Why soundness test of cement

The soundness test is carried out to make sure that the cement does not undergo a harmful change of volume (expansion) after it has set.

Why it is necessary

• Cement with too much free lime (CaO), free magnesia (MgO) or excess sulphates keeps hydrating slowly even after setting. This slow reaction increases the volume, which causes cracks, distortion, disintegration and loss of strength in concrete and mortar.
• The defect cannot be noticed at the time of use, as the effect appears days or months later, when repair is costly. Hence it is checked in advance.
• It confirms that the clinker was properly burnt and ground and that the cement is not adulterated.
• It checks that the cement has not deteriorated through bad storage.

How it is tested

• Le Chatelier test: detects expansion due to free lime. The paste is made with 0.78 P water, filled in the mould, kept under water for 24 hours and then boiled for 3 hours; the expansion between the pointers must not exceed 10 mm.
• Autoclave test: detects expansion due to magnesia; a bar is heated in steam at about 2 MPa for 3 hours, and the expansion is limited to 0.8%.

Remedy

If the cement fails, it may be aerated by spreading it in a layer about 75 mm thick for 7 days and then retested; during aeration the free lime slakes by absorbing moisture (and CO2). If it still fails, it is rejected.

Conclusion on use

Only sound cement is used in structural and hydraulic works, to ensure durability and safety.