Soundness of Cement

Unsoundness occurs when free lime, magnesia, and calcium sulfate hydrate after the cement paste sets, causing destructive expansion. Free lime hydrates slowly and intercrystallizes with other compounds, making it difficult to detect. Magnesia reacts with water to form crystals that disrupt the cement's structure, while calcium sulfate can create ettringite, leading to expansion if not adequately consumed during setting.

In the Le Chatelier test, cement paste is filled in a mold with indicators and stored underwater for a specific time. The mold is then immersed in water and boiled to accelerate hydration of cement. After cooling, the distance between indicators is measured to determine expansion caused by free lime, revealing whether the cement can resist significant volume changes.

The Le Chatelier test primarily detects unsoundness from free lime, while the autoclave test measures unsoundness from both free lime and magnesia. In the autoclave test, a cement paste specimen is cured in humid air, then heated under high-pressure steam. The specimen's length is measured before and after heating to determine total expansion, providing a more comprehensive assessment of cement soundness.

If expansion exceeds 0.8% of the original length after autoclaving, the cement may be considered unsound. This threshold helps identify whether the cement can retain its volume adequately after setting. Exceeding this limit suggests the presence of reactive compounds like free lime or magnesia that will cause structural problems in hardened concrete.

Free lime expansion is difficult to detect because free lime intercrystallizes with other compounds in the cement matrix. This integration masks the presence of free lime, making visual inspection unreliable. Specialized tests like Le Chatelier and autoclave methods are necessary to accelerate hydration and reveal the expansion that would otherwise occur slowly over time in service.

Magnesia reacts with water to form crystals that can disrupt the cement's structure, causing expansion and unsoundness. Magnesium-related issues are more prevalent in US-produced cement due to the raw materials used. Unlike free lime, which is rare as an unsoundness cause, magnesia expansion is a common concern that the autoclave test is specifically designed to detect.

Calcium sulfate can create ettringite, a compound that leads to expansion if not adequately consumed during the cement setting process. When calcium sulfate remains unreacted, it continues to form ettringite crystals that disrupt the cement matrix. Proper cement composition and setting time of cement management help ensure calcium sulfate is consumed appropriately to prevent expansion-related unsoundness.