Concrete mineral admixtures

Admixtures are added in concrete to improve the quality of concrete.
Fly ash (FA), silica fume (SF), ground granulated blast furnace slag (GGBS), Metakaolin (MK), and rice husk ash (RHA)
Possess certain characteristics through which they influence the properties of concrete differently.
Effect of mineral admixtures on the properties of fresh concrete is very important as these properties may affect the durability and mechanical properties of concrete.

1. MINERAL ADMIXTURES IN CONCRETE Prepared By, S.INDHU M.E .-STRUCTURAL ENGINEERING  

2. INTRODUCTION : Admixtures are added in concrete to improve the quality of concrete . Fly ash (FA), silica fume (SF), ground granulated blast furnace slag (GGBS), M etakaolin (MK), and rice husk ash (RHA ) Possess certain characteristics through which they influence the properties of concrete differently. Effect of mineral admixtures on the properties of fresh concrete is very important as these properties may affect the durability and mechanical properties of concrete .

3. POZZOLANIC MATERIALS: Siliceous or siliceous-aluminous materials P ossess little or no cementitious value. These are added to improve many qualities of concrete, such as: Lower the heat of hydration and thermal shrinkage; Increase the water tightness; Reduce the alkali-aggregate reaction; Improve resistance to attack by sulphate soils and sea water; Improve extensibility and workability; Lower susceptibility to dissolution and leaching; Lower costs.

4. PHYSICAL & CHEMICAL CHARACTERISTICS OF MINERAL ADMIXTURES: The physical and chemical properties of mineral admixtures and OPC are generally varied depending on the source from which these mineral admixtures are derived. T he variation in these properties seldom too large and comprehensive comparison is possible through these properties. The first property is the specific gravity. The mineral admixtures have lesser specific gravity than OPC. Therefore , more volume is expected when any one of these mineral admixtures replaces OPC by mass. Generally reduction in Fine aggregate contents is necessary to overcome the volume increase

5. POZZOLANIC MATERIALS CAN BE DIVIDED INTO TWO GROUPS: Natural Pozzolans Clay and Shales Opalinc Cherts Diatomaceous Earth Volcanic Tuffs and Pumicites . Artificial Pozzolans Fly ash Blast Furnace Slag Silica Fume Rice Husk ash Metakaoline Surkhi

6. NATURAL POZZOLANS: Natural pozzolans such as diatomaceous earth, clay and shale, pumicites , opaline cherts , etc., N eeds further grinding and sometimes needs calcining to activate them to show pozzolanic activities . The natural pozzolans have lost their popularity in view of the availability of more active artificial pozzolans .

7. ARTIFICIAL POZZOLANS: 1.FLY ASH: Finely divided residue resulting from the combustion of powdered coal and transported by the flue gases and collected by Electrostatic / Precipitator . M ost widely used pozzolanic material all over the world. FIG 1:Scanning electron microscope of Fly ash

8. ARTIFICIAL POZZOLANS: Effects of Fly Ash on Fresh Concrete: Reduction of water demand for desired slump. With the reduction of unit water content, bleeding and drying shrinkage will also be reduced. Effects of Fly Ash on Hardened Concrete: C ontributes to the strength of concrete due to its pozzolanic reactivity . C ontinued pozzolanic reactivity concrete develops greater strength at later age not at initial stage. Resulting in decrease of water permeability and gas permeability. Application: Many high-rise buildings Industrial structures Water front structures Concrete roads , Roller compacted concrete dams .  

9. ARTIFICIAL POZZOLANS: 2.SILICA FUME : It is a product resulting from reduction of high purity quartz with coal in an electric arc furnace in the manufacture of silicon or ferrosilicon alloy. Contains a t least 85% SiO 2 content with Mean particle size between 0.1 and 0.2 micron . Minimum specific surface area is 15,000 m 2 /kg . Particle shape is Spherical. Fig 2: Silica Fume

10. ARTIFICIAL POZZOLANS: Effect on fresh concrete: F resh concrete sticky in nature and hard to handle. Causes large reduction in bleeding and concrete with microsilica could be handled and transported without segregation to plastic shrinkage cracking. Effect on hardened concrete: Modulus of elasticity of microsilica concrete is less . It causes improvement in durability of concrete and Resistance against frost damage. Application: Conserve cement Produce ultra high strength concrete of the order of 70 to 120 Mpa . Increase early strength of fly concrete. Control alkali-aggregate reaction. Reduce sulfate attack & chloride associated corrosion.

