The exhaustive tests are carried out in laboratory to decide precisely the quality of cement. The extreme care should be taken to collect the sample of cement from site.
In order to make a representative sample, it is desirable to collect in nearly equal portions as follows:
(1) When Cement is Loose:
It should be taken from at least 12 points from heap or heaps of cement.
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(2) When Cement is in Bags:
It should be taken from at least 12 different bags or packages.
The quantity of cement so collected is intimately mixed and the final sample of cement weighing at least 50 N is prepared. It is then stored in air-tight container till the tests are started.
Following are the purposes of testing of cement (BIS- 4031/1968):
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(i) To determine the physical and chemical properties of cement;
(ii) To regulate the various stages in the manufacturing process as a slight difference in the process of manufacture of cement may cause a great difference in the quality of cement; and
(iii) To understand the behaviour of cement after its use on the works.
Following are the standard tests for cement:
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(1) Chemical composition
(2) Fineness
(3) Compressive strength
(4) Tensile strength
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(5) Consistency
(6) Setting times
(7) Soundness
A brief description of each of the above test is given below.
1. Chemical Composition:
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The various tests are carried out to determine the chemical constituents of cement.
Following are the chemical requirements of ordinary cement as per IS: 269-1998:
(i) Ratio of percentage of alumina to that of iron oxide – This ratio should not be less than 0.66.
(ii) Ratio of percentage of lime to those of alumina, iron oxide and silica – This ratio is known as the lime saturation factor (LSF) and it should not be less than 0.66 and it should not be greater than 1.02, when calculated by the following formula –
(iii) Total loss on ignition – This should not be greater than 4 per cent.
(iv) Total sulphur content – The sulphur content is calculated as so3 and it should not be greater than 2.75 per cent.
(v) Weight of insoluble residue – This should not be greater than 1.50 per cent.
(vi) Weight of magnesia – This should not exceed 5%.
2. Fineness:
This test is carried out to check proper grinding of cement. The fineness of cement particles may be determined either by sieve test or by permeability apparatus test.
In sieve test, the cement weighing 100 gm is taken and it is continuously passed for 15 minutes through standard BIS sieve no. 9. The residue is then weighed and this weight should not be more than 10 per cent of original weight.
In permeability apparatus test, specific surface area of cement particles is calculated. This test is better than sieve test and it gives an idea of uniformity of fineness. The specific surface acts as a measure of the frequency of particles of average size. The specific surface of cement should not be less than 2250 cm2/gm.
3. Compressive Strength:
This test is carried out to determine the compressive strength of cement.
Following procedure is adopted:
(i) The mortar of cement and sand is prepared. The proportion is 1:3 which means that x gm of cement is mixed with 3x gm of sand.
(ii) The water is added to the mortar. The water cement ratio is kept as 0.4 which means that x gm of water is added to dry mortar.
(iii) The mortar is placed in moulds. The test specimens are in the form of cubes with side as 70.6 mm or 76 mm. The moulds are of metal and they are constructed in such a way that the specimens can be easily taken out without being damaged. For 70.6 mm and 76 mm cubes, the cement required is 185 gm and 235 gm respectively.
(iv) The mortar, after being placed in the moulds, is compacted in vibrating machine for 2 minutes.
(v) The moulds are placed in a damp cabin for 24 hours.
(vi) The specimens are removed from the moulds and they are submerged in clean water for curing.
(vii) The cubes are then tested in compression testing machine at the end of 3 days and 7 days. The testing of cubes is carried out on their three sides without packing. Thus three cubes are tested each time to find out the compressive strength at the end of 3 days and 7 days. The average value is then worked out. During the test, the load is to be applied uniformly at the rate of 350 kg/cm2 or 35 N/mm2.
(viii) The compressive strength at the end of 3 days should not be less than 115 kg/cm2 or 11.50 N/mm2 and that at the end of 7 days should not be less than 175 kg/cm2 or 17.50 N/mm2.
4. Tensile Strength:
This test was formerly used to have an indirect indication of compressive strength of cement. It is at present generally used for the rapid hardening cement.
Following procedure is adopted:
(i) The mortar of cement and sand is prepared. The proportion is 1:3 which means that x gm of cement is mixed with 3x gm of sand.
(ii) The water is added to the mortar. The quantity of water is 8 per cent by weight of cement and sand.
(iii) The mortar is placed in briquette moulds. A typical briquette is shown in fig. 6-7. The mould is filled with mortar and then a small heap of mortar is formed at its top. It is beaten down by a standard spatula till water appears on the surface. Same procedure is repeated for the other face of briquette. Such twelve standard briquettes are prepared. The quantity of cement may be 600 gm for 12 briquettes.
