The below list some common methods for testing the purity of purchased concrete.
This test is performed to know the amount of uncombined lime in supplied cement. For soundness test on cement, Le Chatelier apparatus is used. This apparatus consists of a brass mould with a diameter 30 mm, and 30 mm in height. In mould there is a split, which does not exceed 0.50 mm. Two indicators are available on each side of split. Following procedure is applied to determine soundness test on supplied cement:
- 1. At first, a cement paste is made adding 30% of water.
- 2. Brass mould is transferred on glass plate and filled with that cement paste.
- 3. Then the mould is covered by another glass plate. Small weight is then placed at top and mould is submerged in tape water for a period of 24 hours when temperature of water is room temperature.
- 4. The distance between two points of indicator is measured and noted.
- 5. Again, mould is submerged in water and water is heated at boiling point for sixty minutes.
- 6. Mould is taken out from hot water and allowed to be cool.
- 7. After cooling mould, distance between two points of indicator is measured and noted.
- 8. The difference between two measures is the expansion of cement, and if it is good quality cement, it will not exceed 10 mm.
To know about grinding of supplied cement, fineness test is performed. This test is done in two different ways: one is done by sieve test and another is by permeability apparatus test.
Sieve test: In sieve test, 100 gm cement is weighed and sieved through a standard sieve no. 9 for fifteen minutes. After that, residue is weighed and residue will not exceed more than 10% if it is quality cement.
Permeability apparatus test: By this test, surface area (specific surface area) of the cement particles is measured. It gives an idea about uniformity of cement particles and better than sieve analysis. Surface area of supplied cement should be more than 2250 cm2/gm.
There are four field tests for testing quality of cement as follows:
- 1. Color: Cement color should be uniform. Typical color of cement is grey with light greenish shade. It is not any reliable test but gives an idea about excess clay or lime and degree of burning as well.
- 2. Physical property: Cement will be smooth in touch. When cement is rubbed in, it will give smooth feelings. But it gives rough feelings when it is adulterated with sand particles. Normally, if you insert your hand in cement bag, you will feel cool. If you feel warm after inserting hand in cement bag, that cement will be poor in quality. Good quality cement sink in water and poor quality cement floats on water surface. So throwing a small amount of cement you can test quality of cement. If thin paste of supplied cement gives sticky feelings between fingers, it is good quality cement. Most important issue is that cement will give bad earthy smell if it contains too much powdered clay as adulterant.
- 3. Presence of small lumps: Good quality cement should be free from lumps. Lumps are formed by absorbing water from sounding environment. If there is any lump in cement, you should avoid using that cement in construction work and making pizza oven.
- 4. Strength: The strength of cement can be measured by following ways:
- i) A briquette is made with a proportion of cement to sand is 1:6. The size is 75mm × 25mm × 12mm. Then briquette is immersed in tape water for three days. If cement is good in quality, you will not able to break it easily. Even, you will not able to ground it in powder form.
- ii) A block of supplied cement is made with a proportion of cement to sand is 1:6 and immersed in water for 7 days. Then 340N pressure is applied on it. If it is not broken down, cement is good in quality.