Portland Cement

Introduction To Portland Cement, Including properties and processes during hydration

1. MILITARY COLLEGE OF ENGINEERING RISALPUR CE 308 – LECTURE 2 PORTLAND CEMENT AND HYDRATION

2. The use of cementing materials is very old. The ancient Egyptian used calcined impure gypsum. The Greeks and the Romans used calcined limestone and later learned to add lime and water, sand and crushed stone or brick and broken tiles. This was the first concrete in history. Historical Note

3. Lime mortar does not harden under water , and for construction under water the Romans ground together lime and a volcanic ash or finely ground burnt clay tiles . The active silica and alumina in the ash and tiles combined with the lime to produce, Pozzolanic cement from name of village of Pozzuloi , near Vesuvius, where the volcanic ash was first found . Some of the Roman structures in which masonry was bonded by mortar, such as the Coliseum in Rome have survived to this day, with the cementious material still hard and firm.

4. Portland cement patented by Joseph Aspdin , in 1824 . Prepared cement is the name given to a cement obtained by intimately mixing together calcareous and argillaceous, or other silica-, alumina- and iron oxide-bearing materials, burning them at a clinkering temperature, and grinding the resulting clinker The name Portland cement---resemblance of color to Portland stone –Limestone quarried in Dorset. Portland cement is a fine powder that when mixed with water becomes the glue that holds aggregates together in concrete . Portland cement

5. Portland cements are hydraulic cements composed primarily of hydraulic calcium silicates . Hydraulic cements set and harden by reacting chemically with water. During this reaction, called hydration, cement combines with water to form a stone like mass, called paste. When the paste (cement and water) is added to aggregates (sand and gravel, crushed stone, or other granular material) it acts as an adhesive and binds the aggregates together to form concrete, the world’s most versatile and most widely used construction material . Fresh cement is like a fine powder which is commonly available in sealed bags. On opening the bag, cement starts absorbing moisture from the air and with the passage of time, small and large lumps are formed, which results in the decline of cement strength. So the cement bag once opened should be used as soon as possible. Portland cement

6. Lime ( CaO ) 60 to 67% Silica (SiO 2 ) 17 to 25% Alumina (Al 2 O 3 ) 3 to 8% Iron oxide (Fe 2 O 3 ) 0.5 to 6% Magnesia (MgO) 0.1 to 4% Sulphur trioxide (SO 3 ) 1 to 3% Alkalis (Na 2 O+K 2 O ) 0.2 to 1.3% The chief chemical constituents of Portland cement

7. Compound Oxide ( Abvn ) %age Tricalcium silicate 3 CaO.SiO 2 ( C 3 S) 40% ( b ) Dicalcium silicate 2CaO.SiO 2 ( C 2 S) 30% ( c ) Tricalcium aluminate 3CaO.Al2O 3 ( C 3 A) 11% ( d ) Tetracalcium aluminate 4CaO.Al 2 O 3 .Fe 2 O 3 (C 3 AF) 11% Component Oxides

8. HYDRATION OF CEMENT The hydration of cement compounds is exothermic. The reaction proceeds slowly for 2 – 5 hours (called induction or dormant period ). Quantity of heat evolved upon complete hydration at a given temperature is known as heat of hydration. For the usual range of Portland cements, about one-half of the total heat is liberated between 1 and 3 days , about three-quarters in 7 days, and nearly 90 percent in 6 months. The temperature at which hydration occurs greatly affects the rate of heat development, which for practical purposes is more important than the total heat of hydration .

9. HEAT OF HYDRATION

10. HYDRATION OF CEMENT

11. HYDRATION PROCESS When portland cement is dispersed in water , the calcium sulfate and the high-temperature compounds of calcium begin to go into solution, and the liquid phase gets rapidly saturated with various ionic species . As a result of interaction between calcium, sulfate, aluminate , and hydroxyl ions within a few minutes of cement hydration, the needle-shaped crystals of calcium trisulfoaluminate hydrate , called ettringite , first make their appearance.

12. HYDRATION PROCESS A few hours later , large prismatic crystals of calcium hydroxide and very small fibrous crystals of calcium silicate hydrates begin to fill the empty space formerly occupied by water and the dissolving cement particles. After some days, depending on the alumina-to-sulfate ratio of the portland cement, ettringite may become unstable and will decompose to form monosulfoaluminate hydrate, which has a hexagonal-plate morphology.

13. TIME LINE OF HYDRATION

14. TIME LINE OF HYDRATION

15. TIME LINE OF HYDRATION

16. TIME LINE OF HYDRATION

17. HYDRATION PRODUCTS

18. HYDRATION PRODUCTS

19. HYDRATED PASTE

20. HYDRATION PROCESS

21. HYDRATION REACTIONS Hydration of Calcium Aluminates Hydration of Calcium Silicates

22. DEVELOPMENT OF STRENGTH When water is added to cement, C3A is the first to react and cause initial set. It generates great amount of heat. C3S hydrates early and develops strength in the first 28 days . It also generates heat. C2S is the next to hydrate. It hydrates slowly and is responsible for increase in ultimate strength. C4AF is comparatively inactive compound . At the age of about one year , the two compounds (C3S AND C2S) contributes approximately equally to the strength of hydrated cement.

