SILICA FUME AS AN ADMIXTURE IN CONCRETE 1. INTRODUCTIONThe use of pozzolanic materials is an old as that of the art of concrete construction. it was recognized long time age, that the suitable pozzolans used in appropriate amount, modify certain properties of fresh and hardened mortar and concretes. It has been amply demonstrated that the best pozzolans in optimum proportions mixed with portland cement improves many qualities of concrete, such as: a) Lower the heat of hydration and thermal shrinkage ;b)

Increase the water tightness;

c) Reduce the alkali- aggregate reaction; d) Improve resistance to attack by sulphate soils and sea water; e) Improve extensibility;f)

Lower susceptibility to dissolution and leaching;

g) Improve workability; h) Lower costs. Pozzolanic materials are siliceous or siliceous and aluminous materials, which in themselves possess little or no cementitious value, but will, be in finely divided form and in the presence of moisture, chemically react with calcium hydroxide liberated on hydration, at ordinary temperature, to form compounds, possessing cementitious properties.

1.1 What is Silica fume..Silica fume is a by-product from electric arc furnaces used in the manufacture of silicon metal of silicon alloys. The material which contains more than 80% silica in no crystalline state and in the form of extremely fine particles (0.1 m average diameters), is highly pozzolanic. This product is excellent for use as a Portland cement supplement in addition to economic and energy saving potential, the use of pozzolanic admixtures in concrete leads to several technical advantages, such as reduction in thermal cracking caused by heat of cement hydration, improved durability to attack by sulphate and acidic waters, and high ultimate strength. Unlike other by- product pozzolans such as fly ashes, a unique feature of cementatious silica fume (CSF) is that it has a better faster pozzolonic1

action.

2. How silica fume works.In cementitious compounds, silica fume works on two levels, the first one described here is a chemical reaction called the "pozzolanic" reaction. The hydration (mixing with water) of Portland cement produces many compounds, including calcium silicate hydrates (CSH) and calcium hydroxide (CH). The CSH gel is known to be the source of strength in concrete. When silica fume is added to fresh concrete it chemically reacts with the CH to produces additional CSH. The benefit of this reaction is two fold; increased compressive strength and chemical resistance. The bond between the concrete paste and the coarse aggregate, in the crucial interfacial zone, is greatly increased, resulting in compressive strengths that can exceed 100 Mpa. The additional CSH produced by silica fume is more resistant to attack from aggressive chemicals then the weaker CH.

3.Characteristics of Cementatious Silica Fume(CSF)3.1 Chemical composition of silica fume Chemical composition of silica fume will mostly depend upon compositon of the principle product being made by furnace.additionally,the composition is also influnced by the furnace design; generally a furnace with a heat recovery system producessilica fume with lower ignition loss.the SiO2 content of silica fume varies with the silica content of alloy being producer.unlike other by product pozzoloans such as fly ash, silica fume from a single source is little or no varition inchemical composition from one day to another, Item Sio2 mgo SO3 H2O K2O Na2O CaO Si Cl Fe2O3 3.2 Physical characteristics: Requirement >90%