The meaning of soundness of cement is the ability to withstand temperature changes. If cement doesn’t possess good soundness then concrete made from it will undergo high volume changes with temperature variation. This will lead to an increase of cracking in the concrete.
The soundness of cement depends on its constituents. The two main constituents responsible for unsoundness are lime and magnesia. Excess content of lime, magnesia, or sulphate will contribute to unsoundness.
Other reasons for unsound cement are inadequate burning of raw material in the kiln during the 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 Le Chatelier Test for Soundness of Cement
The apparatus for the Le Chatelier test 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 in length.
Cement is mixed with 0.78 water required for the standard consistency. Then the mould is kept on a 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 boil 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 has a limitation, that it measures unsoundness due to free lime only but not because of excess magnesia. To overcome this situation Autoclave test is introduced.
Indian standards state that for more than 3 percent magnesia content, the Soundness test is a must.