23. SIZE OF PORES AND VOIDS

24. WATER FOR HYDRATION

25. Setting This is the term used to describe the stiffening of the cement paste. Broadly speaking, setting refers to a change from a fluid to a rigid stage. Although, during setting, the paste acquires some strength , for practical purposes it is important to distinguish setting from hardening, which refers to the gain of strength of a set cement paste.

26. False Set / Flash Set False set is the name given to the abnormal premature stiffening of cement within a few minutes of mixing with water. It differs from flash set in that no appreciable heat is evolved, and remixing the cement paste without addition of water restores plasticity of the paste until it sets in the normal manner and without a loss of strength.

27. ROLE OF GYPSUM

28. ROLE OF GYPSUM

29. CEMENT TESTS Because the quality of cement is vital for the production of good concrete, the manufacture of cement requires stringent control. A number of tests are performed in the cement plant laboratory to ensure that the cement is of the desired quality and that it conforms to the requirements of the relevant national standards. These tests include: Fineness tests Consistency of standard paste Setting times Soundness tests Strength tests

30. CEMENT TESTS Fineness of Cement. Since hydration starts at the surface of the cement particles. Fineness of cement is the total surface area of cement grains available for hydration (expressed as specific surface m 2 /kg ) The rate of hydration depends on the fineness of cement particles, and for a development of strength a high fineness is necessary (e.g. minimum 325 m 2 /kg fineness is required for rapid hardening cement). Greater the fineness more is the surface available for hydration, resulting greater early strength and more rapid release of heat. An increase in fineness increases the amount of gypsum required for retardation because , in a finer cement, more C 3 A is available for early hydration.

31. CEMENT TESTS Consistency of standard paste. Consistency indicates the degree of density or stiffness of cement. For tests like initial setting time, final setting time, and for Le Chatelier soundness test, neat cement paste is required. A certain minimum quantity of water is required to be mixed with cement so as to complete chemical reaction between water and cement, less water than this quantity would not complete chemical reaction thus resulting in reduction of strength and more water would increase water cement ratio and so would reduce its strength. So correct proportion of water to cement is required to be known to achieve proper strength while using cement in structure. This can be found out knowing standard consistency of cement paste.

32. CEMENT TESTS Setting times. This is the term used to describe the stiffening of the cement paste. Broadly speaking, setting refers to a change from a fluid to a rigid stage. Setting is mainly caused by a selective hydration of C 3 A and C 3 S and is accompanied by temperature rises in the cement paste; initial set corresponds to a rapid rise in temperature and final set corresponds to the peak temperature Although, during setting, the paste acquires some strength, for practical purposes it is important to distinguish setting from hardening, which refers to the gain of strength of a set cement paste.

33. CEMENT TESTS Soundness. It is essential that cement paste, once it has set, does not undergo a large change in volume. In particular, there must be no appreciable expansion which could result in a disruption of the hardened cement paste. This is ensured by limiting the quantities of free lime and magnesia which are causing change in volume of cement (known as unsound). Free lime is present in clinker and is intercrystallized with other compounds; consequently it hydrates very slowly occupying a large volume than the original free calcium oxide. Soundness of cement may be tested by Le- Chatelier method .

34. CEMENT TESTS Strength. Strength tests are not made on neat cement paste because of difficulties in obtaining good specimens and in testing with a consequent large variability of test results. Cement-sand mortar and in some cases concrete of prescribed proportions made with specified materials under strictly controlled conditions, are used for the purpose of determining strength of cement. There are several forms of strength tests,: direct tension, compression and flexure. In recent years the tension test has been gradually superseded by the compression test.

35. TYPES OF PORTLAND CEMENT Ordinary Portland Cement (Type I). This is by far the most common cement used in general concrete construction when there is no exposure to sulphate in the soil or in ground water. Over the years there have been changes in the characteristics of the ordinary Portland cement: modern cements have a higher C 3 S content and a greater fineness than 40 years ago. Rapid Hardening Portland Cement (Type III) It is similar to Type I, but the strength of this cement develops rapidly because of a higher C 3 S content (up to 70 %) and a higher fineness (325 m 2 /kg). It is used when formwork is to be removed early for re-use or where sufficient strength for further construction is required quickly.

36. TYPES OF PORTLAND CEMENT Rapid Hardening Portland Cement (Type III). It should not be used in mass construction or in large structural section because of its higher rate of heat of hydration. However, for constructions at low temperatures, the use of this cement may provide a satisfactory safeguard against early frost damage. Special Rapid Hardening Portland Cement It is specially manufactured cement which is highly rapid hardening. For instance, ultra-high early strength Portland cement is permitted to be used in UK. The high early strength is achieved by higher fineness (700 to 900 m 2 /kg) and a higher gypsum but this does not affect the long term soundness Typical uses are urgent repairs and early prestressing .

