Soundness Test of Cement
Soundness Test it is very important which is measure expansion of cement after it start setting. Certain cements have been found to undergo a large expansion after setting causing disruption of the set and hardened mass. This expansion of cement can cause serious problem for durability of structures when such cement is used.
The testing of soundness of cement, to ensure-that the cement does not show any appreciable subsequent expansion of prime importance. The unsoundness is occurring mainly due to excess of lime than that could be combined with acidic oxide at the kiln. This is also due to inadequate burning or insufficient fineness of grinding or thorough mixing of raw materials. It is also may occur due to too high proportion magnesium content or calcium sulphate content may cause unsoundness in cement.
For this reason the magnesia content allowed in cement is limited to 6 per cent, It can be recalled that, to prevent flash set, calcium sulphate is added to the clinker while grinding. The quantity of gypsum added will vary from 3 to 5 per cent depending upon C3A content. If the addition of gypsum is more that could be combined with C3A, excess of gypsum will remain in the cement in Free State. This high percentage of gypsum leads to an expansion and consequent disruption of the set cement paste.
The unsoundness in cement majorly occurs due to excess of lime, excess of magnesia or excessive proportion of sulphates. Unsoundness effect of cement does not come to surface for considerable period of time,
Read More: SETTING TIME OF CONCRETE & METHOD OF TESTING
Therefore, accelerated tests are required to detect it. The most popular and simple test to measure soundness of cement is Le – chatelier’s soundness test.
Le-Chatlier’s Soundness Test on Cement
- Le- chatelier mould
- Glass sheets
- Mixing pan
It consists of a small split cylinder of spring brass or other sulfa metal. Le- Chatelier mould is 30 mm in diameter and 30 mm high. On both side of mould are attached two indicator arms 165 mm long with pointed ends. Water is added in cement as 0.78 P. where P is % of water for standard consistency of cement. Well mixed paste is filled in mould and cover with glass plate on both faces of mould.
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The mould and the glass plates are oiled before conducting the test
1) Take 400 gram of cement sample.
2) For this test to be performed we need standard consistency of cement. Water is taken as 0.78 x P (Where P is water required for Standard consistency in percentage)
(For example, Standard consistency is 30% of water, then take water percentage for soundness is 0.78 x 30% = 23.4%. So water to mixed in 400 gm of cement will be 400 x (23.4/100) = 93.6ml.)
3) Make well mixed paste of cement and fill in Le- Chatelier mould taking care to keep the edges of the mould gently together during the operation.
4) Clean upper surface and make it smooth and place a small weight over the cover plate.
5) Put this assembly quickly in water at a temperature of 27º C + 2ºC and keep it there for 24 hours.
6) Take out mould from water and measure distance between the indicators points as Reading-1 .(Suppose it is 2 mm)
7) Now, again put this assembly in boiling water for 25 to 30 minutes and keep at boiling for 3 hours. The mould should be in boiled water during this period
8) Remove the mould from water and allow it to cool at room temperature.
9) Measure the distance between the indicator points as Reading-2. (Suppose it is 10mm)
10) Soundness of cement = (Reading-2) – (Reading-1)
= 10 mm – 2mm
= 8 mm
The difference between needle readings indicates the soundness of cement. The soundness limit must exceed 10 mm for ordinary, rapid hardening and low heat Portland cements. If in case the c expansion is more than 10 mm as tested above, the cement is said to be unsound.
The Le Chatelier test has drawback that it detect on lime caused unsoundness. This method does not measure unsoundness caused by presence of excess of magnesia.
Indian Standard Specification stipulates that cement having a magnesia content of more than 3 per cent shall be tested for soundness by Autoclave test which is sensitive to both free magnesia and free lime.
In this test a neat cement specimen 25 x 25 mm is placed in a standard autoclave and the steam pressure inside the autoclave is raised in such a rate as to bring the gauge pressure of the steam to 21 kg/sq cm in 1 — 1’/4 hour from the time the heat is turned on. This pressure is maintained for 3 hours, the autoclave is cooled and the length measured again. The Stream pressure applied eventually accelerates the hydration of both magnesia and lime.
To determine the unsoundness due excess of calcium sulphate there is no satisfactory test is available. But, its amount present can be easily determined by chemical analysis.
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