Lecture-1 The Kiln: • • The heart of the cement plant o Largest moving part of any machine o inclined, rotates o up to 50m long and 5m diam. Heated by fire jet The rotary kiln is along steel cylinder lined with refractory brick (length /diameter ~30).Modern kilns may reach 6m in diameter and over 180m in height with a production capacity exceeding 1000 tones a day. • The kiln is inclined a few degrees from the horizontal ( about 4 cm\m ) and is rotated about its axis at a speed of about 60 to 150 revolution \ hour ). • Pulverized coal or gas is used as the source of heat. The heat is supplied from the lower end of the kiln. The max. Temperature near the lower end of the kiln is generally about 1400-1500 OC. • The upper end of the kiln the temperature is around 150 OC. • The mixture of the raw material is fed from the upper end of the kiln.This material move toward the lower end by effect of inclanation and rotation of the kiln. Thus the material is subjected to high temperature at lower end of the kiln. • The materials that are introduced into the rotary kiln are subjected to several distinct process as they move downward. • When the raw materials are fed into the kiln, drying of the material takes place, and any free water in the raw material is evaporated. • Clay losses its water about 150 to 350 OC. • Clay decompose at a range of 350 to 650 OC. • Magnesite in raw material loss about 600 OC.
Lecture-1 • The limestone losses its CO2 at about 900 OC. • At 1250 to 1280 OC some liquid formation begins and compound formation start to takes place. • Clinkering begins at about 1280 OC. The liquid that forms during the burning process causes the charge to agglomerate into nodules of various size, usually 1-25 mm in diameter known as Portland cement clinker. • All exhaust gases produced during the burning process of the materials leave the kiln through the stack. Cooling & grinding • Rapid cool - glassy • Grinding starts golf ball size. Ends about 2-80 microns, 300 m2/kg. Grinding depends on application. Typical plant capacity is about: 1 MT/y • Portland cement is manufactured by inter grinding the Portland cement clinker with some (3 to 6 %) gypsum rock. Cement: Physical, Chemical Properties and Hydration Physical Properties Portland cements are commonly characterized by their physical properties for quality control purposes. Their physical properties can be used to classify and compare Portland cements. The challenge in physical property characterization is to develop physical tests that can satisfactorily characterize key parameters. The physical properties of cement • Fineness • Soundness • Setting Time • Strength Fineness
Lecture-1 • Fineness or particle size of Portland cement affects Hydration rate and thus the rate of strength gain. The smaller the particle size, the greater the surface areato-volume ratio, and thus, the more area available for water-cement interaction per unit volume. The effects of greater fineness on strength are generally seen during the first seven days. • When the cement particles are coarser, hydration starts on the surface of the particles. So the coarser particles may not be completely hydrated. This causes low strength and low durability. • For a rapid development of strength a high fineness is necessary. Soundness When referring to Portland cement, "soundness" refers to the ability of a hardened cement paste to retain its volume after setting without delayed
Lecture-1 expansion. This expansion is caused by excessive amounts of free lime (CaO) or magnesia (MgO). Most Portland cement specifications limit magnesia content and expansion. Setting Time • Cement paste setting time is affected by a number of items including: cement fineness, water-cement ratio, chemical content (especially gypsum content) and admixtures. Setting tests are used to characterize how a particular cement paste sets. For construction purposes, the initial set must not be too soon and the final set must not be too late. Normally, two setting times are defined: • Initial set. Occurs when the paste begins to stiffen considerably. • Final set. Occurs when the cement has hardened to the point at which it can sustain some load. • Setting is mainly caused by C3A and C3S and results in temperature rise in the cement paste. • False set :No heat is evolved in a false set and the concrete can be remixed without adding water • Occures due to the conversion of unhydreous/semihydrous gypsum to hydrous gypsum(CaSO4.2H2O) • Flash Set: is due to absence of Gypsum. Specifically used for under water repair. Strength • Cement paste strength is typically defined in three ways: compressive, tensile and flexural. These strengths can be affected by a number of items including: water-cement ratio, cement-fine aggregate ratio, type and grading of fine aggregate, curing conditions, size and shape of specimen, loading conditions and age Duration of Testing Typically, Durations of testing are: • 1 day (for high early strength cement)