Physical Testing of Cement.
In order to test the suitability of cement for a certain purpose a number of tests are carried out, some of them being:
A known quantity of cement is mixed with a fixed quantity of water and a stop-watch is started. The cement is made into a paste by a standard procedure. The paste is then placed on a glass slab in a cylindrical ring and the Vicat needle allowed to drop under its own weight. At first, the needle can pierce the cement and touch the glass slab, but as time passes it fails to do so. Note the time when that happens. This interval of time since the start is called the initial setting time. If the experiment is allowed to continue, a time would reach when the cement sets and the needle cannot even pierce it, under its own weight. This is called the final setting time.
For a good type of cement initial setting time should not be less than 40-50 minutes and the final setting time out more than 9-10 hours. Gypsum (2%) retards setting, but the excessive amount may hasten it. Other tests which can be carried out for the determination of the quality of the cement are:
- Compression strength.
- Tensile strength.
- Specific gravity.
It is a mixture of cement with sand and crushed stone, gravel, blast furnace slag, etc., all made up to paste with water. It sets into a very hard solid mass and is extensively used in the construction of buildings and roads. Ferro-concrete contains twisted rods of iron or wire embedded in concrete.