Concrete is either Plain or Reinforced. By definition, Plain Concrete is an artificial stone as a result of mixing cement, fine aggregates, coarse aggregates and water. The conglomentation of these materials producing a solid mass is called Plain Concrete. 

Reinforced Concrete on the other hand, is a concrete with reinforcement embedded in such a manner that the two materials act together in resisting forces.

The different types of cement used in construction are: 

1. The ordinary Portland cement
2. The rapid hardening Portland cement which is preferred when high early strength is desired.
3. The blast furnace or sulphate cement which is used on structures designed to resist chemical attack.
4. The low heat Portland cement used for massive section designed to reduce the heat of hydration.
5. The Portland pozzolan cement with a low hardening characteristics concrete.
6. The high alumina cement.
   

The High Alumina Cement is sometimes called Aluminous Cement or Cement Fundu. Its chemical composition is different from that of Portland cement having predominant alumina oxide contents of at least 32% by weight. The alumina lime of 0.85% to 1.3%.

This type of cement has a very high rate of strength development as compared with the ordinary Portland cement. Aside from its rapid hardening properties, it can resist chemical attack by sulphate and weak acids including sea water. It can also withstand prolonged exposure to high temperature of more than 1,000 degree C. Alumina cement however, is not advisable for mixing with any other types of cement. 

The main composition of cement are: 

1.60 to 65% - Lime

18 to 25% - Silica

3 to 8% - Alumina

3 to 5% - Iron Oxide

2 to 5% - Magnesia

1 – 5% - Sulfur Trioxide

Aggregates 

Aggregates that are used  for concrete work are classified into two categories:

1. Coarse Aggregates such as crushed stone, crushed gravel or natural gravel with particles retained on a 5 mm sieve.

2. Fine Aggregates such as crushed stone, crushed gravel, sand or natural sand with particles passing on a 5 mm sieve.

Sizes of Aggregates

For Coarse Aggregate (gravel), the maximum nominal sizes are usually 40 mm, 20 mm, 14 mm or 10 mm diameter. The choice from the above sizes depends upon the dimensions of the concrete member more particularly the spacing of steel bars reinforcement. Good practice however, demands that the maximum size of the coarse aggregate (gravel) should not exceed 25% of the minimum thickness of the member nor exceed the clear distance between the reinforcing bars and the form. 

The aggregate should be small enough for the concrete mixture to flow around the reinforcement. This is referred to as workability which is ready for compacting.