Cement - Chapter 3 Part 1 - Narmeen PDF

12/03/1446 CEMENT PORTLAND CEMENT CONCRETE: PORTLAND CEMENT CONCRETE IS A CONCRETE OR ARTIFICIAL ROCK COMPOSED OF AGGREGATES, WATER, AND A CEMENTING AGENT KNOWN AS PORTLAND CEMENT. PORTLAND CEMENT IS MADE FROM LIMES...

12/03/1446 CEMENT PORTLAND CEMENT CONCRETE: PORTLAND CEMENT CONCRETE IS A CONCRETE OR ARTIFICIAL ROCK COMPOSED OF AGGREGATES, WATER, AND A CEMENTING AGENT KNOWN AS PORTLAND CEMENT. PORTLAND CEMENT IS MADE FROM LIMESTONE (OR SOME OTHER SOURCES OF LIME) AND OTHER MINERALS, WHICH ARE GROUND UP, MIXED, BURNED IN A KILN, AND SUBSEQUENTLY GROUND TO A FINE POWDER THAT WILL HARDEN WHEN MIXED WITH WATER. 1 12/03/1446  The most important and most costly material in this type of concrete is the cementing agent, Portland cement.  The name Portland is not a trade name but the name of a type of cementing materials.  The main minerals required for the production of Portland cement are : 1) Lime (Cao) (main component 60-65%) 2) Silica (SIO2) 3) Alumina (AL2O3) 4) Iron Oxide (Fe2O3) 2 12/03/1446 Manufacturing of Portland Cement: 1) The raw materials are ground up, mixed to produce the desired proportion of minerals. 2) Then burned in a large Kiln (1500 C or 2700 F), this drives off water and gases and produces new chemical compositions in particles called CLINKER. 3) The CLINKER is subsequently ground with about 5% gypsum (to control rate of hardening) and is ready to use as Portland cement. 3 12/03/1446 CLINKER 4 12/03/1446 5 12/03/1446 CONTINUED The Cement compounds Produces:  C3S: Hardens rapidly, and is mainly responsible for the initial set and early strength.tricalcium silicate(3CaO.SiO2)  C2S: Hydrates slowly, and is the main source of increased strength after The first week of hardening. Dicalcium silicate2CaO.SiO2  C3A: Reacts very quickly and adds a small amount of strength.Tricalcium aluminate3CaO.Al2O3  C4AF: Reacts slowly, and its main purpose in Portland cement is to reduce the temperature required during burning in the kiln.Tetracalcium alumino ferrite 4CaO.Al2O3 Note: C3S and C2S are mainly responsible for the strength of the concrete. 6 12/03/1446  Relative amounts of these four chemicals in the final product depend on the desired properties. Such as rate of the hardening, amount of heat given off, and resistance to chemical attack.  Portland cement is a hydraulic cement, that is, one that sets or hardens when mixed with water, Particles of cement take up water, forming a gel that cements the individual particles together, this chemical process is called Hydration. It continues for months or Years as long as water is present.  Moist cured concrete reaches 100% of its design strength in 28 days, continues to increase in strength, reaching about 120 % and 130% of design strength in 90 days and 180 days, respectively.  During hydration, heat (heat of hydration) is given off, and in massive structures such as dams, this heat could result in cracking due to the fairly rapid rise in temperature and the subsequent cooling. So in this case the cement should contain smaller amount of c3s and c3a, both of which harden rapidly and give off heat quickly. 7 12/03/1446  The rate of heat is greatest in C3A, followed by C3S.  About 50 % of the total heat of hydration is released in the first three days.  The total amount of water required to complete the hydration of the cement is about 25% of the mass of the cement. 8 12/03/1446 TYPES OF PORTLAND CEMENT Different types of Portland cement are manufactured to meet various normal physical and chemical requirements for specific purposes. Portland cements are manufactured to meet the specifications of ASTM C 150, AASHTO M 85, or ASTM C 1157. ASTM C 150, Standard Specification for Portland Cement, provides for eight types of Portland cement using Roman numeral designations as follows: Type I Normal Type IA Normal, air-entraining Type II Moderate sulfate resistance Type IIA Moderate sulfate resistance, air-entraining Type III High early strength Type IIIA High early strength, air-entraining Type IV Low heat of hydration Type V High sulfate resistance 9 12/03/1446 Type I Type I Portland cement is a general- purpose cement suitable for all uses where the special properties of other types are not required. Its uses in concrete include pavements, floors, reinforced concrete buildings, bridges, tanks, reservoirs, pipe, masonry units, and precast concrete products. Type II Type II Portland cement against moderate is used where precaution e sulfate attack is important. It is used in normal structures or elements exposed to soil or ground waters where sulfate concentrations are higher than normal but not unusually severe Type II cement has moderate sulfate resistant properties because it contains no more than 8% tricalcium aluminate (C3A). Sulfates in moist soil or water may enter the concrete and react with the hydrated C3A, resulting in expansion, scaling, and cracking of concrete. Some sulfate compounds, such as magnesium sulfate, directly attack calcium silicate hydrate. 