Cement is one of the most used materials in construction and therefore knowledge of cement testing forms an important part. Tests on cement can be divided into two parts i.e., lab tests, and field tests. This blog will explain in detail both the lab and field tests that are performed on cement.
Field Tests on Cement
Field tests are performed to ascertain the quality of cement delivered to the field. These tests will give quick information about the quality of the cement. Below listed are the tests that are to be performed at the site before using the cement for construction.
Visual test: The colour of the cement must be greenish grey
Feel test: Thrusting the hand into the cement should give a cool feeling. Also, it should give a smooth feeling when rubbed between fingers.
Lump formation: Check with your hands for any lumps in the cement. If there are lumps, break them with your fingers. If the lumps don't break then the cement is considered spoiled due to air setting.
Floating test: The cement should float for some time before sinking when thrown into the water.
Lab Tests on Cement
Fineness Test of Cement
The objective of this test is to check the grinding of the cement. It can be checked using two methods i.e.,
Air Permeability Method
The test contains the following steps.
100 grams of cement is taken as a sample
The cement is sieved using a standard IS sieve no. 9 (90 microns)
Ensure that there are no lumps in the sample
Sieve the sample manually or mechanically for 15 minutes
Weigh the residue left on the sieve
This weight shall not exceed 10% for ordinary cement and 5% for rapid hardening or low-heat cement.
Air Permeability Test
This test is done to check the specific surface of the cement based on which the fineness of the cement could be ascertained. Lea and Nurse Air permeability apparatus is used for this test. The test is based on the relationship between the flow of air through the cement bed and the surface area of the particles comprising the cement bed. Blain air permeability apparatus is also used for this test.
Standard Consistency Test of Cement
Standard consistency 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 the Vicat apparatus.
The standard or normal consistency of a cement paste can be defined as that consistency that 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. The following procedure is used to perform this test.
400 gm sieved cement is taken & 100 gm of water is added and mixed thoroughly for about 3 minutes. T
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 of 5 to 7mm is obtained.
Test for Initial and Final Setting Time of Cement
The cement paste remains plastic for a short period of time. As the time lapses, the plasticity gradually disappears and the paste changes into a solid mass. This process is known as the setting of cement. The time to reach this stage is known as setting time. 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 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 the Vicat apparatus. The initial setting time of cement shall be the time 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. The following procedure is used to perform this test.
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.
A stopwatch is started at the instant of adding water. The paste 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 finally set when the final setting needle makes only an impression on the top surface of the paste.
Soundness Test of Cement
The soundness test of cement is performed to identify the presence of excess free lime and magnesia in the cement. Excess lime could produce an expansion in the volume of cement which is undesirable. Therefore, ascertaining the absence of such free lime becomes essential.
This test is performed with the help of the Le-Chatelier apparatus. A Le-Chtalier apparatus consists of a cylindrical brass mould and two indicator points. A glass plate is used along with Le-Chatlier's apparatus for this test. The following procedure is used to perform this test.
Place the Le-Chatlier's apparatus on a lightly oiled glass sheet and fill it with cement paste formed by gauging cement with 0.78 times the water required to give a paste of standard consistency.
Cover the mould with another piece of a lightly oiled glass sheet, place a small weight on this covering glass sheet and immediately submerge the whole assembly in water at a temperature of 27 ± 2°C for 24 hours.
Measure the distance separating the indicator points to the nearest 0.5 mm.
Submerge the mould again in water at the temperature prescribed above.
Bring the water to boiling, with the mould kept submerged, in 25 to 30 minutes, and keep it boiling for three hours.
Remove the mould from the water, allow it to cool and measure the distance between the indicator points.
The difference between these two measurements indicates the expansion of the cement. This must not exceed 10 mm for ordinary, rapid hardening and low heat portland cement.
If in case the expansion is more than 10 mm as tested above, the cement is said to be unsound.
Compressive Strength Test of Cement
Compressive testing forms an important part of understanding the grade of cement. The following procedure is used for this test.
200 grams of cement and 600 grams of sand are taken. The water of quantity (P/4 + 3) % of the total mass of cement and sand (where P is the standard consistency of cement) is taken.
Mix cement and sand in dry condition with a trowel for one minute and then add water and mix until a uniform colour is obtained. The time of mixing shall not be less than 3 minutes and not more than 5 minutes.
Immediately after the mixing, cement mortar is placed in the cube mould. Before placing the mortar, apply oil to the inner surface of the cube mould. For expelling the entrained air and avoiding honeycombing, the mortar shall be prodded 20 times in about 8 seconds and then compacted by vibration.
After compaction, finish the top surface of the cube in the mould by smoothing the surface with the blade of a trowel.
Keep the filled moulds in a moist closet or moist room for 24 hours.
At the end of that period, remove the mortar cube from the moulds and immediately submerge it in clean, fresh water and keep it there until taken out just prior to testing.
A compression testing machine or universal testing machine is used for testing. Test 3 cubes for compressive strength for each time period as per detailed specifications. Such as 3 cubes for 3 days test, 3 cubes for 7 days test and 3 cubes for 28 days test.
The cubes testing shall be conducted on their sides. Do not use any packing between the cube and the steel plates of the compression testing machine, when testing is conducted.
The load shall be steadily and uniformly applied and the rate of loading should be 35 N/mm2/min.
The pressure at which the cube fails gives the compressive strength of the cement.
With this, we hope we had covered all the important topics related to the test on cement. Do you want us to cover a topic of your interest in a simple manner? Let us know in the comments below.