37. Traditional Classification European Classification British American [BS 8500–1: 2006] Ordinary Portland [BS 12] Type I [ASTM C 150] Type (CEM) I Rapid-hardening Portland [BS 12] Type III [ASTM C 150] Type IIA Low-heat Portland [BS 1370] Type IV [ASTM C 150] Modified cement Type II [ASTM C 150] Type IIB-S Several types of Portland cement are available commercially, and additional special cements can be produced for special uses. Table in the slide lists the main types of Portland cement as classified by BS, ASTM and new BS EN Standards. TYPES OF PORTLAND CEMENT

38. TYPES OF PORTLAND CEMENT Traditional Classification European Classification British American [BS 8500–1: 2006] Sulfate resisting Portland (SRPC) [BS 4027] Type V [ASTM C 150] Portland blast-furnace (Slag cement) [BS 146] Type IS, Type S Type I (SM) [ASTM C 595] Type IIB-V High slag blast-furnace [BS 4246] --- Type IIB+SR White Portland [BS 12] --- Type IIIA Portland-pozzolan [BS 6588; BS 3892] Type IP, Type P Type I (PM) [ASTM C 595] Type IIIA+SR

39. TYPES OF PORTLAND CEMENT Low Heat Portland Cement (Type IV). Developed in the US for use in large gravity dams, this cement has a low heat of hydration. Because of low contents of C 3 S and C 3 A there is a slow development of strength than the ordinary Portland cement, but the ultimate strength is unaffected. The fineness must not be less than 320 m 2 /kg to ensure a sufficient rate of gain of strength. Modified cement (Type II) In some applications a very low early strength may be disadvantage and for this reason a modifies cement was developed in the US.

40. TYPES OF PORTLAND CEMENT Modified cement (Type II) This cement has a higher rate of heat development than the type IV cement, and a rate of gain strength similar to that of type I cement. It is recommended for structures where a moderately low heat generation is desirable or where moderate sulphate attack may occur. Sulphate Resisting cement (Type V) This cement has a low C 3 A content so as to avoid sulphate attack from outside the concrete; otherwise the formation of calcium sulphoaluminate and gypsum would cause disruption of the concrete due to an increased volume of the resulting compounds. Sulphate attack is greatly accelerated if accompanied by alternate wetting and drying e.g. in marine structures subjected to tide or splash.

41. TYPES OF PORTLAND CEMENT Sulphate Resisting cement (Type V) To achieve sulphate resistant, the C 3 A content is limited to 3.5 % In US there exists cement with moderate level of sulphate resistance. These are produced by blending Portland cement with pozzolan , they have the C 3 A content limited to 8 %. The heat developed by this cement is not much higher than that of low-heat cement, which is an advantage, but the cost of the former is on higher side, due to the special composition of raw materials. Thus, in practice the sulphate resisting cement should be specified only when necessary; its not a cement for general use.

42. TYPES OF PORTLAND CEMENT Portland Blast-Furnace cement This type of cement is made by intergrinding or blending Portland cement clinker with ground granulated blast furnace slag ( ggbs ), which is a waste product in the manufacture of iron; thus, there is a lower energy consumption in the manufacture of cement. Slag contains lime, silica and alumina but not in the same proportions as in the Portland cement and its composition can vary a great deal. Sometimes this cement is referred as slag cement Typical uses are in mass concrete because of a lower heat of hydration and in sea-water construction because of a better sulphate resistance (due to a lower C 3 A content) than with the ordinary Portland cement. It is commonly used in countries where slag is widely available, and can be considered as a cement for general use.

43. TYPES OF PORTLAND CEMENT White and coloured Portland cements For architectural purposes, white concrete or particularly in tropical countries a colour paint finish is sometimes required. For these purposes white cement is used. It is also used for its low content of soluble alkalis so that staining is avoided. White cement is made from china clay, which contains little iron oxide and manganese oxide, together with chalk or lime stone free from impurities. In addition special precautions are required during the grinding of clinker so as to avoid contamination. For these reasons the cost of white cement is high (twice that of Portland cement).

44. TYPES OF PORTLAND CEMENT Portland- Pozzolan cements This cement is made by intergrinding or blending pozzolans with portland cement. ASTM C 618-93 describes a Pozzolan as a siliceous or siliceous and aluminous material which in itself possesses little or no cementitious value but will, in finally divided form and in the presence of moisture, chemically react with lime at ordinary temperatures to form compounds possessing cementitious properties. It gains strength slowly and therefore require curing over a comparatively longer period but the long term strength is high. It is recommended for general construction. It is cheaper than the normal Portland cement.

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