Soundness of cement: Why is it important and how to test it?

The stability of any cement during the process of setting and hardening is indicated by the soundness of cement. This occurs when the volume of the cement changes. This occurs when the volume of the cement changes. Poor dimensional stability refers to the condition in which a concrete element would break in the event that the volume change in the cement is unstable after it has set and become hard. This can have a negative impact on the quality of the construction and possibly lead to major accidents.

Soundness test of cement is done by both the autoclave test (ASTM-C151, 2015) and EN-196 (1995), which are based on the Le-Chatelier test technique.The autoclave test (ASTM-C151, 2015) additionally applies pressure to the sample while it is being tested.

see also about: Guniting

What is soundness of cement?

Cement’s various properties influence the properties of hardened mortar or concrete. Ideally, when the concrete hardens, it does not experience any contraction or expansion. This means that the volume or mortar or concrete does not change when they settle. The soundness of cement describes the ability of the cement to resin its original volume after the hardening process. Unstable volume change after this process causes concrete structures to crack, adversely affecting the quality of a structure. This is commonly referred to as poor dimensional stability. Any cement product with poor dimensional stability is disposed of and not used in any projects.

See also: Cement test

Measuring the soundness of cement: Le-Chatelier test

Here’s all you need to know about testing the soundness of cement using the Le-Chatelier test.

Testing soundness of cement: Apparatus needed

Le-Chatelier mould 

Cement

Glass sheets

Mixing pan

Trowel

Weigh

Testing soundness of cement: Procedure 

• Prior to carrying out the soundness of cement test, both the mould and the glass plates are treated with oil.
• Take a sample of 400 grammes of cement. 
• In order to carry out this test, we need cement that is of consistent consistency. Water is taken as 0.78 times the water required for the standard consistency of the paste.
• If, for instance, the standard consistency is 30% water, calculating the water percentage for soundness would need 0.78 times 30%, which would be 23.4%. 
• Therefore, the volume of water that should be combined with 400 grammes of cement is equal to 93.6 millilitres. 
• Prepare a cement paste that has been thoroughly mixed, and then carefully pour it into the Le-Chatelier mould, taking care to keep the mould’s edges softly pressed together as you work. 
• Remove any debris and make the surface as smooth as possible, then set a little weight on top of the cover plate. 
• Immediately submerge this assembly in water that is maintained at a temperature of 27 degrees Celsius plus 2 degrees Celsius, and allow it to remain there for a full day. 
• Remove mould from water, and using Reading-1 as a measuring standard, determine the distance between the indication points. 
• (Let’s say it’s 2 millimetres) 
• Now, once more, place this assembly in boiling water for twenty-five to thirty minutes, and then maintain the boiling temperature for three hours. During this time, the mould should be submerged in water that has been boiled. 
• After removing the mould from the water, let it come down to room temperature before using it. 
• Calculate Reading-2 as the distance in feet between each of the indication points. (Let’s say it’s 10 millimetres) 
• The soundness of cement is equal to (Reading-2) minus (Reading-1); 10 – 2 millimetres = 8 mm. 
• The variance in the readings taken by the needle provides information about the quality of the cement. 
• When referring to conventional, rapid-hardening, and low-heat Portland cement, the soundness limit must be more than 10 millimetres. 
• In the event that the cement expands by more than 10 mm, as measured in the preceding test, the cement is considered to be unsound. 
• The Le-Chatelier soundness of cement test has the disadvantage of being able to identify unsoundness that is brought on by lime.
• This approach does not assess the instability that is brought on by an excessive amount of magnesia being present. 
• The Indian Standard Specification mandates that cement with a magnesia concentration of more than 3 percent must undergo the autoclave test for soundness. 
• This soundness of cement test is sensitive to both free magnesia and free lime. Hence it is necessary to do this test.
• In this particular soundness of cement test, a neat cement specimen measuring 25 mm x 25 mm is placed inside a standard autoclave.
• The steam pressure inside the autoclave is then raised at such a rate that it brings the pressure of the steam gauge to 21 kg/sq cm in around 1 — 1’/4 hours from the time the heat is turned on.
• After keeping this pressure for three hours, the autoclave is allowed to cool down before a new length measurement is taken. 
• The constant pressure exerted by the stream eventually speeds up the process of hydration for both magnesia and lime.

The expansion of cement after it begins setting has been measured for the purpose of the soundness test of cement. It has been seen that certain types of cement undergo a significant expansion after setting, which causes disturbance of the mass that has already set and hardened. When such cement is employed, the expansion of the cement might provide significant challenges for the long-term longevity of the buildings.

Testing soundness of cement: Precautions

• The volume of cement mortars and concrete expands because of unburnt lime and magnesium.
• An unburnt lime test is performed using the Le-Chatelier apparatus.
• Unburnt lime causes cracks in the structure primarily because it increases cement volume.
• Water also reacts with lime and magnesia, increasing the volume of cement and resulting in cracks in structures.

Importance of soundness of cement test

Cement is made by combining silica, lime, sulphur trioxide, magnesia, alumina, alkaline, calcium sulphate, and iron oxide. Amongst all these constituents of cement, lime accounts for 60%- 70%. A cement that doesn’t consist of just the right amount of lime sets quickly and adversely affects the quality of cement. Incorrect amount of line makes the cement unsound and has a negative impact on the cement work performed. The influence of cement’s unsoundness does not become readily apparent for a sizable amount of time after it has been applied. This necessitates the soundness test of cement before it is used. With the help of this test, you can ensure that the cement you use will not experience any expansion and compromise the quality of construction. 

Effects of unsound cement on concrete structures

Some of the major problems caused by the use of unsound cement include:

Decreased strength

Concrete structures created using unsound cement are more likely to break and crack, reducing the overall strength of the structure and making it more susceptible to damage by external forces.

Reduces resistance to weather

Concrete made from unsound cement is more susceptible to damage due to the effects of weather, like freeze-thaw cycles that cause surface spalling and scaling. 

Increased permeability

Unsound cement can result in increased permeability of concrete, which makes it more prone to water damage and corrosion of the steel reinforcements.

Reduced resistance to chemical damage

Unsound cement is more prone to chemical damage like that caused by acid attacks, which can weaken and degrade the concrete. 

Causes of unsound cement

• The primary cause of the unsoundness is an abundance of lime, which, when mixed with acidic oxide in the kiln, may produce an unstable compound and may be detected in the soundness test of cement.
• This is also the result of improper burning, grinding to an insufficiently fine consistency, or insufficiently complete mixing of the raw components that may be detected in the soundness test of cement.
• Unsoundness in cement can also be caused by having an excessively high magnesium concentration or calcium sulphate level, both of which can contribute to the problem.
• The primary causes of unsoundness in cement are an excessive amount of lime, an excessive amount of magnesium, or an excessive proportion of sulphates. 

How to prevent unsoundness of cement?

• It is important to remember that calcium sulphate is added to the clinker when it is being ground to avoid flash setting. As a result of this, the amount of magnesium that can be present in cement is capped at 6 percent. 
• Depending on the amount of C3A present, the amount of gypsum that is added will range anywhere from 3 to 5 percent. 
• If the amount of gypsum added to the cement in the free state is more than the amount that might be coupled with C3A, then extra gypsum will be left in the cement. 
• This high amount of gypsum causes the cement paste to expand, which in turn causes the paste to become more difficult to set. 

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