TEST OF CEMENT

TEST OF CEMENT

1. Strength test
2. soundness test
3. fineness test
4. setting time test

Strength test

   The compressive strength of hardened cement is the more important property of the cement that should always checked before used in construction works. Strength test does not made on only cement paste because of difficulties of shrinkage and cracking of only cement paste. So as sand and cement and standard are mixed in proportions and such testing is done. A ratio of cement and standard sand is taken as 1:3. Then water is mixed in that and mixed for  3 to 4 mintutes. Immediately after mixing, the mortar is filled into a cube mould. Keep that cube mould at a temperature of 27* + 2* C for 24 hours. 

        Three cubes are tested for compressive strength at different periods in crushing. testing machine. The compressive strength  should be average of the strength  of the three cubes. The period in case of ordinary Portland cement are 3 and 7 days. The compressive strength for ordinary Portland cement should not less htan 160 kg/cm2 for 3 days and 220 kg/cm2 for 7 days. 

  Soundness test

     Certain cements shows expansion after setting and cause disruption of hardened mass. The testing of soundness of cement is to ensure that the cement does not show any appreciable expansion after setting. Unsoundness of cement is due to excess of lime, magnesia and proportion of sulphates. There is Le chatelier test to findout the soundness of cement. The apparatus is shown in finger It consists of a small split cylinder of spring brass or other suitable metal of 0.5mm thickness forming a mould 30 mm internal diameter and 30 mm high. On either side of the split, two indicators are braced Suitable with pointed ends made of 2mm diameter brass wire AA, such that the distance of these ends to the centre of the cylinder is 165 mm. The split cylinder is kept between two glass plates.

 Now place the mould on the glass sheet and fill it with cement paste formed by gauging cement with 0.78 time the water required to give a paste of standard consistency. Now mould is covered on the top with another glass sheet and immersed in water at a temperature of 27*-32*C and kept there for 24 hours measure the distance between the indicator points. Now whole assembly is immersed in water again.

Fineness test

   The fineness of  cement of cement imparts impact on hydration and also on the rate of gain of strengths. Finer cements offers cement offers greater surface area for hydration and hence the faster and greater developement of strength. The disadvantage of fine grinding is that it is susceptible to air-set and early deterioration. The average size of the cement particles should be 10 microns. The smallest size of the particle should not less that 1.5 microns. The finene test consist of sieve analysis. firstly taken 100 gm of cement and sieve that sample of cement through I.S. sieve number 9 for fifteen minutes. Now weight the residue after sieving. This should not exceed 10% by weight of cement.

Setting Time Test

  The setting time of the cement divided as the initial setting time and final setting time. Initial setting time taken during the operation as the water is added to the cement and when the paste starts losing its plasticity. Final setting time is the time taken during the operation as the water is added to the cement and when the paste lost its plasticity completely. Generally for the ordinary Portland cement, initial setting time 30 minute and final setting time is taken as 10 hours.
   For finding out the initial setting time and final setting time a standard consistency test is carried out. Perpare a test block of cement of standard consistency. Fill the  cylindrical mould having size 40 mm depth and 80 mm diameter completely and smoothen the surface of the paste and level it.

•     Determination of Initial Setting Time
• Determination of Final Setting Time

(a) Determination of Initial Setting Time

          Place the test block on the nonporous plate under the rod bearing needle. Lower the needle in contact with the surface of the test block quickly release, allowing it to penetrate in to the test block. In the beginning, the needle will completely pierce into the test block. But after sometime when the paste starts losing its plasticity, the neeedle may penetrate only to a depth of 33-35 mm from the top. The period elapsing between the when water is added to the cement and the time at which the needle penetrates the test block to a depth equal to 33-35 mm from the  top is taken as initial setting time.

(b) Determination of Final setting Time

        Replace the needle of the vicat apparatus by a circular attatchment as shown in figure and perform the test as for initial setting time. The cement is  considered as finally set when, the paste has attained such hardness that the centre needle does not pierce through the paste more than 0.5 mm.