Types of tests performed on cement
Tests are performed on cement to check the quality, nature, type etc of cement. These tests are very necessary because these tests gives the idea that how cement will actually behave, bind, compact etc after construction.
Tests performed on cement are distributed into two types.
- Field test of cement
- Laboratory tests of cement
Field test of cement
These tests are mainly performed at the site of construction. These tests are mainly performed when the laboratory apparatus are not present in the site.Following tests can be performed to determine the quality of cement -
- The colour of cement should be uniform greenish grey.
- It should feel smooth when touched. If it feels rough then the cement is not appropriate.
- If small quantity of cement is thrown into the bucket of water, it should float if it sinks then the cement is not of desired quality.
- There should not be any lumps, it there are any lumps they should break on pressing between the thumb & finger (Index of middle finger). If it does not turn into power on pressing then the cement is not appropriate because of presence of moisture in cement.
- When heated it should not change its colour.
- When hand is inserted into a bag of cement it should feel cool instead of warm.
Also read - Various uses of cement
Laboratory tests of cement-
Various tests are performed to determine the quality of cement. Some tests which are generally performed are given below.- Fineness test
- Consistency test
- Initial and final setting time test
- Chemical composition test
- Soundness test
- Compressive strength test
1. Fineness test –
This test is done to check the proper grinding of cement (in ball mills or tube mills). This test is very important because the strength of cement greatly depends on the fineness of cement. Finer the cement more will be the strength of cement. This is because fine cement provides more surface area for water to react.Fineness of cement can be determined from sieve test or by permeability test.
Sieve test: -
In this test 100 g of cement is taken. It is continuously sieved for is min through IS sieve number 9 (90 microns). The residue left on the sieve when weighed should not be more than 10% of cement taken (10% of 100 g cement).Permeability test: -
This test is better than sieve test because it gives an idea of uniformity of fineness of cement. Specific surface area of cement can be calculated in permeability apparatus. For cement permeability should not be less than 2250 cm²/g.2. Consistency test –
This test is very useful because it helps in determining the percentage of water required for preparing cement pastes for other useful tests like setting time, soundness and compressive strength of cement. The apparatus used for performing this test is Vicat apparatus. This apparatus is designed such that it can be used for determining the water required for determining the percentage of water required to produce a cement paste of standard or normal consistency.Vicat Apparatus:
Vicat apparatus consists of a metal (gun metal or aluminium) frame having a movable rod, with a cap on its top and a plunger is attached at the bottom end. The total weight of movable rod including cap is limited upto 300 g. This apparatus is provided with a releasing pin having a movable rod to fix the rod or to make it slide down freely.
There are three attachments in Vicat’s apparatus namely plunger, square needle and needle with annular collar which can be fixed in the lower end of movable rod in Vicat’s apparatus. The uses of different attachments are as follows –- Plunger is used for finding consistency test.
- Square needle is used for initial setting time.
- Needle with annular collar is used for final setting time of cement.
An indicator is provided in movable rod which moves on the vertical scale to give the penetration value. The vertical scale is graduated from 0-40 mm in either direction. This apparatus is also provided with a hollow conical split type mould of inner diameter 80 mm and 40 mm depth. This mould is held in the position with the help of a clamp ring. This mould is used for placing the sample in the form of cylinder and it can be split into two halves, this mould is also known as Vicat mould.
Method of finding consistency of cement paste
Following procedure is adopted for finding out consistency of cement paste.- Take about 500 g of cement and sieve it from IS sieve No. 9. Add approximately 125 g of water (say 25% by weight of cement) for the first trail of cement.
- Mix water and cement in a non-porous surface, so that loss of water can be minimized.
- The paste must be prepared in a standard manner and filled into the Vicat mould within 3-5 minutes i.e. mixing of cement and water should be done within 3-5 minutes (not less than 3 minutes and not more than 5 minutes).
- After completely filling the mould, slightly shake and tap the mould to expel air. The top layer of cement paste is made level with the help of trovel and the excess paste is removed.
- A standard plunger, 10 mm diameter, 50 mm long is attached and brought down to touch the surface of cement paste in the test block and the reading on scale is noted.
- The rod is then quickly released without any jerk allowing it to sink into the paste by its own weight.
- Take the reading by noting the depth of penetration of plunger.
- If the rod penetrates into the paste by 33 mm – 35 mm from the top of the mould, the mould is said to be of normal consistency.
