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  1. Cement Production CE 611 Advanced Concrete Technology
  2. Modern cement….  John Smeaton, while planning the building of Eddystone lighthouse tower in 1756, discovered that the best limes for mortar contained a high degree of clayey matter  Ultimately, such a lime was used along with pozzolana in equal quantities
  3. Eddystone Lighthouse Tower •Completed in 1759 •72 feet tall; 93 steps •Newer lighthouse constructed in 1882; Smeaton’s tower was moved stone-by-stone to Plymouth, where it is still the most major landmark
  4. Portland Cement  L. J. Vicat: Prepared artificial hydraulic lime by calcining an intimate mixture of limestone (chalk) and clay – principal forerunner to Portland Cement  1824 – Joseph Aspdin, while obtaining a patent for his hydraulic cement, termed it as Portland cement, upon Portland stone (limestone from Dorset, UK), which had a high quality and durability and a similar appearance
  5. Aspdin’s Creation Kiln for burning A – Alite, or C3S B – Belite, or C2S
  6. Portland Cement • An unusual industrial product produced in huge quantities in special plants that can produce nothing else • The product is produced by a combination of unusual unit operations involving mining, very fine scale blending of raw materials, very high temperature clinkering reactions, controlled cooling, grinding, blending, and finally shipping under controlled conditions • Chemical composition is maintained within narrow limits despite huge tonnages
  7. Portland cement production • Typical plant costs range upwards of $250 million - a fairly substantial fixed investment. • Plant must produce continuously to pay off capital costs • Plant must also produce continuously to maintain kiln integrity - 3 shifts per day! • Plant must comply with severe environmental constraints • All this must be done to produce a commodity product that sells for Rs. 3.2 / kg
  8. Raw Materials for Cement  Calcareous material – Containing CaCO3 (primary source – limestone); impurities such as iron and alumina are sometimes present  Argillaceous material – Containing clayey matter, source of SiO2, Al2O3 and Fe2O3  Gypsum – Added in the final stages of manufacture as a set regulator  Sometime, ground limestone is also added to cement
  9. Location of cement plants  Outskirts of the city  Primarily, where raw material sources are easily available  Necessary infrastructure (power, equipment, manpower, access) should be available
  10. Schematic depiction of process
  11. Pulverization  Raw material feedstock should be pulverized to the right size  Reduces overall power consumption  Better blending and burning possible with reduced size of material
  12. Blending of raw materials • Choice of blending process - Wet or dry • Wet process – more uniform mixing • Dry process – higher output, lower power consumption • Dry process with precalciners are the order of the day
  13. Blending – Wet Vs. Dry  When moisture content of raw materials is > 15%, wet blending (in slurry form) is preferred  When MC < 8%, dry blending is done  For 8% < MC < 15%, dry blending with precalciners used  Wet blending – better blend
  14. Picture of a cement plant, showing a precalciner and rotary kiln
  15. Burning in kiln • Only rotary kilns used nowadays • Typical kilns are long ~ 30 – 40 m • Length of kiln also depends on blending process • Temperature inside kiln varies from 850 (at inlet) to 1450 oC (at the outlet) • Reactions are not completed inside kiln; some require cooling to occur • What comes out of kiln is called ‘clinker’
  16. Reactions in the kiln • The clinkering reactions involve conversion of mixtures of calcium carbonate and silica and alumina- bearing components to a mixture of special crystalline components capable of reacting with water to produce controlled setting and strength gain • The major components in clinker are impure but well crystallized fine (ca. <50 m) crystals of tricalcium silicate and dicalcium silicate
  17. Kiln reactions (continued) • Minor but important crystalline components are extremely fine crystals of tricalcium aluminate and calcium aluminate ferrite solid solution (ferrite) • Of great importance despite minor amount present are deposits of soluble crystalline components (alkali sulfates and calcium alkali sulfates) on the surfaces of clinkers
  18. From P. C. Hewlett's 'Lea's Chemistry of Cement and Concrete' Up to 700 oC: activation of silicates through removal of water and changes in crystal structure 700 – 900 oC: dacarbonation of CaCO3, initial combination of A, F, and activated silica with lime 900 – 1200 oC: Belite (C2S) formation > 1250 oC (more particularly, > 1300 oC): liquid phase appears and promotes the reaction between belite and free lime to form alite (C3S) Cooling stage: molten phase (containing C3A and C4AF) gets transformed to a glass; if cooling is slow, C3A crystallizes out (causes setting problems), or alite converts to belite and free lime
  19. cement-rotary-kiln.html Some cement plants
  20. Intergrinding with gypsum • Final step in cement manufacture • Gypsum added as a set regulator (absence  flash set) • Strict control on temperature required • Done in ball mills • Cement of required fineness produced
  21. Other issues • Cement manufacture today is a highly controlled process • However, there is lot of variation in quality of cements (between brands, in the same brand, sometimes in batches produced on the same day!) • Quality control during cement manufacture  done at every stage in the process
  22. Quality control • Sampling and evaluation should be performed after excavation from the quarry, before and after blending the feedstock, after formation of clinker, after intergrinding clinker with gypsum, and finally before packaging in the bags and drums
  23. Quality control parameters Lime saturation factor (LSF) = C/(2.8S + 1.2A + 0.65F), where C, S, A, and F are the % amounts of CaO, SiO2, Al2O3, and Fe2O3, respectively. Silica ratio (or modulus) = S/(A + F) Alumina ratio (or modulus) = A/F Potential C3S from Bogue formulation The LSF is particularly important because it dictates the amount of free lime that will be present in the product. Too much free lime can cause unsoundness of the cement.