(iv) The briquettes are kept in a damp cabin for 24 hours.
(v) The briquettes are carefully removed from the moulds and they are submerged in clean water for curing.
(vi) The briquettes are tested in testing machine at the end of 3 days and 7 days. Six briquettes are tested in each test and average is found out. During the test, the load is to be applied uniformly at the rate of 35 kg/cm2 or 3.50 N/mm2.
(vii) It may be noted that cross-sectional area of briquette at its least section is 6.45 cm2. Hence the ultimate tensile stress of cement paste is obtained from the following relation –
(viii) The tensile stress at the end of 3 days should not be less than 20 kg/cm2 or 2 N/mm2 and that at the end of 7 days should not be less than 25 kg/cm2 or 2.50 N/mm2.
5. Consistency:
The purpose of this test is to determine the percentage of water required for preparing cement pastes for other tests.
Following procedure is adopted:
(i) Take 300 gm of cement and add 30 per cent by weight or 90 gm of water to it.
(ii) Mix water and cement on a non-porous surface. The mixing should be done thoroughly.
(iii) Fill the mould of Vicat apparatus. The interval between the addition of water to the commencement of filling the mould is known as the time of gauging and it should be 3 ¾ to 4 ¼ minutes.
(iv) The Vicat apparatus is shown in fig. 6-8. It consists of a frame to which is attached a movable rod weighing 300 gm and having diameter and length as 10 mm and 50 mm respectively. An indicator is attached to the movable rod. This indicator moves on a vertical scale and it gives the penetration. The Vicat mould is in the form of a cylinder and it can be split into two halves.
The Vicat mould is placed on a non-porous plate. There are three attachments — square needle, plunger and needle with annular collar. The square needle is used for initial setting time test, the plunger is used for consistency test and the needle with annular collar is used for final setting time test.
(v) The plunger is attached to the movable rod of Vicat apparatus. The plunger is gently lowered on the paste in the mould.
(vi) The settlement of plunger is noted. If the penetration is between 5 mm to 7 mm from the bottom of mould, the water added is correct. If penetration is not proper, the process is repeated with different percentages of water till the desired penetration is obtained.
6. Setting Times:
This test is used to detect the deterioration of cement due to storage. It may however be noted that this is purely a conventional type of test and it has got no relation with the setting or hardening of actual concrete. The test is carried out to find out initial setting time and final setting time.
(i) Initial Setting Time:
Following procedure is adopted:
(a) The cement weighing 300 gm is taken and it is mixed with percentage of water as determined in consistency test.
(b) The cement paste is filled in the Vicat mould.
(c) The square needle of cross-section 1 mm x 1 mm is attached to the moving rod of the Vicat apparatus.
(d) The needle is quickly released and it is allowed to penetrate the cement paste. In the beginning, the needle penetrates completely. It is then taken out and dropped at a fresh place. The procedure is repeated at regular intervals till the needle does not penetrate completely. The needle should penetrate upto about 5 mm measured from bottom.
(e) The initial setting time is the interval between the addition of water to cement and the stage when needle ceases to penetrate completely. This time should be about 30 minutes for ordinary cement.
(ii) Final Setting Time:
Following procedure is adopted:
(a) The cement paste is prepared as above and it is filled in the Vicat mould.
(b) The needle with annular collar is attached to the moving rod of the Vicat apparatus. This needle has a sharp point projecting in the centre with annular collar as shown in fig. 6-8.
(c) The needle is gently released. The time at which the needle makes an impression on test block and the collar fails to do so is noted.
(d) The final setting time is the difference between the time at which water was added to cement and time as recorded in (c). This time should be about 10 hours for ordinary cement.
7. Soundness:
The purpose of this test is to detect the presence of uncombined lime in cement. This test is performed with the help of Le Chatelier apparatus as shown in fig. 6-9. It consists of a brass mould of diameter 30 mm and height 30 mm. There is a split in mould and it does not exceed 0.50 mm. On either side of split, there are two indicators with pointed ends. The thickness of mould.
Following procedure is adopted:
(i) The cement paste is prepared. The percentage of water is taken as determined in the consistency test.
(ii) The mould is placed on a glass plate and it is filled by cement paste.
(iii) It is covered at top by another glass plate. A small weight is placed at top and the whole assembly is submerged in water for 24 hours. The temperature of water should be between 24°C to 35°C.
(iv) The distance between the points of indicator is noted. The mould is again placed in water and heat is applied in such a way that boiling point of water is reached in about 30 minutes. The boiling of water is continued for one hour.
(v) The mould is removed from water and it is allowed to cool down.
(vi) The distance between the points of indicator is again measured. The difference between the two readings indicates the expansion of cement and it should not exceed 10 mm.