Cement, The Popular Binding Material in Building Construction.

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Cement is produced by burning a mixture of calcium and clay material at very high temperature then grinded to produce fine powder.

Types of cement.

Cement comes in many varieties. Some are briefly explained below.

Ordinary portland cement;

The most used type of cement in general concrete construction. It is suitable when there is no danger of exposure to sulphates in the soil.

Quick setting cement;

Quick setting cement is produced by reducing the percentage of gypsum and adding a small amount of aluminium sulphate during production. This type of cement starts setting around 5 minutes and hardens within 30 minutes. Quick  setting cement is used to produce concrete for underwater construction.

White cement;

it is made from oxides of manganese,iron and chlorium. It is used for plastering, ornamental works, fixing of marbles and tiles.

Rapid hardening cement;

This type of cement gains strength earlier than ordinary Portland cement and consequently speeds up construction activity.

Coloured cement;

Coloured cement is produced by mixing pigments with ordinary Portland cement. Pigments such as colbalt produces blue colour, iron oxide produces brown, red or yellow colours.

Low heat cement;

When heat produced by hydration of cement does not get easily dispersed, it may give rise to cracks, hence in such situations it is preferable to use low heat cement.

High alumina cement;

This type of cement is produced from a mixture of lime and banxite. It is resistant to sulphate and acid attack.it is used in underwater construction.

Acid resistant cement;

This is cement made by adding acid resistant materials such as quartz, quartzite, sodium silicate or soluble glass. Acid resistant cement is commonly used in construction of chemical factories.

Common Problems of Fresh Concrete

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Common problems of fresh concrete will be discussed under 4 modes which are: inadequate workability, segregation, bleeding and harshness.

Inadequate workability:

Workability is the ease at which concrete can be placed and compacted. When inadequate, it leads to segregation and bleeding. Workability depends on quantity of water, shape of aggregates and their percentage in concrete.

Segregation:

This is the separation of coarse particles in green (fresh) concrete. It happens due to insufficient finer particles. Segregation can also be caused by throwing concrete from great heights at time of placing. Segregation causes loss of cohesiveness in concrete which results in honey combing and loss of strength.

Bleeding:

The appearance of water with cement particles on surface of freshly laid concrete. Caused by excessive quantity of water in the mix or too much compaction. Bleeding causes pores to form on concrete and renders it weak.

Harshness:

Fresh concrete is said to be harsh when it becomes difficult to obtain a smooth surface finish. It is caused by insufficient quantity of fine aggregates or insufficient quantity of water.

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Plain Concrete

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Plain concrete is known as concrete without steel reinforcement. It is an intimate mixture of cement, fine aggregates, coarse aggregates and water. It can be easily molded to desired shape and size before it hardens. Plain concrete is strong in compression but weak in tension.

Major ingredients of plain concrete are;

  • Binding materials such as cement, lime, polymer.
  • Fine aggregate material such as sand.
  • Coarse aggregate materials like crushed stone, gravel, and granite.
  • Clean water.

Common cement, sand and coarse aggregate ratios are;

1:1:2, 1:2:4, 1:3:6, 1:4:8

Constituents of plain concrete.

Cement:

Cement is the binding material. It binds and hardens when water is added. Generally a concrete mix with more cement gives more strength.

Coarse aggregate:

Coarse aggregates consists of crushed stones of igneous origin. They should be clean, hard, angular and sharp. They give mass to the concrete and prevent shrinkage of cement.

Fine aggregates:

River sand is the common fine aggregate for plain concrete. Its gives mass to concrete and fills voids created by coarse aggregates.it adds density to concrete. A denser concrete is higher in strength.

Water:

Water used for making concrete should be clean. It hydrates the cement to form a plastic mass which hardens gradually. Water makes it possible to mix the concrete with ease and place it in final position. The more water the better, however, excess water reduces the strength of concrete. A water – cement ratio of 0.5-0.6 is used to achieve desired concrete strength when hand mixed.

Preparing and placing of concrete.

Batching:

Batching by volume or weight are the 2 common methods of measurement.

Mixing: 

Thorough mixing to produce good quality concrete. Cement, sand and coarse aggregates is mixed in dry condition, then in wet condition after adding water. Hand and machine mixing are 2 common methods of mixing of concrete mixing

Transport and placing:

Fresh concrete is transported on site using head pans, wheel barrow on small scale work while concreting chutes, bell conveyors or pumps are used in large scale construction.

Compaction:

Compaction of concrete is necessary to remove entrapped air during placement of concrete. Compaction can be carried out by hand or with help of  concrete vibrators

Properties and Uses of Cement Mortar

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Cement mortar is an intimate mixture of cement , sand and water. Cement is the binding material, and sand is the fine aggregrate. The binding material (cement) develops when water is added to the mixture. Sand suitable for cement mortar include, river sand, gutter or pit sand.

Sand used for cement mortar must have the following properties;

  • should be chemically inert.
  • It should be free from organic or vegetable matter.
  • Should not contain salt.
  • Should contain sharp, angular, coarse grain that is well graded and hard.

Preparing cement mortar.

Cement and sand in the required ratio is first thoroughly mixed in dry condition then water gradually added and mixed until required consistency is achieved.

Cement to sand ratio for various works;

  • Masonry Works :   1:6 to 1: 8
  • Plastering:    1:3 to 1:6

Mortar Curing.

Curing is the process of ensuring sufficient moisture during hardening of mortar. Curing is ensured by spraying of water. It starts 6 – 24 hours after mortar has been applied

Characteristics of Cement Mortar;

  • Cement sand ratio mix richer than 1:3 is prone to shrinkage.
  • Mortar with well-proportioned mix gives an impervious surface.
  • Mortar with leaner mix may become porous.
  • Strength of mortar depends on the cement sand ratio.

Compressive Strength of various cement sand ratios;

  • 1:3 –   10N/mm2
  • 1:4 –  7.5N/mm2
  • 1:5 –   5.0 N/mm2
  • 1:6 –   3.0 N/mm2
  • 1:8 –    0.7 N/mm2

Uses of cement mortar.

  • To bind masonry units like blocks and bricks.
  • Plaster walls and slab.
  • Give neat finishes to walls and concrete works.
  • Fill joints and cracks in walls.
  • As filler material in stone masonry.