chemistry

Information about chemistry

Published on March 8, 2014

Author: nimisha_singhal04

Source: authorstream.com

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Cement Chemistry: Cement Chemistry CEL 510 Advanced Concrete Technology 1. Cement Hydration: 1. Cement Hydration Cement hydration: Cement hydration Reaction of cement with water Exothermic; heat released is called ‘Heat of Hydration’ Rate of heat evolution is faster if the reaction is quicker Heat evolved depends on heat of hydration of individual compounds PowerPoint Presentation: Heat of hydration of pure cement compounds Compound HOH (J/g) C 3 S 502 C 2 S 260 C 3 A 867 C 4 AF 419 Bogue: ½ of total heat is evolved between 1 and 3 days, about ¾ in 7 days, and 83 – 91% in 6 months PowerPoint Presentation: From Cement Chemistry, by H. F. W. Taylor Typical heat evolution pattern PowerPoint Presentation: Peak I: ‘Heat of wetting’ + some early C-S-H formation Dormant period: Very slow rate of heat evolution Peak II: Main peak; associated with the rapid dissolution of C 3 S to form CSH and CH, and formation of ettringite (AF t ) from C 3 A. A slowdown of the hydration process beyond the main peak leads to lower rates of heat evolution. A broader peak (III) is associated with the conversion of ettringite to monosulphate (AF m ). Heat evolution pattern explanation PowerPoint Presentation: Suggested new heat evolution pattern that shows an initial Endothermic peak due to the dissolution of alkali sulphates From Lea’s Chemistry of Cement and Concrete, Edited by P. C. Hewlett Other facts: Other facts Difficult to obtain the correct relationship between heat evolution and temperature unless the system is perfectly insulated Dependence on the water to cement ratio: Water has a much higher specific heat than cement, thus when more water is present, a higher degree of heat will be required to increase the temperature of the system. Cement contains highly soluble alkali oxides (Na 2 O and K 2 O). The dissolution of these compounds is responsible for the high alkalinity (pH 12 – 13) of the pore solution. Thus, the hydration of cement actually takes place in the pore solution, and not in water. Temperature evolution: Temperature evolution Why dormant period?: Why dormant period? Several theories proposed These theories basically point to the fact that a layer (either of hydrates or ions) is created on the surface of cement particles Further wetting of the cement particle is possible only by diffusion across this layer Therefore rate slows down End of dormant period: End of dormant period Weakening of barrier by aging Increased rates of diffusion Weakening of ionic strength around the hydrating particle Hydration reactions: Hydration reactions Silicates (C 3 S and C 2 S) hydrate to produce Calcium-silicate-hydrate (C-S-H) gel and calcium hydroxide (CH) 3 times as much CH produced by C 3 S hydration compared to C 2 S C-S-H does not have a well-defined composition; C/S varies from 1.5 to 2 Hydration reactions: Hydration reactions Aluminates (C 3 A and C 4 AF), in the absence of gypsum, hydrate rapidly to produce Calcium-aluminate-hydrates (C-A-H) In the presence of gypsum, ettringite (AF t ) and monosulphate (AF m ) are produced (depending on the C 3 A to SO 3 ratio) Ettringite formation is known to be expansive (numerous mechanisms suggested) Reactions - Specifics: Reactions - Specifics 2 C 3 S + 6 H  C 3 S 2 H 3 + 3 CH 2 C 2 S + 4 H  C 3 S 2 H 3 + CH 2 C 3 A + 21 H  C 4 AH 13 + C 2 AH 8 Flash set reaction! C 2 AH 8 is a metastable phase that deposits as hexagonal platelets (similar to CH). Above 30 o C, it is converted to cubic hydrogarnet (C 3 AH 6 ). In the presence of gypsum, C 3 A + 3 C S H 2 + 26 H  C 6 A S 3 H 32 PowerPoint Presentation: Nearly all the SO 4 2- gets combined to form ettringite in an ordinary Portland cement. If there is still C 3 A left after this reaction, it can combine with ettringite to form monosulphate (or AF m phase) which has a stoichiometry of C 4 A S H 12-18 . If there is sufficient excess C 3 A, then C 4 AH 13 can also form as a hydration product, and can exist in a solid solution with AF m . C 4 AF produces similar hydration products as C 3 A, with the Al 3+ being partly replaced by Fe 3+ . The final hydration product depends on the availability of lime in the system. In the presence of gypsum, C 4 AF produces an iron-substituted ettringite. Higher the ratio C 4 AF/C 3 A, lower is the conversion of ettringite to monosulphate. Kinetics of cement hydration: Kinetics of cement hydration The progress of cement hydration depends on: ·  Rate of dissolution of the involved phases (in the initial stages), and at later stages, ·  Rate of nucleation and