11. ARTIFICIAL POZZOLANS: 3. RICE HUSK ASH: Rice husk ash is obtained by Burning rice husk in a controlled manner. Material of future as mineral additives. It is added to 10% by weight of cement. It greatly enhances the workability and impermeability of concrete . It contains Amorphous silica (90% SiO2) in very high proportion when burnt in controlled manner, 5% carbon, 2% K 2 O . Fig 3: Rice Husk Ash

12. ARTIFICIAL POZZOLANS: Effects: It reduces susceptible to acid attack and improves resistance to chloride penetration and reduces large pores and porosity resulting very low permeability and free lime present in the cement paste . It Decreases the permeability of the system and Improves overall resistance to CO 2  attack Improves capillary suction and accelerated chloride diffusivity . It also Enhances resistance to corrosion of steel in concrete. Reducing micro cracking and improving freeze-thaw resistance.

13. ARTIFICIAL POZZOLANS: 4. GROUND GRANULATED BLAST FURNACE SLAG: N onmetallic product consisting essentially of silicates and aluminates of calcium and other bases. The molten slag is rapidly chilled by quenching in water to form a glassy sand like granulated material. The granulated material when further ground to less than 45 micron will have specific surface of about 400 to 600 m2/ kg (Blaine).   Fig 4: Blast Furnace Slag

14. ARTIFICIAL POZZOLANS: Effects on fresh concrete: It r educes the unit water content necessary to obtain the same slump . Water used for mixing is not immediately lost, as the surface hydration of slag is slightly slower than that of cement. It reduction of bleeding. Effects on hardened concrete: It Reduced heat of hydration . Refinement of pore structures greatly r educed permeability to the external agencies and increased resistance to chemical attack.

15. ARTIFICIAL POZZOLANS: 5 . METAKAOLIN: Highly reactive metakaolin is made by water processing to remove unreactive impurities to make 100 % reactive pozzolan Such a product, white or cream in colour , purified, thermally activated is called High Reactive Metakaolin (HRM). Fig 6 : Metakaolin

16. ARTIFICIAL POZZOLANS: Effects of Metakaolin : High reactive metakaolin shows high pozzolanic reactivity and reduction in Ca (OH) 2 even as early as one day. The cement paste undergoes distinct densification. Densification includes an increase in strength and decrease in permeability . The high reactive metakaolin is having the potential to compete with silica fume.

17. ARTIFICIAL POZZOLANS: 6. SURKHI: It has been used along with Lime in many old Structures. It was one of the main Surkhi is an artificial pozzolana made by powdering bricks or burnt clay balls. Its characteristics are greatly influenced by constituent mineral composition of soil, degree of burning and fineness of grinding. Fig 7: Surkhi

18. ARTIFICIAL POZZOLANS: Effects: Surkhi makes cement mortars and concretes more water proof, more resistant to alkalies and to salt solutions than those in which no surkhi is used. A surkhi concrete of 25 to 50 mm slump is just as readily placed as a corresponding straight cement of much higher slump. Surkhi is added both in mortar and concrete. The addition of surkhi is accompanied by slight reduction in strength as surkhi attains its full strength only after one year. Surkhi concrete is subject to a slightly higher shrinkage than ordinary concrete.

19. CONCLUSION : Chemically active mineral admixtures increase the cohesiveness of concrete and require more water to maintain workability however , the requirement of water may be offset by adding plasticizer. Heat of hydration increases with the use of chemically active mineral admixtures and decreases with the use of microfiller mineral admixtures. Initial and Final setting time of concrete depends on water content, initial and curing temperature, dosage of superplasticizer , and the reactivity of mineral admixture. With the increase of mineral admixtures, all the important parameters of the fresh and hardened concrete are improved. Mineral admixtures reduce bleeding in concrete with correct proportion of all ingredients.

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