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Cement Notes-2012.pdf

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Civil Engineering
Ghani Razaqpur

CEMENT AND CEMENTITIOUS MATERIALS Cement is a material that exhibits characteristic properties of setting and hardening when mixed to a paste with water. In the genera l sense of the word, it can be described as a material with adhesive and cohesive properties, which make it capable of bonding mineral fragments into a compact whole. For construction purposes, the term ‘cement’ is restricted to the bonding materi als used with stones, sand, bricks, building blocks, etc. The principal constituents of this type of hydraulic cement are the compounds of lime (calcareous cements). These types of cements have the property of setting and hardening under water, therefore they are called ‘hydraulic cements’. They include hydraulic limes, pozzolan cements, slag cements, natural cem ents, Portland cements, Portland-blast- furnace-slag cements, alumina cements and expansive cements. Non-Hydraulic cements, for example lime, do not harden under water because they require air to harden. Lime: Lime is produced by burning limestone with certain impurities, such as silica, magnesia and iron, in a rotary kiln at 980 oC. The principal constituent of limestone is cacium carbonate, CaCO . The burning results in the following reaction: 3 CaCO →3CaO + CO 2 The calcium oxide thus formed is called quicklime, which reacts with water to form calcium hydroxide, Ca(OH) 2 This process is called slaking and the Ca(OH) . is2called slaked lime. Hydrated lime is produced by adding water to quicklime approximately 1/3 of its weight. Lime putty is produced when significantly more water is added. Both hydrated lime and putty lime are used to make mortar for masonry. Usually the lime is mixed with mortar sand in the ratio of 1 to 3 by volume, and the sand helps to prevent excessive shrinkage. For the lime to harden, it must be exposed to air because the hydrated lime reacts with the CO 2 in the air and essentially re turns to its initial calcium carbonate form. The harden calcium carbonate holds the masonry units together. Since the hardening requires air rather than water, lime is not a hydraulic cement. Types of Hydraulic Cement 1. Pozzolan cements: A pozzolan is a siliceous or s iliceous and aluminuous material which has little or no cementitious value al one, but when finely divided and in the presence of moisture, react chemically with calcium hydroxide at ordinary temperature to form compounds with cementitious properties. Pozzolans may be natural or artificial. Natural Pozzolans: (a) Those made up of pumicite, obsidian, tuff, and other minerals, which are derived f from volcanic rocks. (b) Those with large quantities of finely dispersed amorphous silica, which react with lime in the presence of water to form hydrated silicates. Artificial Pozzolans: They include fly ash, boiler slag, and by-products of bauxite ore. Pozzolan cements are made by grinding the volca nic rocks directly or by calcinating and grinding clays, shales, and diatomaceous ea rth. Diatoms are microscopic unicellular marine or fresh water algae having siliceous shell walls. 2. Slag Cements: Slag cements consist mainly of a uniform blend of granulated blast- furnace slag and hydrated lime, with slag being at least 60% of the total mass. Slag cements are classified as Type S and Type SA, with SA being air-entrained. Both can be used in combination with Portland cemen t to make concrete or with lime to make masonry mortar. BFS (blast-furnace-slag) is a non-metallic compound, produced during the production of steel. It consists principally of silicates and aluminosilicates of calcium and of other bases. Slag cements are not normally in used structural concrete, but may be used in large masses of concrete masonry where weight rather than strength may be an important consideration. 3. Natural Cements: Natural cements are produced by the calcination of natural clay limestone, which contains 13-35% clay material, 10-20% silica and the remaining fractions being alumina and iron oxide. Afte r calcinations, the clinker is ground into a fine powder and is called natural cement. Na tural cement can be used in mortars or instead of Portland cement in concrete, provid ed the natural cement concrete structures are not exposed to the elements or subjected to high stresses. It can also be used in mortars. 3. Portland Cement (PC): is the most important type of cement in construction and is used in all types of construction in concrete, mortar, plaster, stucco and grout. PC is a hydraulic cement produced by pulverizing clinker that mainly consists of hydraulic calcium silicates. The name ‘P ortland Cement’ was in itially suggested by Joseph Aspdin due to the resemblance of the colour and quality of the hardened cement grout to the natural limestone quarried on Isle of Portland in England. It is a cement obtained by thoroughly mixing calcareous and argillaceous, or other silica-, alumina-, an0 iron-oxide bearing materials, bu rning them at a clinkering temperature of about 1450 C, and grinding the resulting clinker into a fine powder. Gypsum is added to the pulverized clinker to produce the final cement product. The approximate composition of PC is as follows: Lime (CaO) : 60 - 75 % (SilcaO 2) : 20 - 25 % Iron oxide (Fe 2 )3 : 3.0 – 8.0 % Aluminum-oxide (Al O ) : 0.5 - 0.6 % 2 3 %4M .0gO:0.5 Alkalis (K 2, Na O2 : 0.3 – 1.2 % %3S .O–32.0 Raw Materials and Manufacturing: 1. Calcareous :CaCO 3> 75 % (Limestone, chalk) 2. Argillo-calcareouCaCO 3= 40-75 % (Clayey limestone, clayey chalk, clayey marl) 3. Argillaceous :CaCO 3< 40 % (Slate-which is not used in making PC, shale, clay) Cement Making: Raw Products: Lime (CaO) : from limestone and chalk (ilcaO ) : from clay and shale 2 Alumina (Al O2)3 : from clay and shale Manufacturing Process of Portland Cement: (1) The raw mixture of lime and clay (+ so me other minor ingredients) are ground into a mixture. Either a wet or dry mix is ma de. The wet form is called the “slurry” process. (2) The mixture is fed into the upper end of a sl ightly inclined rotary kiln operating at a high temperature. (3) The material passes from the upper to the lower end of the kiln at a controlled rate. (4) As the temperature of the kiln reaches the point of incipient fusion, i.e. the olinkering temperature, it is kept at that temperature until the mixture forms at 1500 C the Portland cement pellets. The pellets are from 1.5 to 50 mm and called clinker. (5) The clinker is cooled and ground to powder. (6) A small quantity of gypsum is added during the grinding process to control or retard the setting time of concrete. (7) The cement is stored and packed. General Composition of Portland Cement: The major compounds of cement are: Tricalcium silicate (C3C)a: .SiO 2] Dicalcium silicate (C2C) : .SiO 2] Tricalcium aluminate [3C3:). 2O 3 Tetracalcium alumino-ferrite (C4AF) : [4CaO. Al2O 3 Fe2O 3 Note that a letter describes each oxide. C = CaO S = SiO 2 A = Al 2O3 F = Fe O 2 3 H = H 2 Almost 75-80% of cement is made of calcium silicates. Aluminates, alumino- ferrites and gypsum make up the rest. Calcium sili cates are the principal strength producing components. In reality, the silicates in cement are npure compounds, but contain minor oxides in solid solution. These oxides have significant effects on the atomic arrangements, crystal form and hydraulic properties of the silicates. Characteristics of the Major Compounds in Portland Cement Property C 3 C 2S C 3A C 4AF Cementing Good Good Poor Poor value Rate of Medium Slow Fast Slow reaction Heat of Medium Small Large Small Reaction Calcium silicates are the principal strength producing components. In reality, the silicates in ceme
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