- If the normal consistency is not obtained, conduct a 2nd trial (say with 24 or 26% of water) and find out the depth of penetration of plunger.
- Similarly, conduct trials with higher or lower water/cement ratios till such time the plunger penetrates for a depth of 33-35 mm from the top.
The percentage of water in which normal/standard consistency is obtained i.e. that particular percentage of water that allows plunger to penetrate only to a depth of 33-35 mm from the top is known as the percentage of water required to produce a cement paste of standard consistency. This percentage is usually denoted as ‘P’. This test should be conducted in a constant temperature (27° + 2°C) and constant humidity (90%).
Let W be the weight of cement taken and W1 be the weight of water required for normal consistency or weight of water required for desired penetration.
Then, percentage of water required for normal consistency
P= (W1/W) x 100
The percentage of water required for normal consistency varies from 25% to 35%.
Also read - Raw materials required for manufacturing of cement
3. Initial and final setting time test
When the cement is mixed with water the cement paste is formed, as the time passes the cement paste converts into hard mass. The time elapsed to reach this stage is called setting time of cement. The time is counted from the moment when the water is added to the cement. The setting time of cement is divided into two parts known as initial setting time and final setting time. Initial and final setting time test is carried out in Vicat apparatus.a) Initial setting time test
The time elapsed between the moment when the water is added to the cement, to the time that the paste starts losing its plasticity is called initial setting time.Initial setting time is determined to give the sufficient time for mixing, transportation, placing, compaction and finishing of cement mortar or cement concrete.
Vicat apparatus is used for determining initial setting time. Following procedure is adopted for determining initial setting time.
Procedure
- Take 500 g of cement sample and sieve it through I.S. Sieve No. 9. Water is added at the rate of 0.85P (0.85 times the water required to produce the cement of normal/standard consistency) by the weight of cement. (i.e. Weight of water to be added = 0.85P X 500∕100)
- The stopwatch should be immediately started at the moment water is added to the cement.
- Clean the Vicat’s mould properly and lubricate it from inside so that cement paste does not stick to it.
- The paste shall be guaged and filled into the Vicat mould in specified manner within 3-5 minutes (Mixing process of cement should be done within 3-5 minutes).
- The temperature of water and that of the test room, at the time of gauging shall be within 27°C ± 2°C.
- The square needle of cross-section 1 mm x 1 mm is attached to the moving rod of Vicat apparatus.
- Lower the square needle (also known as initial setting time needle) gently and bring it in contact with the surface of the cement paste in test block.
- Note down the initial reading in the graduated vertical scale.
- Quickly release the needle. Allow it to penetrate into the cement paste in test block. Note the time on the stop watch.
- In the beginning, the needle will completely penetrate through the test block. But after some time when the paste starts losing its plasticity, needle may penetrate only to a depth of 33-35 mm from the top or 5 mm to 7 mm from bottom of the mould.
- Note down the time from stopwatch when the needle penetrates the cement paste in test block to a depth equal to 33-35 mm from the top. The reading obtained is taken as initial setting time.
Therefore, Initial setting time may also be defined as the time consumed between the addition of water to cement and the stage when the square needle fails to penetrate the cement paste in the test block to a depth of 33-35 mm from the top or 5-7 mm from the bottom of mould.
b) Final setting time
The time elapsed between the moment when the water is added to the cement, and the time when the paste has completely lost its plasticity and has attained sufficient firmness to resist certain definite pressure is termed as final setting time of cement.Vicat apparatus is used for determining final setting time. Following procedure is adopted for determining final setting time.
Procedure
The cement paste for final setting time is made in similar manner as in case of initial setting time test.
- Replace the initial setting time needle with final setting time needle (i.e. needle with annular collars).
- The needle is released on the top surface of prepared cement paste sample in Vicat’s mould at regular intervals and the time is recorded.
- Initially both needle and the collar will make an impression on the top surface of cement paste sample.
- The time is noted when center needle makes an impression, while the circular cutting edge of the attachment fails to do so. The time thus recorded is the final setting time of cement. In other words the paste has attained such hardness that the center needle does not pierce through the paste more than 0.5 mm.
Therefore, the final setting time may be defined as the time elapsed between the moment when water is added to the cement and the stage when the needle makes an impression in cement sample while the collar fails to do so.