crystal growth of hydrates ·  Rate of diffusion of water and dissolved ions through the hydrated materials already formed Factors affecting hydration rate: Factors affecting hydration rate The phase composition of cement The amount and form of gypsum in the cement: Whether gypsum is present in the dihydrate, hemihydrate, or the anhydrite form. Fineness of cement: Higher the fineness, higher the rate of reaction due to availability of a larger surface area. w/c of mix: At high w/c, hydration may progress till all of the cement is consumed, while at low w/c the reaction may stop altogether due to lack of water. Factors…(contd.): Factors…(contd.) Curing conditions: The relative humidity can have major effects on the progress of hydration. Hydration temperature: Increase in temperature generally causes an increase in the rate of the reaction, although the hydrated structure can be different at different temperatures. Presence of chemical admixtures: For example, set controllers, and plasticizers. Composition of pore solution: Composition of pore solution The evolution of pore solution composition for a typical cement (0.6% equivalent Na 2 O, 3% SO 3 , 0.5 w/c) is shown here. By 1 week, the only ions remaining in appreciable concentration are Na + , K + , and OH - . 2. Structure of hydrated cement paste: 2. Structure of hydrated cement paste Hydrated cement paste: Hydrated cement paste Hydrated cement paste is composed of capillary pores and the hydration product. The pores within the structure of the hydration product are termed ‘gel’ pores. This hydration product includes C-S-H, CH, AF t , AF m , etc. Gel pores are included within the structure of hydrated cement. According to Powers, 1/3 of the pore space is comprised of gel pores, and the rest are capillary pores. PowerPoint Presentation: Gel-like C-S-H observed Bright particles: Unhydrated cement; Gray regions: C-S-H; white rim around aggregates: CH PowerPoint Presentation: C 3 S mortar showing white unhydrated cement particles and gray C-S-H, along with white rims of CH Porosity of paste in concrete is visible in this picture Water within cement paste: Water within cement paste Capillary water: Present in voids larger than 50 A o . Further classified into: (a) free water, the removal of which does not cause any shrinkage strains, and (b) water held by capillary tension in small pores, which causes shrinkage strains on drying. Adsorbed water: Water adsorbed on the surface of hydration products, primarily C-S-H. Water can be physically adsorbed in many layers, but the drying of farther surfaces can occur at about 30 % relative humidity. Drying of this water is responsible for a lot of shrinkage. Interlayer water: Water held in between layers of C-S-H. The drying of this water leads to a lot of shrinkage due to the collapse of the C-S-H structure. Bound water: This is chemically bound to the hydration product, and can only be removed on ignition. Also called ‘non-evaporable’ water. 2 and 3 are together called ‘gel’ water. Calculation of hcp structure: Calculation of hcp structure See handout! Structure of hydration products: Structure of hydration products CH: hexagonal crystals, generally oriented tangentially to pore spaces and aggregates along the longitudinal axis Ettringite: Acicular, columnar, hexagonal crystals. The presence of tubular channels in between the columns can lead to high water absorption and swelling by ettringite. This is one of the theories explaining the expansion caused by ettringite formation. Structure…(contd.): Structure…(contd.) Ettringite demonstrates a trigonal structure, while monosulfate is monoclinic The structure of C-S-H is best described by the Feldman-Sereda model. It consists of randomly oriented sheets of C-S-H, with water adsorbed on the surface of the sheets (adsorbed water) , as well as in between the layers (interlayer water), and in the spaces inside (capillary water). Structure of C-S-H: Structure of C-S-H The Feldman-Sereda model implies a very high surface area for the gel. Using water sorption and N 2 sorption measurements, a surface area of 200000 m 2 /kg is reported (ordinary PC has a fineness in the order of 225 – 325 m 2 /kg). Small angle X-ray scattering measurements show results in the range of 600000 m 2 /kg. The corresponding figure for high pressure steam-cured cement paste is 7000 m 2 /kg, which suggests that hydration at different temperatures leads to different gel structures. The structure of C-S-H is compared to the crystal structure of Jennite and Tobermorite. A combination of the two minerals is supposed to be the closest to C-S-H. Feldman-Sereda model: Feldman-Sereda model

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