07-08-2012, 12:40 PM
SUPPLEMENTARY CEMENTITIOUS MATERIALS USED IN HIGH PERFORMANCE CONCRETE
SUPPLEMENTARY CEMENTITIOUS MATERIALS USED IN HIGH PERFORMANCE CONCRETE.DOC (Size: 85 KB / Downloads: 74)
ABSTRACT
Development and use of High Performance Concrete (HPC) is growing mainly due to the considerations of cost saving, energy saving, environmental protection and conservation of resources. In order to give these results to HPC, the addition of supplementary cementitious material is essential. This paper deals with the different types of supplementary cementitious materials used in HPC to achieve the desired characteristics.
1. INTRODUCTION
High Performance Concrete (HPC) is a concrete, which satisfies certain criteria to overcome limitations of conventional concrete. HPC is not a commodity, but a range of products, each specially designed to satisfy the performance requirements for the intended application in the most effective way. HPC should necessarily satisfy both strength and durability requirements. The high strength and durability of high performance concrete is obtained by the application of a reactive silica based complex admixture. This technique permits the use of the required amount of supplementary cementing material in the cement composition.This leads to achieve the desired characteristics of hardened concrete.
2. SUPPLEMENTARY CEMENTITIOUS MATERIAL
Supplementary cementing materials are mineral admixtures , which can be used individually in combination with portland cement to achieve the desired characteristics of hardened concrete.These materials act as a filler due to the fine particle size that enables their penetration between cement grains, this results in a reduction in water cement ratio to achieve a given workability. Modern high performance concrete contain atleast one supplementary cementing material like
1. Silica Fume
2. Fly ash
3. Ground Granulated Blast Furnace Slag
4. High Reactive Metakaolin
2.1 Silica Fume
Silica fume is a byproduct resulting from the reduction of high purity quarts with coal or coke with wood chips in an electric arc furnace during the production of silicon metal or ferrosilicon alloys.The silica fume content of concrete generally ranges from 5 to 10 percent of the total cementitious material content. The silica fume, which condenses from the gases escaping from the furnace, has a very high content of amorphous silicon dioxide and consists of very fine spherical particles (0.1*10-6m average diameter). The silicon dioxide content of silica fume varies depending on the type of alloy produced. Higher the silica contents of the alloy, the higher the silicon dioxide content of silica fume.(Shannag, 2000).
Alloy Type SiO2 content in silica fume (ACI 234R-96)
50% Ferrosilicon 61%-84%
75% Ferrosilicon 84%-91%
Silicon metal (98%) 87%-98%
The silica content of silica fume is as high as 86-96%. It is 100 times finer than cement and is highly pozzolanic. In itself silica fume it does not have any binding properties, but it react with Ca (OH) 2 or CH on hydration of cement and produces calcium- silicate-hydrate (C-S-H), which has good binding properties. The specific gravity of silica fume is about 2.2.The specific surface area (by nitrogen absorption test) of silica fume varies from 15000 to 25000m2/ kg.
Silica fume is available at present in four different forms. The characteristics of these forms are outlined below:
a) As-produced silica fume
1. Extremele fine powder
2. Available in bulk or in bags
3. Serious handling problem
4. Bulk density 130 to 430 kg/m3
b) Slurried silica fume
1. Water based slurry
2. 42 to 60% silica fume by mass
3. Easy transportation
4. Available with and without chemical admixtures
5. Bulk density 1320 to 1440 kg/m3.
c) Densified (compacted) silica fume.
1. Reduces the dust
2. Easy to handle
3. Agglomerates when compressed air is blown from bottom of silo storing silica fume.
4. Densification by mechanical compression
5. Available with or without chemical admixtures
6. Bulk density 480 to 640kg/m3
d) Palletized silica fume
1. Mixing with small amount of water
2. Not a reversible process
3. Used for manufacturing blended cements by grinding with Portland cement clinker