Manufacture|Hydration|Water required|Tests on Cement

Manufacturing of Cement

Manufacturing of cement involves various raw materials and processes. Each process is explained as chemical reactions take place for the manufacture of Portland cement. The Portland cement can be manufactured by two processes, Wet process and dry process depending upon whether the ingredients are mixed in wet or dry state. The manufacture procedures of Portland cement is described in following four steps, they are

1. Mixing of raw material

2.Burning

3.Grinding

4. Storage and packaging

1.Mixing of raw material

The major raw materials used in the manufacture of cement are Lime, Silica, Iron and Aluminum.

The mixing procedure of the manufacture of cement is done in 2 methods,

a) Dry process

b) Wet process

Dry Process

a)The both calcareous and argillaceous raw materials are firstly crushed in the gyratory crushers to get 2-5 cm size pieces separately. The crushed materials are again grinded to get fine particles into a ball or tube mill. Each finely grinded material is stored in a hopper after screening. Now these powdered

minerals are mixed in required proportion to get dry raw mix which is then stored in silos and kept ready to be sent into a rotary kiln.

Wet Process

The raw materials are firstly crushed and made into powdered form and stored in silos. The clay is then washed in washing mills to remove adhering organic matters found in clay. The powdered limestone and water washed clay are sent to flow in the channels and transfer to grinding mills

where they are completely mixed and the paste is formed, i.e., known as slurry. Then the slurry is led into a collecting basin where composition can be adjusted. The slurry contains around 38-40% water that is stored in storage tanks and kept ready for the rotary kiln for burning.

2.Burning

The burning process is carried out in the rotary kiln which is made up of steel tubes having the

diameter of 2.5-3.0 meter and the length 90-120 meter. The inner side of the kiln is lined with refractory bricks. The kiln is supported on the columns of masonry or concrete and rested on roller bearing in a slightly inclined position at the gradient of 1 in 25 to 1 in 30. The raw mix of dry process of corrected slurry of wet process is injected into the kiln from the upper end. The kiln is heated with the help of powdered coal or oil or hot gasses from the lower end of the kiln so that the long hot flames are produced. As the kiln position is inclined and it rotates slowly, the material charged from the upper end moves towards the lower end at the speed of 15 m/hr. In the upper part, water or moisture in the material evaporates at 400°C temperature, so this process is known as the Drying pm Zone. The central part i.e., calcination zone, the temperature is around 1000°C, where decomposition of limestone takes

place. The remaining material is in the form of small lumps known as nodules after the CO₂ is released.

3. Grinding of Clinkers

The cooled clinkers are received from the cooling pans and sent into mills. The clinkers are grinded finely into powder in a ball mill or tube mill. Powdered gypsum is added around 2-3% as a retarding agent during final grinding. The final obtained product is cement.

4.Storage and packaging

The grinded cement is stored in silos, from which it is marketed either in container load or 50 kg. bags.

Hydration of Cement

The Chemical reaction that takes place between cement and water is called hydration of cement.This reaction is exothermic in nature, due to which a considerable amount of heat is released during hydration of cement. This is called the 'heat of hydration'. The hydration of cement is not a sudden process. This reaction is faster in the early period and continues indefinitely at a decreasing rate. During Hydration of cement, C₂S and C₂S react with water and calcium silicate hydrate (C-S-H) is formed along with calcium hydroxide Ca(OH)2.

Calcium silicate hydrate is one of the most important products of the hydration process and it determines the good properties of cement. It can be seen from the above reactions that C3S produces more

quantity of calcium hydroxide than C₂S.

Water Required for Complete Hydration

It has been estimated that on an average 23% of water by weight of cement is required for chemical reaction with Portland cement compounds. As this 23% of water chemically combines with cement, it is called bound water. A certain quantity of water is absorbed by the gel pores. This water is known as gel water. The bound water and gel water are complementary to each other. It has been

estimated that 15% water by weight of cement is required to fill up the gel pores. Therefore a total of 38% of water (bound water + gel water) by weight of cement is required for the complete chemical reaction of cement and occupy the space within gel pores.

Tests on Cement

1.Fineness Test

This test is carried out to check the proper grinding of cement. Fiber cement produces stronger mortar and can be mixed with a larger volume of sand than coarser grain 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 IS sieve no. 9. The residue is then weighed and this weight should not be more than 10% of the original weight for OPC and 5% for RHC. 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. Specific surface area of OPC = 2250 cm²/gm PPC-3000 cm³/gm. LHPC-3200 cm²/gm, RHPC-3250 cm²/gm and SRPC-4000 cm²/gm.

