The meaning of soundness is the ability to withstand temperature changes. If cement doesn’t possess good soundness then concrete made from it will undergo high property changes.
Soundness of cement depends on its constituents. The two main constituents are lime and magnesia. Excess content of lime, magnesia or sulphate will contribute to unsoundness.
Other reasons of unsound cement are inadequate burning of raw material in the kiln during making of cement of insufficient grinding of raw material.
There are two methods to find the soundness of cement viz, Autoclave test and Le Chatelier test.
The apparatus consists of a small split cylinder of spring brass or other suitable metal which is 30 mm in diameter and 30 mm in height. On either side of the split, two indicator arms are attached of 165 mm length.
Cement is mixed with 0.78 water required for the standard consistency. Then the mould is kept on glass plate and also covered with a glass plate too.
This whole assembly kept immersed in water for 24 hours and at a temperature range of 27°C
– 32°C. keep note of the distance between two indicator points.
After this process, make this whole assembly immersed in water for 3 hours and boiling this water at its boiling temperature for 3 hours. After that, let the assembly cool down and then note down the distance between indicator arms.
Calculate the difference between these two distances. If it is more than 10 mm then cement is unsound and not suitable for making durable concrete. If the difference between these two distances is less than 10 mm then cement is ok and sound.
The Le Chatelier test
The Autoclave test has a limitation that it measures unsoundness due to free lime only but not because of excess magnesia. To overcome this situation Le Chatelier test is introduced.
Indian standards state that for more than 3 percent magnesia content, the Soundness test is a must.