Also read - How is cement manufactured
4. Chemical composition test
Chemical composition of cement mainly defines the property of cement. Several tests are performed to determine the chemical composition of cement. The chemical composition of cement should be strictly as per following guidelines.(i). Ratio of percentage of lime to percentage of silica, alumina and iron oxide when calculated by the formula given below should not be greater than 1.02 and not less than 0.66. This ratio is called lime saturation factor percent or cementation index.
(ii). Ratio of percentage of alumina to iron oxide should not be less than 0.66.
(iii). Magnesia content should not be more than 5%.
(iv). The loss in ignition should not be more than 4%.
(v). The insoluble residue left should not be more than 2%.
(vi). The total sulphur content (SO3) should not be more than 2.80%.
5. Soundness test
This is very important that the cement after setting shall not undergo any appreciable change of volume or expansion in cement after setting. Some cement undergoes a large expansion after setting causing disruption of the set and hardened mass. When such cement is used serious difficulties are faced in fields of durability of structures. The purpose of this test is to ensure that the cement does not show any appreciable subsequent expansion. The unsoundness in cement is due to the presence of excess of lime and magnesia. The cement mortar prepared from such cement is liable to expand after final setting is complete. Unsoundness in cement is the major reason for cracking of cement.So, we can say that soundness test detect the presence of uncombined lime and magnesia in cement.
Symptoms of unsoundness in cement does not come to surface for a considerable period of time. Therefore, accelerated tests are required to detect it.
There are number of such tests used for detecting unsoundness in cement. But here we will discuss about soundness test by using Le-Chatelier’s apparatus.
Procedure for doing this test:
(i). Take 100 g of neat cement and sieve it through I.S. Sieve No. 9. Then add water at the rate of 0.72P (Where P = Percentage of water required for making cement paste of normal consistency) by the weight of cement.
(ii). Thoroughly mix the cement water matrix for about 3 minutes.
(iii). The mould is placed on a glass plate.
(iv). The prepared cement paste is then filled into the mould.
(v). The mould is covered at top by another glass plate. A small weight is placed at the top and whole assembly is submersed in water for 24 hours. The temperature of water should range between 27° C to 32° C.
(vi). Take the mould out and note the distance between the points of indicators.
(vii). The mould is again placed in water and the water is heated such that the boiling point of water is reached in about 25-30 minutes. The boiling of water is continued for 3 hours.
(viii). The mould is removed from water and it is allowed to cool down.
(ix). Again measure the distance between the two points of indicators.
(x). The difference between these two measurements indicates the expansion of cement. For ordinary portland cement, rapid hardening cement and low heat portland cement it should not exceed 10 mm.
6. Compressive strength test
Cement is known for its compressive strength. So, we can say that compressive strength of hardened concrete or cement mortar is the most important property among all the properties of cement. Therefore it is very necessary to determine compressive strength of cement at the laboratory before using cement in important works.
Compressive strength test is performed to determine the compressive strength of cement. Compressive strength is the final test in finding out quality of cement.
Procedure(i). Take 600 g of standard sand (Ennore sand), 200 g of cement (i.e., ratio of cement to sand is 1:3) in a non-porous enamel tray and mix them with a trowel for one minute. The standard sand should conform to IS 650 – 1991 specifications.
(ii). Then add water at the rate of P/ 4 + 3.0 percent of combined weight of cement and sand. Remember if ordinary sand is used add P/4 + 3.5 percent.
(iii). Mix all the three ingredients thoroughly until the mixture is of uniform colour. The time of mixing should not be less than 3 minutes nor more than 5 minutes.
(iv). Clean the cube mould of size 7.06 cm and lubricate the inner surface of cube mould with oil so that cement would not stick on the mould.
(v). Place the mould on a non porous base plate and fill the cube mould with cement mortar immediately after mixing process. The area of each face of the cube will be equal to 50 sq cm.
(vi). Compact the mortar either by hand compaction in a standard specified manner or on the vibrating machine (12000 RPM) for 2 minutes.
(vii). Compacted cube in the mould are kept at the temperature of 27°C ± 2°C and at least 90 per cent relative humidity for 24 hours to set and harden.
(viii). After 24 hours the cubes are removed from the mould and submersed in clean fresh water until taken out for testing.
(ix). Three cubes are tested in a compression testing machine at the end of three days and the remaining cubes are tested after 7 days of curing.
(x). The compressive strength shall be the average of the strengths of the three cubes for each period respectively.
(xi). Compressive strength of ordinary portland cement should not be less than the values given below.
