Cement is a powder made by heating up raw materials containing calcium in a kiln to create dark pellets called clinkers; which are then mixed with gypsum to create a fine grey coloured powder.
The raw materials may be various combinations of limestone, sea shells, chalk, clay and other materials including and, shale and iron. These are ground into small pieces up to 2cm diameter; before heating to temperatures as high as 3,000 degrees Fahrenheit.
The final product commonly contains a mix of calcium, silicon, aluminum, and iron and is called Portland cement or type 1 cement.
Cement can become useless if it gets wet before use, do bags should be stored as follows:
- in dry water proof areas
- where humidity is low
where there is a dry floor
- stack in a header footer fashion
- stack on raised boards
- use oldest bags first
Different types of cement can have different characteristics. Physical requirements that may be variable between different cements include strength, heat of hydration, setting time, air content, expansion and fineness.
Rapid Hardening Cement
Has increased lime content which causes it to become stronger faster. This is useful when form work is being used and needs to be removed earlier.
Quick Setting Cement
This contains a small % of aluminium sulphate which helps speed up the setting. It is useful in wet conditions or where a job needs to be completed as fast as possible.
This is a term used to describe cement that sets so rapidly that it can be used to stop water leaks. It is similar to mortar, but sets very fast once combined with water.
Low Heat Cement
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Made by reducing tricalcium aluminate -good for large scale construction
Sulphates Resistant Cement
Tricalcium aluminate level is kept under 6% -which in turn increases power against sulphates. Used for concreting where the action of sulphates is extreme and could over time damage the concrete (eg. in flowing water).
Blast Furnace Slag Cement
Created by grinding 40% clinkers with 60% slag to produce a material with similar properties to Portland cement. This may be used if cheaper to obtain.
High Alimina Cement
Made by melting mixture of bauxite & lime & grinding with clinker -it creates a rapid hardening cement – good to use in extremely high or low temperature conditions.
Iron oxide is kept from any of the raw materials
Pigment added to the cement to give it colour
Created by grinding & mixing pozzolanic clinker with portland cement
Air Entraining Cement
Components such as resins, glues or sodium salts of sulphates are added to improve characteristics including frost resistance and work-ability
Water repellent materials mixed into the cement