Understanding Cement
A cement is a binder, a substance used for construction that sets, hardens, and adheres to other materials to bind them together. Cements are dry powders and should not be confused with concretes or mortars, but they are an important constituent of both of these materials, in which they act as the ‘glue’ that gives strength to structures.
Because it is a major component of both of these building materials, cement is an extremely important construction material. It is used in the production of the many structures that make up the modern world including buildings, bridges, harbours, runways and roads.
Cements used in construction can be characterized as non-hydraulic or hydraulic.
Non-hydraulic :- cement does not set in wet conditions or under water. Rather, it sets as it dries and reacts with carbon dioxide in the air. It is resistant to attack by chemicals after setting.
Hydraulic cements :- (e.g., Portland cement) set and become adhesive due to a chemical reaction between the dry ingredients and water. The chemical reaction results in mineral hydrates that are not very water-soluble and so are quite durable in water and safe from chemical attack. This allows setting in wet conditions or under water and further protects the hardened material from chemical attack. The chemical process for hydraulic cement was found by ancient Romans who used volcanic ash (pozzolana) with added lime (calcium oxide).
Modern cement has come a very long way from its origins. Archaeological evidence has been found that a form of crude concrete was used in hut construction in the areas now covered by Serbia, dating from around 5600BC and Israel (7000BC).
There are five types of Portland cement with the following features:
Type I:- cement is good for general purposes where special properties are not required.
Type II:- has a moderate sulfate resistance a
Type III :- cement has a relatively high early strength meaning that it gains strength faster than Type I, enabling forms to be removed sooner.
Type IV:- has a low heat of hydration and is used in massive construction projects. It develops strength at a slower rate than other types of cement.
Type V cement is only used for severe sulfate resistance as it develops strength at a slower rate than other types of cements.
It is necessary to check the quality of cement on site at the time of preliminary inspection.
- DATE OF PACKING
Date of manufacture should be seen on the bag. It is important because the strength of cement reduces with age.
- COLOUR
The cement should be uniform in colour. In general the colour of cement is grey with a light greenish shade. The colour of cement gives an indication of excess lime or clay and the degree of burning.
- RUBBING
Take a pinch of cement between fingers and rub it. It should feel smooth while rubbing. If it is rough, that means adulteration with sand.
- HAND INSERTION
Thrust your hand into the cement bag and it should give cool feeling. It indicates that no hydration reaction is taking place in the bag.
- FLOAT TEST
Throw a small quantity of cement in a bucket of water. It should sink and should not float on the surface.
- SMELL TEST
Take a pinch of cement and smell it. If the cement contains too much of pounded clay and silt as an adulterant, the paste will give an earthy smell.
- PRESENCE OF LUMPS
Open the bag and see that lumps should not be present in the bag. It will ensure that no setting has taken place.
Cement when mixed with fine aggregate (sand), coarse aggregate (boulders or gravels) and water which solidifies into a strong mass due to the Hydration of Cement. This is called as concrete. Apart from these components, large number of additives called admixtures are also added to enhance the properties of concrete like workability, durability, time of setting etc. Cocnrete possess a strong compressive strength but week tensile strength. Tensile strength is defined as the ability to resist bending forces. For the same reason plain concrete is not used where bending action can occur.
This is the reason we place steel in concrete. It allows the concrete to withstand tensile forces that would otherwise crack apart unreinforced concrete. Concrete is said to be reinforced when it contains steel bar. The amount, size, configuration, pattern, and the format for tying the bars together all effect the concrete’s ability to with stand these forces.
The steel bars compensate the concrete’s incapability for tensile resistance, effectively taking up all the tension, acting monolithically with concrete. So, when concrete and steel bars combine to act against forces result in good strength. Similar to how human bones reinforces the flesh and mass around it.