concretebasics.org: your guide to cement & concrete

Definitions & History

What is Soundness?

Cement soundness can be considered to be a resistance to, or the lack of, any swelling, cracking, or disintegration resulting from an expansive chemical reaction such as the hydration of free lime (CaO) or crystalline free magnesia (MgO) in cement paste, mortar, or concrete.

A “Sound Cement” basically is one which does not undergo a large change in volume, once it has set.

Unsoundness of cement may cause disruption of the hardened cement paste under conditions of restraint.

Unsoundness of cement occurs due to delayed /slow hydration or other reaction of some compounds present in the hardened cement viz. free lime, magnesia and calcium sulphate.

In the United States, the Autoclave expansion test (ASTM C 151) is used to provide an index of potential delayed expansion caused by the hydration of CaO and MgO in hydraulic cements. In other countries, cement soundness may be monitored by cement paste expansion in boiling water with the Le Chatelier test apparatus. However, the Le Chatelier procedure determines unsoundness from the hydration of free CaO only, whereas the autoclave procedure measures cement paste expansion from either free CaO, free MgO, or both.

FAQ

Why presence of free lime causes unsoundness?

FUAQs

Why lime does not cause unsoundness whereas free lime does?

Can we determine free lime by conducting chemical analysis of cement?