Concrete is most widely used material for various applications in this world. India is second largest producer of concrete in world. As this is used in various applications its base properties which are obtained by preparing concrete with required raw materials must be enhanced by adding some other material to it. There comes the role of admixtures by which one can extract required property from concrete. There are various two basic classifications of admixtures. Chemical and Mineral admixture. There are various types of mineral admixtures. Most of mineral admixtures are by-products available from some production industry. Silica fume is a mineral admixture which is obtained ...view middle of the document...
2 to 2.3
Specific Surface area – 15000 – 30000 m2/kg (BET test)
Colour varies from light to dark grey depending on percentage of nonsilica content in it.
It is also called as microsilica (MS), condensed silica and fumed silica.
It is synthetically produced ultrafine siliceous material containing amorphous silica. It is 1000 times smaller than cement particle and 10 times smaller than silica fume.
Effect of addition of NS/MS on properties of cement mortars and concrete
It increases by increasing the content of NS. As these particles are very fine particles with higher surface area demands more water than normal cement pastes . According to Berra addition of NS to cement paste decreases the availability of free water which is the effect of denser particle packing and results in more demand of water to attain required workability .
Water demand increases by increasing the content of MS in concrete.
Workability decreases due to increase in cohesion . In order to improve workability superplasticizers are added to cement paste. According to Berra delayed additions of water aliquots showed the gain in workability in cement mixes having NS 
Workability decreases by adding MS and concrete becomes sticky. One has to increase initial slump of MS incorporated concrete by 50mm in order to attain similar workability of normal concrete.
It increases by addition of NS to cement mortar but the effect is not that much significant .
It increases by addition of NS to cement mortar. At constant water content and with increasing amounts of NS results in denser packing in cement paste which decreases the availability of free water and results in increase of yield stress .
Both initial and final setting time decreases due to addition of NS which is due to accelerated hydration. Dormant period length was also decreased due to acceleration of hydration. 
There is no significant effect on setting times with MS which may be due to high range water reducers that are used.
Bleeding and Plastic Shrinkage
Reduced bleeding and high plastic shrinkage is observed in concretes having NS or MS in order to counteract this one should provide proper curing.
As there is accelerated hydration, the heat evolved was more when compared to cement paste without NS. 
Heat evolved is more when we add MS to cement mix similar to NS but total heat evolved is less when compared with NS + cement paste.
Air entrainment admixture
As these are very fine particles the amount of air entrainment admixture required is increased when compared to normal concretes. Studies showed that increase in dosage will be around 125% than that required for normal concretes.
Apparent density reduces by incorporating NS to cement paste which may be due to air entrainment effect and by replacing more denser...