10 12/03/1446 Use of Type II cement in concrete must be accompanied by the use of a low water to cementitious materials ratio and low permeability to control sulfate attack. Fig. 2-13 (top) illustrates the improved sulfate resistance of Type II cement over Type I cement. Concrete exposed to seawater is often made with Type II cement. Seawater contains significant amounts of sulfates and chlorides. Although sulfates in seawater are capable of attacking concrete, the presence of chlorides inhibits the expansive reaction that is characteristic of sulfate attack. Chloride competes with sulfate for the aluminate phases, after which they exist together in the concrete 11 12/03/1446 Type II continued The reaction products of sulfate attack are also more soluble in a chloride solution and can leach out of the concrete. Observations from a number of sources show that the performance of concretes in seawater with Portland cements having C3A contents as high as 10%, have shown satisfactory durability, providing the permeability of the concrete is low and the reinforcing steel has adequate cover.. Type II cements specially manufactured to meet the moderate heat option of ASTM C 150 (AASHTO M 85) will generate heat at a slower rate than Type I or most Type II cements. The requirement of moderate heat of hydration can be specified at the option of the purchaser 12 12/03/1446 A cement in which heat-of-hydration maximums are specified can be used in structures of considerable mass, such as large piers, large foundations, and thick retaining walls (Fig. 2-16). Its use will reduce temperature rise and temperature related cracking, which is especially important when concrete is placed in warm weather. Because of its increased availability, Type II cement is sometimes used in all aspects of construction, regardless of the need for sulfate resistance or moderate heat generation. Some cements may be labeled with more than one type designation, for example Type I/II. This simply means that such a cement meets the requirements of both cement Types I and II. 13 12/03/1446 conclusion 1)Type II (20): Moderate resistance to sulphate (over 150 ppm) in the water or 0.1 % in the soil), or slightly lower heat of hydration is required. Type III Type III portland cement provides strength at an early period, usually a week or less. It is chemically and physically similar to Type I cement, except that its particles have been ground finer. It is used when forms need to be removed as soon as possible or when the structure must be put into service quickly. In cold weather its use permits a reduction in the length of the curing period (Fig. 2-17). Although higher-cement content mixes of Type I cement can be used to gain high early strength, Type III may provide it easier and more economically. 14 12/03/1446 Type IV  Type IV portland cement is used where the rate and amount of heat generated from hydration must be minimized. It develops strength at a slower rate than other cement types. Type IV cement is intended for use in massive concrete structures, such as large gravity dams, where the temperature rise resulting from heat generated during hardening must be minimized (Fig. 2- 16). Type IV cement is rarely available. Used where a lower rate of heat increase is required, as in dams.  It contains smaller amount of C3S and C3A both of which harden rapidly and give off heat quickly. Type V Type V portland cement is used in concrete exposed to severe sulfate action—principally where soils or ground waters have a high sulfate content. It gains strength more slowly than Type I cement. Table 2-2 lists sulfate concentrations requiring the use of Type V cement. The high sulfate resistance of Type V cement is attributed to a low tricalcium aluminate content, not more than 5%. Use of a low water to cementitious materials ratio and low permeability are critical to the performance of any concrete exposed to sulfates. 