Cement is a adhesive and cohesive materials which is capable of bonding together particles of solid matter into a compact durable mass. It is obtained by mixing lime stone and clay burning them and grinding to fine power.
Chemical composition of cement:
The raw materials used for manufacture of cement consists mainly of Silica, Lime, and Alumina. Iron oxide, Sulfur trioxide and other alkalies are also present but in very small proportions. These material are intimately mixed and burnt at a temperature of about 1400°C in a kiln to form clinkers which are complex chemical compounds. The relative proportions of these oxides are responsible for affecting the various properties of cement. Approximate composition of raw materials used in the manufacturing of ordinary land cement is as follows:
Final Compounds of Cement (Bogue Compound)
All the compounds undergo some chemical combination during the manufacturing process and form clinker. The constituents of the final products is known as bogue compounds. The materials, when subjected to high temperature, combine with each other to form complex compounds. The four compounds usually regarded as major compounds are:
Physical Properties of Cement compounds
Tri-calcium silicate (C3S) is responsible for most of the early strength of the concrete. Strength acquired during the first 7 days is mostly due to the hydration of tri-calcium silicate. Di-calcium silicate reacts with water at a very slow rate, and hence, the strength of the concrete, after the 7 days, is mainly due to the hydration of this compound. Di-calcium silicate starts contributing strength after 7 days of its placement and continues for about one year, though the rate of gaining strength from this compound goes on decreasing with the passage of time. Tetra-calcium aluminoferrite is more or les inactive compound and does not play any significant role in the setting properties or hardening properties of the cement. After addition of water in cement, tri-calcium aluminate (C,A) Is the first compound which starts hydration and is responsible for early setting of the cement. Hydration of tri-calcium aluminate generates considerable heat, which may be responsible for certain undesirable properties of the concrete. Hydrations of tri-calcium aluminate do not contribute any strength to the concrete. It is mainly responsible for the setting action of the cement.
Setting and Hardening of Cement: although the process of setting and hardening of cement take place simultaneously, yet thère is a difference between the two. Setting of the cement involves changing the plastic state cement paste to a stiff solid state which is not strong enough and has a very small compressive strength. Hardening of the cement concrete is the rate of gaining strength. With the passage of time, as the hydration proceeds, the compressive strength of the product goes on increasing. The sequence of hydration of compounds is listed below
Tri-calclum Aluminate:As soon as water is added to the cement, hydrolysis of this compound starts. This process may be delayed for about 30 to 40 minutes, by adding 210396 gypsum Powder This delay is essential to gain time for midng waterwith the cement, transportation of that mix to the place of use and for the finishing process. It is the compound responsible for the setting of cement. It does not contribute any strength to the concrete