Monday, July 9, 2012

Silica Fume in Concrete

Silica fume is not a new product by one company but is a major raw material that has been available over 40 years. Silica fume has become the world standard for chloride corrosion situations and also to improve dramatically the water tightness of concrete structures. Waterproofers are viewed suspiciously by the international research industry because of their side affects on strength and porosity.

Below we highlight the positive effects of silica fume and superplasticizer on pore size of cement paste, and porosity, water permeability and water absoption of concrete.

Silica fume in concrete and concrete porosity

Silica fume has an average particle size of less the one micron (100 times finer than cement). In concrete these fine particles fill the gaps between cement grans and react with free lime released during cement hydration. This produces strong calcium silicate hydrates to replace weak lime and voids found in normal concrete.

The effect on the water penetrability is related to the pores size. It has has been shown that water does not penetrate pores less than 6– Angstroms nominal diameter. Silica fume modifies concrete’s pore structure such that few pores are larger than 600 Angstroms.

Because of the fine pore structure water permeability is reduce by 100 times in laboratory test. In reality on site 20 times is more likely.

The cause of porosity is rarely to be found in the aggregate, it is on the contrary a property of the cement paste.

The porosity can be reduced by:

good compaction
use of admixtures – superplasticizer
precise grading of the aggregates
filling og all voids with admixture (silicafume) that react with the cement (dense packing)
Dense packing depends theoretically only on the shape and size distribution of the individual particles. A packing of spheres of the sme size has 48% voids with cubic packing and 26% voids with hexagonal packing independently of their size. But the packing density depends strictly on th particle shape alone, broken particles cannot be so well compacted as round ones.

The Silica fume technology is based on the following knowledge:

the use of chosen cements
filling the voids between the cement particles with micro-silica (silica fume)
reduce the water content as far as possible with superplasticizers
use of best grading sand and gravel mix
use of good quality aggregates
supervision of concrete compaction
good curing
The addition of silica fume with superplasticizer significantly reduces the total porosity of the cement paste.

Water absorption of concrete – effect of silica fume

In concrete constantly immersed in water the pores will reach supersaturation within a few months. After that time absorption becomes immaterial. A waterproofer with reduce the depth to which water is absorbed into the concrete but more significance is increased porosity. Tests have shown that waterproofers may reduce absorption but increae porosity.

Silica fume reduce absorption of concrete by reducing the pore size.  Waterproofers increase pore size (because of interference with cement hydration), however improved mix design and supervision may lead to improvement of concrete.


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