4.7 : Soundness of Cement

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Occasionally, after the cement paste sets, the free lime, magnesia, and calcium sulfate may hydrate, causing a destructive expansion in the hardened cement.

Soundness is the ability of the cement paste to resist significant expansion after setting.

Le Chatelier and autoclave accelerated tests are used to determine the soundness of cement.

In a Le Chatelier test, the cement paste is filled in the mold and stored underwater for a specific time.

Afterward, the distance between the indicators is measured. Next, the mold is immersed in water and is boiled for a designated time to accelerate cement hydration.

The setup is then cooled, and the distance between the indicators is measured again to determine the specimen's expansion due to free lime.

Alternatively, in the autoclave test, a cement paste specimen is prepared and cured in humid air.

The specimen's length is measured before placing it inside the autoclave for high-pressure steam heating.

After heating, the specimen is cooled, and its length is measured to determine its expansion resulting from free lime and magnesia.

The soundness of cement refers to the ability of cement paste to retain its volume after setting. Unsound cement can lead to expansion and structural damage due to the presence of free lime, magnesia, and calcium sulfate. Free lime hydrates very slowly, expanding and causing unsoundness, which is difficult to detect because it intercrystallizes with other compounds. Magnesia also reacts with water, forming crystals that can disrupt the cement's structure. Calcium sulfate can create ettringite, which leads to expansion if not adequately consumed during the setting process.

The standard tests for cement soundness are Le Chatelier's and autoclave tests. Le Chatelier's test primarily detects unsoundness from free lime, and the autoclave test measures the unsoundness from free lime and magnesia. If expansion is more than 0.8% of the original length after autoclaving, the cement may be considered unsound. Unsoundness from free lime is rare, with magnesium-related issues more prevalent in US-produced cement due to the raw materials used.

Tags

Cement Soundness Unsound Cement Expansion Structural Damage Free Lime Magnesia Calcium Sulfate Ettringite Le Chatelier s Test Autoclave Test Hydration Cement Paste

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