1Test on properties of cement; The manufacture of cement requires stringent control, and a number of tests are performed in the cement plant laboratory to ensure that the cement is of the desired quality and that it conforms to the requirements of the relevant national standards. The tests are as follows: Consistency of standard paste For the determination of the initial and final setting time and for the Le Chatelier soundness test, a neat paste of standard consistency has to be used. It is, therefore, necessary to determine for any given cement the water content of the paste which will produce the desired consistency. The consistency is measured by the Vicat apparatus, using a 10mm diameter fitted into the needle holder. A trial paste of cement and water is mixed in a prescribed manner and placed in a mould. The plunger is then brought into contact with the top surface of the paste and released. Under the action of its weight the plunger will penetrate the paste; the depth of the penetration depends on the consistency. This is considered to be standard, in the meaning of EN(European standards) 196-3:1987, when the plunger penetrates the paste to a point 6 +/- 1mm from the bottom of the mould. The water content of the standard paste is expressed as a percentage by mass of the dry cement, usually range of values being between 26 and 33 percent. Setting time The setting time of cement is measured using the Vicat apparatus with different penetrating attachments. The method is prescribed by European standards (EN 196-3: 1987). For the determination of the initial set, a round needle with diameter 1.13 +/- 0.05 is used. This needle, acting under the prescribed weight, is used to penetrate a paste of standard consistency placed in the special mould. When the paste stiffens sufficiently for the needle to penetrate no deeper than to a point 5 +/- 1mm from the bottom, the initial set is said to have taken place. The initial set is expressed as the time elapsed since the mixing water was added to the cement. A minimum of 60 minutes is prescribed for cement for cement with strengths up to 42.5 Mpa, and 45 minutes for cement with higher strengths. Final set is determined by a similar needle fitted with a metal attachment hollowed out so as to leave a circular cutting edge 5mm in diameter and set 0.5mm behind the tip of the needle. Final set is said to have taken place when the needle, gently lowered to the surface of the paste ,penetrate it to a depth of 0.5mm but the circular cutting edge fails to make an impression on the surface of the paste. Soundness It is essential that the cement paste, once it has set, does not undergo a large volume change. In particular, there must be no appreciable expansion which, under conditions of restraint, could result in a disruption of the hardened cement paste. Such expansion may take place due to the delayed or slow hydration, or other reaction, of some compounds present in hardened cement, namely free lime, magnesia, and calcium sulfate. Strength of cement The strength of mortar or concrete depends on the cohesion of cement paste, on its adhesion to the aggregate particles, and to a certain extent on the strength of aggregate itself. Strength test are not made on a neat cement paste because of difficulties of moulding and testing with a consequent large variability of the test results, cement-sand mortar and, in some cases, concrete of prescribed proportions and made with specified materials under strictly controlled conditions, are used for the purpose of determining the strength of cement. There are several forms of strength tests: direct tension, direct compression, and flexure. European Standard EN 196-1: 1987 prescribes a compressive strength test on mortar specimens. The specimens are tested as 40mm equivalent cubes; they are derived from 40mm x 40mm x 160mm prism, which are first tested in flexure so as to break into halves, or areas otherwise broken into halves. Thus an optional centre-point test over a span of 100mm is possible. The test is performed on mortar of fixed composition, made with ‘CEN standard sand’. (CEN is the acronym of the French name of the European committee for standardization.) The sand is natural, siliceous rounded sand which can be obtained from various sources. The sand/cement ratio is 3 and the water/cement ratio is 0.50.The mortar is mixed in a cake mixer and compacted on a jolting table with a drop of 15mm;a vibrating table can also be used, provided it results in similar compaction. The specimens are demoulded after 24 hours in a moist atmosphere and thereafter cured in water at 20 degrees Celsius. Fineness of cement Because hydration starts starts at the surface of the cement particles, it is the total surface area of cement that represents the material available for hydration. Thus, the rate of hydration depends on the fineness of the cement particles and, for a rapid development of strength, high fineness is necessary; the long-term strength is not affected. The fineness test would be carried out by sieve test; but, since this test does not give information on the size of grains smaller than 45µm sieve, and it is the finer that play the greatest part in early hydration. For this reason, modern standards prescribe a test for fineness by determination of the specific surface of cement expressed as the total surface area in square metres per kilogram. A direct method is to measure the particle size distribution by sedimentation or elutriation: these methods are based on the dependence of the rate of free fall of particles on their diameter. Stroke’s law gives the terminal velocity of fall under gravity of a spherical particle in a fluid medium; the cement particles are, in fact not spherical. This medium must of course be chemically inert with respect to cement. It is also important to achieve a satisfactory dispersion of cement particle as partial flocculation would produce a decrease in the apparent specific surface.