Builiding Technology 2008 Question 6A

What is cement? State the functions of cement in concrete mix.

Answer:

Cement is the material used to bind concrete. There are many varieties although Portland cement are the most important and widely used.  They depend on water to set, harden and as known as hydraulic cements when water is added to the cement, chemical reactions take place. The process is exothermic, meaning that heat is given off. The stiffening process subsequently begins and the material begins to harden and develop strength. A mixture of cement and sand and water to form a paste is known as cement mortar, while the product obtained by mixing cement, water, sand and aggregates is cement concrete.  

The ingredient of cement are clay (silica, aluminum oxide and iron oxide) and chalk or limestone. All these materials are finely and intimately mixed and burnt at a chinkering temperature of about 1400c. The chinkens resulting are ground to a fine powder to get cement. During burning and fusion, all the ingredients under chemical combination and from what are known as Bogue compounds. Bogue Compounds are:

1. Tri-calcium silicate, C3S
2. Di-calcium silicate, C2S
3. Tri-calcium aluminate, C3A
4. Tetra-calcium alumino-ferrite, C4AF

The proportion of above stated Bogue compounds vary in various types of cements. Tri-calcium silicate and Di-calcium silicate are the compounds mostly responsible for the strength characteristics of the cement. Concrete is manufactured using cement, aggregates and water. The aggregates usually contain a range of particle sizes, from fine to coarse. Fine aggregate as referred to as sand. The coarse aggregates are usually crushed rock or limestone. The chemical reaction between cement and water is responsible for setting and hardening of cement concrete. Although all materials that go into a concrete mixture are essential, cement is the most important constituent because it is usually the delicate link in the chain.

The function of cement is:

1. Although cement constitutes only about 10% of volume of the concrete mix, its the active portion of the building medium and the only scientifically controlled ingredient of concrete.
2. The extent of hydration of cement and resultant hydrated cement influences the physical properties of concrete. This hinges on the fineness of the cement portion of water and types of cement used. The amount of water in a cement mix must be carefully controlled. if high quality concretes with uniform strength are to be achieved. For this reason, the amount of water contained in the aggregates before they are mixed must also be carefully determined before adding the extra water required for mixing.
3. Bind the sand and course aggregates together. Aggregates help reduce the cost of materials as aggregates are cheaper that cement. They reduce the dry shrinkage and thermal movement and improves resistance to wear by both abrasion and weathering.
4. Fill the voids in between the sand and coarse aggregates particles to form a compact mass.