fresh water for curing. Curing water should be changed every week. As far as possible the curing temperature should be kept at 27 ± 2°C.
Test:
The tensile strength test of cement should be done at 3 and 7 days age. For testing, the briquette is taken out of curing water and excessive water wiped out and fixed into the clamp.
This machine works on the principle of compound lever. Before starting the test, the jaws D-D of the machine are greased and the briquette is fixed and the lead shot are allowed to fall in container 3 from container A. The change of shots from A to B should be so regulated that the rate of increase in load on briquette should increase at the rate of 35 kg/cm2 per minute or 7 kg/cm2 per 12 seconds. The breaking stress of briquette can be obtained by dividing the weight of lead shots in chamber B at the time of breaking by the area of cross section.
Result:
For one age, the average of six briquette strength should be taken. The individual strength of any briquette should not differ by 15% from the average value.
For different cements as per I.S. 269-1958, the tensile strength should be as follows:
7. Determination of Soundness of Cement:
Cements which exhibit expansions due to delayed or slow hydration of free lime, magnesia and calcium sulphate are known as unsound cements. As unsoundness of cement is not apparent for months together after adding water to cement, therefore it is essential to test the soundness of cement in accelerated manner. Le Chatelier was first to provide a test for soundness of cement. He used a very simple apparatus.
Le Chatelier apparatus consists of a small brass split cylinder or other suitable metal of 0.5 mm thickness. The internal diameter and its height is 30 mm. On either side of the split, two indicators with pointed ends AA are attached to the cylinder. The length of these pointers from the centre of the cylinder is 165 mm. The split of the cylinder should not be more than 0.5 mm.
Test:
Weigh 100 grams of cement carefully and add 0.78 times water needed for preparing a paste of standard consistency. Now place the cylinder on a glass plate and fill it with the cement paste prepared and cover it with an other glass plate and put a small weight on this covering sheet or plate. Immerse the whole assembly in water at a temperature of 27°C to 32°C and keep it there for 24 hours. At the end of this period i.e., after 24 hours of immersion, measure the distance between the indicator points and submerge the whole assembly in water again at the temperature as above (27°C – 32°C).
Now heat the water in such a way that water may start boiling within 25 to 30 minutes. Keep the mould assembly in boiling water for one hour. After boiling for one hour, the assembly is taken out of water and allowed to cool. After cooling the distance between the indicators is measured again. The increase in this distance represents the expansion of the cement. The expansion of aerated cement should not exceed more than 5 mm.
Le Chatelier test detects unsoundness due to free lime only; but if magnesia is also present in the cement, then soundness should be tested with the help of autoclave test, which is sensitive to both free lime and free magnesia.
8. Heat of Hydrated Cement:
Autoclave:
It is a high pressure steam boiler, equipped with an automatic pressure control and safety valve. The capacity of heating unit should be such that with the maximum load (water plus specimen etc.) the pressure of the saturated steam in the autoclave may be raised to a gauge pressure of 21 kg/cm2 in about 60 ± 15 minutes time from the start of heating. It should be capable of maintaining the pressure of 21 ± 1 kg/cm2 corresponding to a temperature of 215.7°C.
Further it should be capable to drop pressure from 21 kg/cm2 to 0.7 kg/cm2 in 60 minutes after shutting off the supply of heat. The pressure gauge has a dial diameter of about 11.5 cms and graduated from zero to 42 kg/cm2. The least count of the gauge is 0.4 kg/cm2. The error in the gauge should not be more than 0.2 kg/cm2.
Test:
500 gram of cement is weighed carefully and sufficient water is mixed to make a paste of standard consistency. The paste is filled in the mould of cross section 25 x 25 mm and 250 mm length. The mortar is filled in the mould by pressing it with thumb or fore finger in on or two layers. After compacting the mortar, the surface is flushed with the help of trowel.
After moulding, the specimens are stored in a moist room. After 24 ± 1/2 hours of moulding, the mould is removed and the length of the specimen measured carefully, and placed in the autoclave at room temperature and about 7 to 10% volume of autoclave is filled with water.
To permit air to escape from the autoclave during the early portion of heating period, the vent valve is left open till the steam begins to escape. Now the valve is closed and the temperature of the autoclave is raised in such a way that a pressure of 21 kg/cm2 is developed in the autoclave at a temperature of 216°C in about 60 ± 15 min. from the start of heating and this pressure is maintained for 3 hours. At the end of 3 hours, heat supply is stopped and autoclave is allowed to cool at a rate such that the pressure falls less than 1 kg/cm2 at the end of one hour.
Now the autoclave is opened and test specimens are immediately placed in water at a temperature above 90°C and cooled by adding cold water in such a way that the temperature of the whole assembly including water falls down to 27 ± 2°C in 15 minutes. After maintaining this temperature for further 15 minutes, the specimens are taken out of water, surface dried and their lengths measured. The expansion of the bar should not be more than 0.8% but preferably 0.5%.