3 Soundness – The soundness of cement refers to the stability of the volume change in the process of setting and hardening. If the volume change is unstable after setting and hardening, the concrete structures will crack, which can affect the quality of buildings or even cause serious accidents, known as poor dimensional stability.
The cement product whose dimensional stability is poor will be disposed as spoiled product, not used in projects. The reasons for poor dimensional stability are: the free calcium oxide ( f -CaO) in the clinker is too much, or the free magnesium oxide in the clinker ( f -MgO) is quite a little, or the gypsum mixed in the clinker is excessive.
f -CaO and f -MgO in the clinker are all sintered, so their ageing speed is very slow. They start ageing slowly after the setting and hardening. CaO + H 2 O = Ca OH 2 MgO + H 2 O = Mg OH 2 In the ageing process, there is volume expansion which causes the cracking of cement.
The excessive amount of gypsum will react with the solid calcium aluminate hydrate to generate crystals of calcium sulfoaluminate hydrate. Thus, the volume will expand 1.5 times, which leads to the cracking of cement paste matrix. The national standards require: boiling method can be used to inspect the poor dimensional stability of the cement caused by the free CaO.
The so-called boiling method includes Pat test and Le Chatelier test. Pat test is to make the cement paste of normal consistency into cement cake, boil it for 3 h, and then observe it by naked eyes. If there is no crack and no bending by ruler inspection, it is called qualified soundness.
- Le Chatelier test is to measure the expansion value after the cement paste is boiled and get hardened on Le Chatelier needles.
- If the expansion value is within the required value, its stability is qualified.
- If there is contradictory between the results measured by Pat test and Le Chatelier test, Le Chaterlier test should prevail.
The hydration of free magnesium oxide is slower than that of free calcium oxide. Therefore, its harm can be inspected only by autoclave test. The harm of gypsum will be found by immersing in room-temperature water for a long time. Then the poor dimensional stability caused by magnesium dioxide and gypsum is inconvenient to be tested rapidly.
- Thus, they should be controlled strictly in the production of cement.
- The national standards require: the content of free magnesium oxide in cement should not be more than 5.0%, and the content of sulfur trioxide in slag cement should not be more than 4.0% and that in other kinds of cement should not exceed 3.5%.
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What is the soundness of cement?
Soundness Of Cement – its 2 | May 3, 2020 |, | The soundness of cement is an ability of hardened paste after setting to retain its volume and it is very important that the cement shall not undergo any appreciable change of volume after setting. Certain types of cement after setting have been found to undergo a large expansion causing disruption of hardened mass and the set. In soundness of cement test a specimen of hardened cement paste is boiled for a fixed time and by doing this any tendency to expand is speeded up and can be detected and most used test for the soundness of cement is “Le Chatelier Apparatus Test”. On the setting of cement, it is understood that, its capacity to form a non-disintegrating, hard uniform strong mass by the soundness of cement test.
Why does hardened cement have no sound?
Causes of Lack of Soundness in Cement – The inept chemical composition of cement is held responsible for the lack of soundness in hardened cement. This is because excess hard-burned free lime or magnesia propel the expansion of cement, thereby changing its volume. Fig.: Cracking due to unsoundness of cement
What is the difference between sound cement and unsound cement?
Importance of soundness of cement Cement is most recognized construction material. It is very useful for concrete, mortar, plaster, etc. Prior to apply cement for concrete construction, the quality of the cement should be checked thoroughly. Soundness of cement is one of such vital properties that should be examined properly prior to construction or else the cracks may develop in concrete and its durability will also be reduced.
- Given below, the details on the soundness of cement.
- Soundness of Cement The properties of hardened concrete or mortar are mainly affected by the different properties of cement.
- Usually, once the concrete or mortar becomes solidified, they do not set out under expansion or contraction.i.e.
- The volume of concrete or mortar remains unchanged, as soon as they are set.
Soundness of cement means the strength of cement to maintain its volume as soon as it is solidified. Once the cement mortar or concrete gets dried, there should not be any alteration in volume. If any alteration in the volume occurs, it will allow the cement to inflate or contract.
So, the cracks may form. Such cement is known as unsound cement. If unsound cement is applied in the construction work, the stability of the structures is significantly affected. On the other hand, as soon as sound cement is set, no alternation in volume occurs. Besides, once the sound cement is solidified, no expansion or contraction happens.
Reasons for unsoundness of cement 01. Surplus Lime: With the existence of extra lime (CaO) in the cement, soundness of cement is impacted. This extra lime hydrates gradually and develops slaked lime that absorbs a bigger volume as compared to the original free calcium oxide.
Due to the slow hydration process, the properties of hardened concrete are changed. The variation in the rate of hydration of free lime and slaked lime results in modifying the volume of hardened concrete. The cement that shows this type of volume changes is known as unsound cement. So, a limit is set in the ordinary Portland cement concerning the existence of free lime & magnesia in cement content.02.
Surplus Magnesia: Cement can also be unsound because of the existence of magnesium oxide (MgO), that reacts with water in a manner identical to the lime and influences the rate of hydration.03. Insufficient Burning: While the manufacturing process of cement is going on, raw materials are supplied into the kiln.
Different types of raw materials like lime (Cao) and some acidic oxide get amalgamated inside the kiln. If the burning and cooling processes are not performed in a proper manner, then surplus lime will stay in a free condition. Therefore, it will produce the unsound cement further.04. Surplus Calcium Sulphate: The expansion is caused by the third compound alias calcium sulphate (CaSo4).
Gypsum is included to cement clinker to resist flash setting of cement. But if excessive gypsum exists it will create reaction with C3A throughout setting and develops calcium sulfoaluminate. This sulfoaluminate creates expansion in hardened concrete. Due to this, the standards limit the amount of gypsum to be included to clinker.
- The limits are settled in such a manner that the soundness of cement is retained and it does not become unsound.
- The soundness of cement should be examined prior to utilize cement in construction.
- In Indian standards, the soundness of cement is examined through Le Chatelier apparatus test.
- The manufacturers of cement should also perform the quality control, in order to develop sound cement.
If unsound cement is applied for building up your house, the setting time of cement is raised. Under such situation, if formwork is detached before time, the cracks may develop in due course of time. Consequently, the soundness of cement should be examined prior to utilize it in developing concrete. : Importance of soundness of cement
What causes unsoundness of cement?
Especially excess of lime and/ or magnesia oxide present in the cement causes unsoundness. this materials expands in structure and thus the concrete or mortar also expands, causing disintegration. the test is designed to accelerate the expansion in cement paste by application of heat. expansion beyond certain limit indicates unsound cement.