Tests for cement: Field tests and Laboratory tests

 

Tests for cement 

1. Field tests

2. Laboratory tests

 

1. Field tests of Cement -

Following tests may be carried out in the field :

A.     Colour

B.     Physical properties

C.     Presence of lumps

D.    Strength

E.     Date of Packing

 

A. Colour -

• The colour of cement should be uniform.
• It should be typical cement colour i.e. grey colour with a light greenish shade.
• This is not not always a reliable test. But it gives an indication of excess lime or clay and the degree of burning.

B. Physical properties -

• Cement should feel smooth when touched or rubbed in between fingers.
• If the hand is inserted in a bag or heap of cement, it should feel cool and not warm.
• If a small quantity of cement is thrown into the water, it should float some time before finally sinking. Then, it should sink and should not float on the surface.
• A thin paste of cement with water should feel sticky between the fingers. If the cement contains too much-pounded clay and silt as an adulterant, the paste will give an earthy smell.

 

C. Presence of lumps -

• Cement should be free from hard lumps.
• Such lumps are formed by the absorption of moisture from the atmosphere.
• Any bag of cement containing such lumps should be rejected.

 

 

D. Strength -

• A thick paste of cement with water is made on a piece of a glass plate and it is kept under water for 24 hours. It should set and not crack.
• A block of cement of dimension 25 mm ×25 mm and length of 200 mm is prepared. It is immersed for 7 days in water. It is then placed on supports 150mm apart and it is loaded with a weight of about 34kg/ 340 N. If the block will not show any failure, then the cement is of good quality.

 

E. Date of Packing -

Strength of cement reduces with time, so it is important to check the manufacturing date of the cement. The general fact is that the cement should be used before 90 days from the date of manufacturing.

 

2. Laboratory tests -

Laboratory tests are conducted mainly for assessing the quality of cement. These are:

A.     Fineness

B.     Compressive Strength

C.     Consistency

D.    Setting Time

E.     Soundness

F.      Tensile Strength

 

 

A. Fineness Test -

• The fineness of cement is responsible for the rate of hydration, rate of evolution of heat and the rate of gain of strength. Finer the grains more is the surface area and faster the development of strength.
• This test is carried out to check the proper grinding of cement.
• The fineness of cement particles may be determined either by the sieve test or permeability apparatus test.
• In the 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% of the original weight.
• In the permeability apparatus test, a specific area of cement particles is calculated. This test is better than the sieve test. 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.

 

B. Compressive Strength test -

• This test is carried out to determine the compressive strength of cement.
• The mortar of cement and sand is prepared in ratio of 1:3.
• Water is added to mortar in water-cement ratio 0.4.
• The mortar is placed in moulds. The test specimens are in the form of cubes and the moulds are of metals. For 70.6 mm and 76 mm cubes, the cement required is 185gm and 235 gm respectively.
• Then the mortar is compacted in a vibrating machine for 2 minutes and the moulds are placed in a damp cabin for 24 hours.
• The specimens are removed from the moulds and they are submerged in clean water for curing.
• The cubes are then tested in a compression testing machine at the end of 3days and 7 days.
• The load is to be applied uniformly at the rate of 35 N/mm².
• The compressive strength at the end of 3 days should not be less than 11.50 N/mm² and that at the end of 7 days should not be less than 17.50 N/mm².

C. Consistency test on cement -

• This test is conducted to find water content of cement using a standard consistency test apparatus, Vicat's apparatus.
• Standard consistency of cement paste is defined as that water content which will permit a Vicat plunger of 10 mm diameter and 50 mm length to penetrate depths of 5 mm to 7 mm from the bottom of the mould within 3-5 minutes of mixing.
• The test has to undergo three times, each time the cement is mixed with water varying from 24 to 27% of the weight of cement.
• This test should be conducted at a constant temperature of 25°C or 29°C and at a constant humidity of 20%.
• The purpose of this test is to determine the percentage of water required for preparing cement pastes for other tests.
• Take 300 gm of cement and add 30 per cent by weight or 90 gm of water to it.
• Mix water and cement thoroughly.
• Fill the mould of the Vicat apparatus and the gauging time should be 3.75 to 4.25 minutes.
• Vicat apparatus consists of a needle is attached to a movable rod with an indicator attached to it.
• There are three attachments: square needle, plunger, and needle with annular collar.
• The plunger is attached to the movable rod. the plunger is gently lowered on the paste in the mould.

• The settlement of the plunger is noted. If the penetration is between 5 mm to 7 mm from the bottom of the mould, the water added is correct. If not the process is repeated with different percentages of water till the desired penetration is obtained.

D. Setting Time -

• Vicat's apparatus is used to find the setting times of cement i.e., initial setting time and final setting time.
• Initial Setting Time: For this test, a needle of 1 mm square size is used. The needle is allowed to penetrate into the paste (a mixture of water and cement as per the consistency test). The time taken to penetrate 33-35 mm depth is recorded as the initial setting time.
• Final Setting Time: After the paste has attained hardness, the needle does not penetrate the paste more than 0.5 mm. The time at which the needle does not penetrate more than 0.5 mm is taken as the final setting time.
• This test is used to detect the deterioration of cement due to storage. The test is performed to find out the initial setting time and final setting time.
• Cement mixed with water and cement paste is filled in the Vicat mould.
• The square needle is attached to the moving rod of the Vicat apparatus.
• The needle is quickly released and it is allowed to penetrate the cement paste. In the beginning, the needle penetrates completely. The procedure is repeated at regular intervals until the needle does not penetrate completely. (up to 5mm from bottom)
• Initial setting time =<30min for ordinary Portland cement and 60 min for low heat cement.
• The cement paste is prepared as above and it is filled in the Vicat mould.
• The needle with the annular collar is attached to the moving rod of the Vicat apparatus.
• The needle is gently released. The time at which the needle makes an impression on the test block and the collar fails to do so is noted.
• Final setting time is the difference between the time at which water was added to cement and time as recorded in the previous step, and it is =<10 hours.

 

E. Soundness -

• The purpose of this test is to detect the presence of uncombined lime in the cement.
• The cement paste is prepared.
• The mould is placed and it is filled by cement paste.
• It is covered at the top by another glass plate. A small weight is placed at the top and the whole assembly is submerged in water for 24 hours.
• The distance between the points of the 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.
• The mould is removed from the water and it is allowed to cool down.
• The distance between the points of the indicator is again measured. The difference between the two readings indicates the expansion of cement and it should not exceed 10 mm.

F. Tensile Strength test -

• This test is carried out using a cement-mortar briquette in a tensile testing machine.
• The mortar of cement and sand is prepared. The proportion is 1:3 which means that (x) gm of cement is mixed with 3(x) gm of sand.
• The water is added to the mortar. The quantity of water is 5 per cent by weight of cement and sand.
• The mortar is placed in briquette moulds. A typical briquette is shown in below figure. 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 ml for 12 briquettes.
• The briquettes are kept in a damp cabin for 24 hours.
• The briquettes are carefully removed from the moulds and they are submerged in clean water for curing.
• 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.

• 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:
• Ultimate tensile stress = failing load / 6.45
• 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.