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Garikapati Rambabu
Garikapati Rambabu
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UNIT-I CEMENTS & ADMIXTURES Ordinary Portland Cement (OPC): Ordinary Portland cement is the best type of cement so far. There was only one grade of OPC cement is available. Later, high grade cements were introduced in India. The OPC was classified into three grades, namely 33 grades, 43 grades and 53 grades depending upon the 28 days compressive strength of cement. If the 28 days compressive strength is not less than 33N/mm², it is called as 33 grade cement. If strength of cement is not less than 43N/mm², it is called as 43 grade cement. If the strength of cement is not less than 53N/mm², it is call as 53 grade cement. As OPC is free from sulphate attack, more than 70% of cement is OPC only and it is best suitable for all construction works. Chemical composition of cement: According to chemical analysis Portland cement should have the following proportion of various ingredients. (But main constituents for practical purpose are lime, silica alumina and Iron oxide). S. No. Name of the Ingredient Approx. Proportion Percentage 1 Lime (CaO) 60 to 67 2 Silica (SiO2) 17 to 25 3 Alumina (Al2 O3) 4 Iron oxide (Fe2 O3) 0.5 to 6 5 Magnesium oxide (Mg O) 0.1 to 4 6 Alkalis(K2O, Na2O) 0.4 -1.3 7 SO3 1.3 -3.0 3 to 8 Hydration of cement: If the cement is mixed with water (25 to 35% by weight) a paste is formed. This cement paste remains plastic for a short period. But when time passes, the paste becomes harder. This is due to the chemical reaction between water and cement. This reaction is also called as “hydration of cement”. Thus the plastic sticky cement paste turns into a hard solid stiff mass. This process of paste changing into stiff mass is called as "setting of cement ".

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Structure of Hydrated Cement: If we want to know about behavior of concrete, we should know the structure of hardened cement paste. If concrete is considered as two phase material, namely, the paste phase and the aggregate phase. It is very important to understand the paste phase, because, it can influence the whole behavior of the concrete. Strength, the permeability, the durability, the drying shrinkage, the elastic properties, the creep and the volume change properties of concrete greatly influenced by the paste structure. The aggregate phase has the lesser influence on the properties of concrete than the paste phase. Therefore it is very important to understand the structure of hydrated cement paste. Tests on cement Testing of cement can be brought under two categories: (a) Field testing (b) Laboratory testing. (a) Field testing: It is sufficient to subject the cement to field tests when it is used for minor works. The Following are the field tests: (i) Open the bag and take a good look at the cement. There should not be any visible lumps. The color of the cement should normally be greenish grey. (ii) Thrust your hand into the cement bag. It must give you a cool feeling. There should not be any lump inside. (iii) Take a pinch of cement and feel-between the fingers. It should give a smooth and not a gritty feeling. (iv)Take a handful of cement and throw it on a bucket full of water, the particles should float for some time before they sink. (b) Laboratory testing: The following tests are usually conducted in the laboratory. (a) (b) (c) (d) (e) Fineness test. Standard consistency test. Setting time test. Strength test. Soundness test. (a)Fineness Test: The fineness of cement is one of the major influencing factor on the rate of hydration and hence on the rate of gain of strength and also on the rate of evolution of heat. Finer cement offers a greater surface

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area for hydration and hence fast development of strength. Fineness test is carried out to determine the grinding property of cement. Different cements are ground to different fineness. The disadvantage of fine grinding is that it is susceptible to air set and early deterioration. Maximum number of particles in a sample of cement should have a size less than about 100 microns. The smallest particle may have a size of about 1.5 microns. An average large size of the cement particles may be taken as about 10 micron. The particle size fraction below 3 microns has been found to have the predominant effect on the strength at one day while 3-25 micron fraction has a major influence on the 28 days strength. Increase in fineness of cement is also found to increase the drying shrinkage of concrete. In commercial cement it is suggested that there should be about 25-30 per cent of particles of less than 7 micron in size. Fineness of cement is tested in two ways: (i) By sieving. (ii) By determination of specific surface (total surface area of all the particles in one gram of cement) by air-permeability apparatus. Generally Blaine Air permeability apparatus is used. (i)Sieve Test: Weigh correctly 100 grams of cement and take it on a standard IS Sieve No. 9 (90 microns). Break down the air-set lumps in the sample with fingers. Continuously sieve the sample giving circular and vertical motion for a period of 15 minutes. Mechanical sieving devices may also be used. Weigh the residue left on the sieve. This weight shall not exceed 10% for ordinary cement. (b)Standard Consistency Test: For finding out initial setting time, final setting time and soundness of cement, and strength parameter known as standard consistency has to be used. The standard consistency of a cement paste is defined as that consistency which will permit a Vicat plunger having 10 mm diameter and 50 mm length to penetrate to a depth of 33-35 mm from the top of the mould. The apparatus is called Vicat Apparatus. This apparatus is used to find out the percentage of water required to produce a cement paste of standard consistency. The standard consistency of the cement paste is some time called normal consistency (CPNC). The following procedure is adopted to find out standard consistency. Procedure: Take about 500 gms of cement and prepare a paste with a weighed quantity of water (say 24 per cent by weight of cement) for the first trial. The paste must be prepared in a standard manner and filled into the Vicat mould within 3-5 minutes. After completely filling the mould, shake the mould to expel air. A standard plunger, 10 mm diameter, 50 mm long is attached and brought down to touch the surface of the paste in the test block and quickly released allowing it to sink into the paste by its own weight. Take the reading by noting the depth of penetration of the plunger. Conduct a 2nd trial (say with 25 per cent of water) and find out the depth of penetration of plunger. Similarly, conduct trials with higher and higher

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