Concrete Technology: Chemical and Mineral Admixtures Unit 2 Part 2

Que2.10. bandy fly ash in concrete. Give the advantages and  disadvantages of cover ash. 

Answer  Fly Ash Fly ash is one of the  remainders generated in combustion, and  comprises the fine  patches that rise with the stovepipe  feasts. In an artificial   environment, fly ash  generally refers to ash produced during combustion of  coal.  Advantages of Fly Ash in Concrete Following are the advantages  of cover ash  1. Lower permeability and better resistance to sulphate attack.  2. Lower  loss and porosity as a result of the lower water content.  3. bettered long term strength and  continuity performance.  4. The rate of bleeding is reduced while plasticity is increased.  5. Reduced water content for a given plasticity or  bettered plasticity  at the same water content.  Disadvantages of Fly Ash in Concrete Following are the  disadvantages of cover ash  1. It’s more  delicate to control the colour of concrete containing cover ash  than  fusions with Portland cement only.  2. Fly ash reduces the  quantum of air entrainment, and concrete  fusions  high in cover ash  frequently bear more air- enplaning   amalgamation.  3. Fly ash  cocktails can  outstretch the time it takes for concrete to set.  4. Concrete changes from a liquid to a solid a many hours after pouring, but  the curing process may take much longer.  

Que2.11. What are the  groups of cover ash? Also give the use  of cover ash concrete. 

Answer  Types of Fly Ash Following are the two types of cover ash  1. Class C Fly Ash  i. This class of cover ash has a high CaO content and used as a standalone  stabilizing agent.  ii. The strength characteristics of class C cover ash having a CaO  lower than 25  percent can be  bettered by adding lime.  2. Class F Fly Ash  i. This class of cover ash has a low CaO content.  ii. Class F fly ash has an  inadequate CaO content for the pozzolanic  response  to  do.  iii. It isn’t effective as a stabilizing agent by itself  still, when mixed  with either lime or lime and cement, the cover ash admixture becomes an  effective agent.  Uses of Fly Ash Concrete Fly ash concrete are used in  1. Pumped concrete.  2. Road stabilization.  3. Tunnelling concrete.  4. Self compacting concrete.  5. Water retaining structure.  6. Marine  terrain concretes.  7. Ready  blend and precast  operation.  8. Mass concrete section.  

Que2.12. What are the  goods of cover ash on  colorful  parcels of  concrete? 

Answer  goods of Fly Ash on Concrete Following are the  goods of cover ash  on concrete  1. On quantum of Mixing Water  i. The use of cover ash in limited  quantities as a  relief for cement or as  an addition to cement requires a little  further water for the same depression  because of fineness of the cover ash.  ii. It’s generally agreed that the use of cover ash, particularly as an  amalgamation  rather than as a  relief of cement, reduces,  isolation and  bleeding.  iii. If the beach is coarse the addition of cover ash produces  salutary results;  for fine beach, its addition may increase the water  demand for a  given plasticity.  2. On Compressive Strength An addition of cover ash up to 30 per cent  may affect in lower strength at 7 and 28 days, but may be about equal at  months and may further increase at  periods lesser than 3 months   handed curing is continued.  3. On Modulus of Pliantness It’s lower at early  periods and advanced at  latterly   periods.  4. On Curing Condition It’s  analogous to Portland cement concrete.  5. On loss of Concrete Coarse cover ash and those having high  carbon content are more liable to increase drying  loss than the  finer cover ashes and those having low carbon content.  6. On Permeability The permeability of concrete reduces on addition of  fly ash to cement.  7. On Resistance to Chemical Attack Fly ash slightly improves the  resistance of concrete to sulphate attack.  8. On Heat of Hydration Fly ash reduces the heat of hydration in  concrete. A negotiation of 30 cover ash may affect in a reduction of  50- 60 heat of hydration.  9. On Air Entrainment The presence of cover ash reduces the  quantum of  air enplaning  agent.  10. On Setting Time A 30 negotiation of cover ash may affect in an  increase of  original setting time over to 2 hours. 

Que2.13. What’s silica cloud? How is it produced? Give the  chemical composition of it. 