15 12/03/1446 Even Type V cement concrete cannot withstand a severe sulfate exposure if the concrete has a high water to cementitious materials ratio (Fig. 2-15 top). Type V cement, like other portland cements, is not resistant to acids and other highly corrosive substances. ASTM C 150 (AASHTO M 85) allows both a chemical It contains smaller amount of C3S and C3A both of which harden rapidly and give off heat quickly. The amount of C3A in this type only about one- third of that in normal cement. 16 12/03/1446 17 12/03/1446 18 12/03/1446 19 12/03/1446 20 12/03/1446 21 12/03/1446 22 12/03/1446 23 12/03/1446 24 12/03/1446 PHYSICAL TESTS 1-FINENESS TEST  Importance: Finer the cement, more is the strength since surface area for hydration will be large. With increase in fineness, the early development of strength is enhanced but the ultimate strength is not affected. An increase in the fineness of the cement increases the cohesiveness of the concrete mix and thus reduces the amount of water which separates to the top of a lift (bleeding),. particularly while compacting with vibrators. However, if the cement is ground beyond a certain limit, its cementative properties are affected due to the pre hydration by atmospheric moisture. Finer cement reacts more strongly in alkali reactive aggregate. Also, the water requirement and workability will be more leading to higher drying shrinkage and cracking 25 12/03/1446 SIEVE METHOD  100 g of cement sample is taken and air-set lumps, if any, in the sample are broken with fingers. The sample is placed on a 90 micron sieve and continuously sieved for 15 minutes. The residue should not exceed the limits specified below: 26 12/03/1446 DETERMINATION OF INITIAL AND FINAL SETTING TIME  When water is added to cement, the resulting paste starts to stiffen and gain strength and lose the consistency simultaneously. The term setting implies solidification of the plastic cement paste. Initial and final setting times may be regarded as the two stiffening states of the cement. The beginning of solidification, called the initial set, marks the point in time when the paste has become unworkable. The time taken to solidify completely marks the final set, which should not be too long in order to resume construction activity within a reasonable time after the placement of concrete. Vicat’s apparatus used for the purpose is shown in Fig. 5.9. The initial setting time may be defined as the time taken by the paste to stiffen to such an extent that the Vicat’s needle is not permitted to move down through the paste to within 5 ± 0.5 mm measured from the bottom of the mould. The final setting time is the time after which the paste becomes so hard that the angular attachment to the needle, under standard weight, fails to leave any mark on the hardened concrete. Initial and final setting times are the rheological (‫)االنسيابية‬properties of cement. 27 12/03/1446 IMPORTANCE It is important to know the initial setting time, because of loss of useful properties of cement if the cement mortar or concrete is placed in moulds after this time. The importance of final setting time lies in the fact that the moulds can be removed after this time. The former defines the limit of handling and the latter defines the beginning of development of mechanical strength. SOUNDNESS TEST  It is essential that the cement concrete does not undergo large change in volume after setting. This is ensured by limiting the quantities of free lime and magnesia which slake‫ يخمد اويطفأ‬slowly causing change in volume of cement (known as unsound). Soundness of cement may be tested by LeChatelier method or by autoclave method. For OPC, RHC, LHC and PPC it is limited to 10 mm, whereas for HAC and SSC it should not exceed 5 mm. 28 12/03/1446 IMPORTANCE It is a very important test to assure the quality of cement since an unsound cement produces cracks, distortion and disintegration, ultimately leading to failure. 29 12/03/1446 DETERMINATION OF STRENGTH  Cement hydrates when water is added to it and cohesion and solidity is exhibited. It binds together the aggregates by adhesion. The strength of mortar and concrete depends upon the type and nature of cement. So, it should develop a minimum specified strength if it is to be used in structures. Cement is tested for compressive and tensile strengths COMPRESSIVE STRENGTH Compressive strength is the basic data required for mix design. By this test, the quality and the quantity of concrete can be controlled and the degree of adulteration‫ غش‬can be checked. 30 12/03/1446 TENSILE STRENGTH  The tensile strength may be determined by Briquette test method or by split tensile strength test. 31 12/03/1446 32