VARIOUS TESTS OF CEMENT

Basically two types of tests are under taken for assessing the quality of cement. These are either field test or lab tests. The current section describes these tests in details.

Field test:

There are four field tests may be carried out to as certain roughly the quality of cement. There are four types of field tests to access the colour, physical property, and strength of the cement as described below.

Colour:

The colour of cement should be uniform.

It should be typical cement colour i.e. grey colour with a light greenish shade.

Physical properties:

Cement should feel smooth when touched between fingers.

If hand is inserted in a bag or heap of cement, it should feel cool.

Presence of lumps:

Cement should be free from lumps.

For a moisture content of about 5 to 8%, this increase of volume may be much as 20 to 40 %, depending upon the grading of sand.

Strength:

A thick paste of cement with water is made on a piece of thick glass and it is kept under water for 24 hours. It should set and not crack.

Laboratory tests:

Six laboratory tests are conducted mainly for assessing the quality of cement. These are: fineness, compressive strength, consistency, setting time, soundness and tensile strength.

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 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% of original weight.

In permeability apparatus test, specific area of cement particles is calculated. This test is better than sieve test. The specific surface acts as a measure of the frequency of particles of average size.

Compressive strength:

This test is carried out to determine the compressive strength of cement.

The mortar of cement and sand is prepared in ratio 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 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 compression testing machine at the end of 3days and 7 days. Thus compressive strength was found out.

Consistency:

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 percent by weight or 90 gm of water to it.

Mix water and cement thoroughly.

Fill the mould of Vicat apparatus and the gauging time should be 3.75 to 4.25 minutes.

Vicat apparatus consists of a needle is attached 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 plunger is noted. If the penetration is between 5 mm to 7 mm from the bottom of mould, the water added is correct.

If not process is repeated with different percentages of water till the desired penetration is obtained.

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 percent by weight or 90 gm of water to it.

Mix water and cement thoroughly.

Fill the mould of Vicat apparatus and the gauging time should be 3.75 to 4.25 minutes.

Vicat apparatus consists of a needle is attached 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 plunger is noted. If the penetration is between 5 mm to 7 mm from the bottom of mould, the water added is correct.

If not process is repeated with different percentages of water till the desired penetration is obtained.

Setting time:

This test is used to detect the deterioration of cement due to storage. The test is performed to find out initial setting time and final setting time.

Cement mixed with water and cement paste is filled in the Vicat mould.

Square needle is attached to moving rod of 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 till 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 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 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 previous step, and it is =<10hours.

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 top by another glass plate. A small weight is placed at top and the whole assembly is submerged in water for 24 hours.

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.

The mould is removed from water and it is allowed to cool down.

Tensile strength:

This test was formerly used to have an indirect indication of compressive strength of cement.

The mortar of sand and cement is prepared.

The water is added to the mortar.

The mortar is placed in briquette moulds. 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.

The briquettes are kept in a damp for 24 hours and carefully removed from the moulds.

The briquettes are tested in a testing machine at the end of 3 and 7 days and average is found out.

Compressive strength:

This test is carried out to determine the compressive strength of cement.

The mortar of cement and sand is prepared in ratio 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 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 compression testing machine at the end of 3days and 7 days. Thus compressive strength was found out.

Grade of cement:-

Grade of cement represents the specific 28 days compressive strength. The following three grades are given along with their compressive strengths

33 Grade OPC –33 MPa

43 Grade OPC – 43 MPa

53 Grade OPC – 53 MPa