Answer  Silica Cloud  Silica cloud, also known as micro silica, is a derivate of the reduction of  high-  chastity quartz with coal in electric furnaces in the  product of  silicon and ferrosilicon  blends.  ii. Silica cloud is also collected as a derivate of the  product of other  silicon  blends  similar as ferrochromium, ferromanganese, ferro magnesium,  and calcium silicon.  Chemical Composition  1. It’s  substantially made of silica having silica percent  further than 80.  2. The other chemical composition includes Fe2O3, Al2O3, CaO, MgO, Na2O,  K2O in small  probabilities.  

Que2.14. What’s the physical characteristics and functions of  silica cloud? 

Answer  Physical Characteristics Following are the physical  parcels of  silica cloud  1. It should be in  decoration white and standard  slate colour.  2. The specific  graveness of the silica cloud concrete is2.2.  3. flyspeck size is  lower than 1 micron with average periphery of0.1 micron.  4. Its specific  face area is to be,000 m2/ kg.  5. The shape of the  flyspeck is  globular.  6. It should be in  unformed in nature.  Functions of Silica Cloud Following are the  colorful functions of  silica cloud  1. The hydration of Portland cement produces  numerous  factors, including  calcium silicate hydrates( CSH) and calcium hydroxide( CH).  2. The  fresh calcium silicate hydrates produced by the silica cloud is  more resistant to attack from aggressive chemicals  also the weaker  calcium hydroxide.  3. The silica cloud is added to the calcium hydroxide for produce the   fresh calcium silicate hydrates to  gain a  veritably good compressive  strength can exceed 15000 psi.  

Que2.15. Explain the  goods of silica cloud on concrete  parcels. 

Answer  Effect of Silica Cloud on the parcels of Fresh Concrete  1. Plasticity  Reduced plasticity.  ii. Water demand increases in proportion to silica cloud added. Water  demand is 1 for every 1  relief of cement.  iii. Lower depression and  further cohesive  blend.  2. Bleeding and isolation  Bleeding is reduced as silica cloud  patches find their way in between  two cement grains.  ii. isolation is reduced has the concrete  blend is more cohesive due to  increase in number of solid to solid contact points.  3. Time of Setting The  original setting time and final setting time is not  greatly  told . The increase may be 30 min or so.  4. Plastic loss Since silica cloud concrete show no bleeding, fresh  concrete is  subordinated to plastic  loss.  goods of Silica Cloud on the parcels of Hardened Concrete  1. Drying loss Long term  loss of concrete isn’t affected  significantly by the addition of silica cloud.  2. Creep The creep of concrete containing silica cloud will be lower than  corresponding Portland cement concrete.  3. Chemical Resistance A major reason for the  bettered resistance of  concrete to acidic and sulphate waters is the reduction in the Ca( OH) 2  content of the cement paste, which decreases linearly with the  quantum  of silica cloud added.  4. Alkali Aggregate response lower than 10 of silica cloud is  set up  acceptable for reducing the alkali  total expansion as compared to fly  ash which requires 30-40  relief.  5. Strength Strength of 62- 80 MPa can be  fluently achieved.  6. Permeability Silica cloud reduce the size of voids in doused  cement  paste,  therefore making them  nearly impermeable indeed at early  periods with  addition of silica cloud by weight of cement.  7. snap and thaw Effect The effect of silica cloud concrete on freezethaw  affect isn’t  veritably significant.  

Que2.16. What are the advantages, disadvantages and uses of silica cloud? 

Answer  Advantages of Silica Cloud Following are the advantages of silica  cloud  1. Lowers concrete permeability.  2. Significantly increases concrete  continuity.  3. Increases ultimate strength gain.  4. salutary in all types of high strength concrete  operations.  5. Improves bond strength to  sword.  6. Significantly reduces alkali- silica reactivity.  7. Provides excellent resistance to sulphate or seawater attack.  8. Reduces  sword  erosion.  9. Improves  snap/ thaw  continuity of concrete.  Disadvantages of Silica Cloud Following are the disadvantages of  silica cloud  1. Silica cloud concrete  loss rate is a large.  2. Silica cloud concrete plasticity is poor.  3. It’s easy to produce temperature cracks.  Uses of Silica Cloud  1. For  product of high strength concrete,  erosion- resistant concrete,  bruise- resistant concrete, and low permeability concrete.  2. Used to make seamster and manhole  form products. Reduces answer in  shotcrete  operation.

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