Cement was made out of lime also in the part. It is still lime, but processed to become portland cement.

--By Abhinay Jayaswal

Cement is a fine grained powder that, when mixed with water, turns into a solid. Because of its strength, cement is widely used for construction projects. It is simply a binder, which sets after it dries and reacts with carbon dioxide in the open air and hence binds other materials together.

The basis of cement is lime; it has been used as a binding material in construction for thousands of years. In the past, lime was simply heated by itself in order to drive out the carbon dioxide and leave calcium oxide, which is what was used to make cement. These days they use what is called Portland cement. This is made by heating lime, bauxite, iron ore and clay together. They then crush it and add gypsum to the mixture. The gypsum will slow down the rate at which the cement hardens making it easier to use.

Cement is produced burning calcium carbonate found in lime stone, silica, alumina and iron oxide found in clay, bauxite, iron ore respectively at about 14000 C to form clinker and after grinding with a mixture of gypsum, OPC (Ordinary Portland Cement) is produced.

Raw Materials of Cement Industry

1. Aluminia or Clay: Aluminia is responsible for the setting action of cement. The larger the amount of aluminia present in cement, the quicker it will start setting. Excess quantity of aluminia weakens the cement

2. Silica (SiO2): It also goes in to the chemical combination with calcium and forms hard silicates which are responsible for imparting strength to the cement.

3.  Lime (CaO): It is the most important ingredient of cement and its bulk in cement is above 60 percent of the total contents. Its proportion should be carefully decided. 

4.  Iron Oxide: This ingredient mainly imparts colour to the cement. Besides this, it also goes into the chemical combination and helps increase the strength and hardness of the cement.

5.  Magnesium Oxide (MgO): It also imparts strength and hardness to the cement, but only when present in small amounts.

6.  Sulphur Trioxide: A small percentage of sulphur renders cement sound. An excess amount of it may make it, unsound.

7.  Alkalies: Alkalies present in cement are Na2O, K2O. Alkalies present in cement may react with reactive silica present in aggregate to form sodium silicate which has a capacity to absorb an excess amount of water causing volume change in hardened cement, mortar/cement concrete.

Reaction is very slow and may take years together to be completed and can expand 130 percent.

8.  Calcium Sulphate (CaSO4) or Gypsum: This ingredient is used to retard or prolong the initial setting action of the cement.

There are different types of cement and they have been classified according to their use and strength. The different types of cement are:

- Ordinary Portland Cement 
- Modified Portland Cement
- Quick Setting Cement
- Low Heat Portland Cement 
- Water Proof Portland Cement
- High Alumina Cement
- Portland Slag Cement
- Portland Pozzolana Cement

The Composition of different types of cement used in Nepal

The compound composition
As we know, the reaction is an exothermic reaction and produces heat because while burning at 1400° C it absorbs lot of energy and can dissipate by reacting with water, and chemical potential is raised and comes out of the stable stage by hydration.

C3A (Tricalcium aluminate) reacts first with water. It gives initial setting with the help of Gypsum.

C3S (Tricalcium silicate) hydrates more quickly. So high strength is gained in 28 days.

C2S (Dicalcium silicate) reacts slowly and gives maximum strength till 2-3 year.

The chemical characteristics for cement as per NS Specification

When water is added in cement it turns into the plastic stage (no liquid/solid), it does not gain any shape and is a workable paste to be used. From plastic stage to almost solid stage it is called initial setting (dominant period) which is of minimum 45 min for OPC and it becomes stiff (non-workable) and turns into hardening to gain strength.

The writer is Project Sales Head, Arghakhanchi Cement Pvt Ltd.