2.Consistency Test

It is done by Vicat's Mold to determine the proper amount of water required for preparing cement pastes for other tests. Vicat apparatus consists of a needle is attached to a movable rod with an

indicator attached to it. Length of Vicat's needle is 40 to 50 mm and its diameter is 10 mm.

Take 300 gm of cement and add 30 percent by weight or 90 gm of water to it and Mix thoroughly.Then Fill the mold of the Vicat's apparatus with this mix, put the mold on the base plate of apparatus and the plunger is gently lowered on the paste in the mold. The normal consistency of

cement is percentage of water corresponding to penetration of Vicats needle between 33 to 35 mm

3.Setting Time Test

It is also done by Vicat's apparatus. The test is performed to find out the initial setting time and final setting time. Initial setting time is the time at which any crack on the surface do not reunite whereas

Final setting time is the time at which concrete gains sufficient strength and hardness.

Initial Setting Time: 300 g of cement is thoroughly mixed with 0.85 times the water for standard consistency and Vicat mold is completely filled and top surface is levelled. 1 mm square needle is

fixed to the rod and gently placed over the paste. Then it is freely allowed to penetrate. In the beginning the needle penetrates the paste completely. As time lapses the paste start losing its

plasticity and offers resistance to penetration. When the needle can penetrate up to 5 to 7 mm above the bottom of the paste experiment is stopped and time lapsed between the addition of water and end of the experiment is noted as the initial setting time.

Final Setting Time: The square needle is replaced with an annular collar. Experiment is continued by allowing this needle to freely move after gently touching the surface of the paste. Time lapsed between the addition of water and the mark of needle but not of annular ring is found on the paste.

4. Soundness test

Uncombined lime and magnesium makes the cement unsound. So, a soundness test is carried out to detect the presence of uncombined lime and magnesia. Soundness test of cement is carried out in the Le chatelaine apparatus as shown in the figure. The apparatus consists of a small brass cylinder of

30mm diameter and 30mm height and 0.5 mm thick and split as shown in the figure. This split should not exceed 0.5mm. Two indicators are attached to the cylinder on either side of the split. Cement paste at

Normal consistency is prepared and the apparatus is filled with it after placing it over a glass plate. Then the upper surface is also covered with another glass plate and a small weight is placed over it. Then the whole assembly is immersed in water and heated to boil in half an hour. Boiling is done for 3 hours and removed from water and cooled. Then the distance between two indicators is again measured. The differences between two measurements represent the expansion of the sand. This value should not be more than 10mm

in the case of OPC. Le Chatelier's test does not consider the expansion due to magnesia. Autoclave test is carried out to consider the effect of both lime and magnesia.

5.Compressive Strength Test

For this 200 gm of cement is mixed with 600 gm of standard sand in 1:3. After mixing dry condition for a minute distilled potable water P/4 + 3 percentage is added where, P is the water required for the Normal consistency. They are mixed with trowel for 3 to 4 minutes to get uniform

mixture. The mix is placed in a cube mold of 70.6 mm size (Area 5000 mm²) kept on a steel plate and prodded with 25 mm standard steel rod 20 times within 8 seconds. Then the mold is placed on

a standard vibrating table that vibrates at a speed of 12000 ± 400 vibration per minute. The mold is vibrated for two minutes and the hopper is removed. The top is finished with a knife or with a trowel and

leveled. After 24 ± 1 hour mold is removed and the cube is placed under clean water for curing.. After a specified period cubes are tested in a compression testing machine. Average of three cubes is reported as crushing strength. The compressive strength at the end of 3 days should not be less than 115 N/mm² and that at the end of 7 days not less than 17.5 N/mm².

6.Tensile Strength Test

It is done by briquette Testing Machine (area of briquette = 6.05 sq. cm) to determine tensile strength of cement, generally used for RHC. 6 Cube are prepared in 1:3 cement sand mortar and placed in mold to prepare briquette for testing. The briquettes are kept in a damp for 24 hours and carefully removed from the molds. The briquettes are tested in a testing machine at the rate of loading 1.2-2.4  N/mm²/min. Tensile strength of Good cement should not be less than 20 kg/cm² after 3 days .