- Classification of Lime:
Friday, September 6, 2013
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Introduction- Lime as a cementing material:
Lime has been used as a cementing material since the ancient times in India and abroad. Ancient Egyptians and Romans used this material for various constructional processes. Even in India, various engineering structures like palaces, bridges, temples etc were constructed with lime mortar and still are in shape.
After the invention of cement in 1824 lime has been replaced by the cement to a large extent, however it is still used at certain places, like for the repairing of the structures which were originally built with the lime mortar and at the places where lime is freely available and cement has its acute shortage.
Broadly lime is classified into three categories:
(1) Fat Lime (2) Hydraulic lime (3) Poor Lime
(1) Fat Lime/Pure Lime:- Fat lime is also known as pure lime, white lime or rich lime, this is manufactured using the purest form of the limestone. It is popular with its name as fat lime since its volume is increased to about 2 to 2.5 times its originally volume after getting slaked. It slakes vigorously.
(2) Hydraulic Lime: This lime has the hydraulic property, means it can set under the water also. It contains clay and some amount of ferrous oxide also. This is also known as water lime.Depending upon the amount of clay hydraulic lime is classified into further three categories:
(a) Feebly hydraulic lime
(b) Moderately hydraulic lime
(c) Eminently hydraulic lime
Increase in the percentage of clay makes the slaking difficult and thus increases the hydraulic property of lime. With about 30 per cent of clay lime resembles the color of cement.
The color of fat lime is not white therefore fat lime looks more sanitary than the hydraulic lime.
(3) Poor Lime: Poor lime contains more than 30 percent of clay therefore it is also known as impure lime. It slakes very slowly and also does not dissolve into water. It has poor binding property and its color is muddy white.
This lime forms very poor mortar and so such lime can be used for inferior types of work or at places where good lime is not available.
BIS: 712 - 1984 classifies lime under six categories, namely Class A, Class B, Class C, Class D, Class E and Class F.
Class A lime is used for structural purposes because it is eminently hydraulic lime and has the property of setting even in the absence of air. It has to be supplies in the hydraulic form only. Its minimum strength with lime sand mortar of proportion(1:3) by weight at the end of 14 days and 28 days should be respectively 1.75 N/mm2 and 2.80 N/mm2.
Class B lime is the semi-hydraulic lime which is used for mortars for masonry and it can be supplied either as quick lime or as hydrated lime. Its minimum compressive strength with lime sand mortar of proportion(1:3) by weight at the end of 14 days and 28 days should be respectively 1.25 N/mm2 and 1.75 N/mm2.
Class C lime is the fat lime which is used mainly for finishing coat in plastering, whitewashing and with suitable admixture such as surkhi or any other pozzolanic material to produce artificial hydraulic mortars. It is to be supplied in hydraulic or quick form.
Class D lime is the magnesium or dolomitic lime which is used for finishing coat in plastering, whitewashing, etc. It is to be supplied in the hydrated or quick form.
Class E lime is the kankar lime and is used for the masonry works, it is to be supplied in the hydraulic form only.
Class F lime is also known as Siliceous dolomitic lime which is used for undercoat and finishing coat of plaster. It is to be supplied in the hydrated or quick form.
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Monday, September 2, 2013
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- Vicat's Apparatus
is used to find out the consistency, initial setting time and final setting time of the cement. In the normal consistency test we have to find out the amount of water to be added to the cement to form a cement paste of normal consistency.
Vicat's apparatus consists of an arrangement to hold the plunger of 10 mm diameter and two other needles which are made to freely fall into a mould filled with the cement paste and the amount of penetration of the needles of plunder can can be noted using the vertical graduations from 0 mm to 50 mm.
- Consistency Test:
To find out the consistency test you have to take a sample of dried cement of about 400 g weight which must pass through the 90 micron IS Sieve. Then mix in it about 25% of water by weight a form a uniform paste withing 2 minutes of time.
Fill the Vicat's mould with this paste and make the 10 mm plunger fixed to the arrangement to just touch the top surface of the cement paste. Make it freely fall and note the amount of penetration.
When the penetration is of about 42 to 45 mm or when the reading on the vertical graduation is about 5 mm to 7 mm that means cement is of normal consistency. Generally the water required to form a paste of normal consistency is 30%.
|Vicat's Apparatus with all the accessories|
- Initial Setting Time:
Initial time of Cement is the time required by the cement for its early setting. Cement must be applied to the place of its use before its initial setting so it is necessary to find out the initial setting time that is available with us.
Vicat's apparatus is the standard apparatus used to find out this initial setting time. Look in the figure above, there is a needle of diameter 1 mm. This needle is fixed to the movable rod weight.
The cement paste of normal consistency is formed and is filled in the mould. Now the needle is made just touch the top surface of the cement paste and made freely fall in it. Initial setting time is the time from the mixing of the cement and the water to the time when the penetration of the needle is just above 5 mm from the bottom of the base plate or mold.
Generally the initial setting time of the ordinary Portland cement is 30 minutes. For Slow setting cement this time may be increased by adding the admixtures or Gypsum up to 60 minutes.
Similarly, for the final setting time we have to use the third needle which has a enlarged 5 mm hollow cylindrical base. The final setting time is the time from the mixing of the water to the time when this needle just makes the impression on the surface of the cement but do not penetrate into it. Generally the final setting time of cement (OPC) is 10 hrs to 12 hrs.
|Photograph of the Vicat's Apparatus|
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