Tel:0086 15960276038
Fax:0086 592 3519825
Email:info@fairmain.com
Skype:fairmain
-
Microsilica Pozzolans
2017-3-3 10:49:28 Pozzolans not only strengthen and seal the concrete, they have many other beneficial features you will realize the moment you add them to the mix. microsilica (SF)
microsilica is a waste product of the silicon metal industry, and is a super-fine powder of almost pure amorphous silica. Though difficult (and expensive) to handle, transport and mix, it has become the chosen favorite for very high-strength concretes (such as for high rise buildings), often in combination with both cement and fly ash.
Silica fume is a by-product resulting from the production of silicon or ferrosilicon alloys or other silicon alloys. microsilica is light or dark gray in color, containing typically more than 90% of amorphous silicon dioxide. microsilica powder collected from waste gases and without any further treatment is generally called ³undensified microsilica², to distinguish it from other forms of silica fume.
Undensified silica fume consists of very fine vitreous spherical particles with an average diameter about 0.1µm, which is 100 times smaller than the average cement particle. The undensified microsilica is almost as fine as cigarette ash and the bulk density is only about 200 - 300 kg/m3. The relative density of typical microsilica particles is 2.2 to 2.5. Because the extreme fineness(!) and high silicon content, microsilica is generally a very effective pozzolan.
High-strength microsilica concretes of up to 300Mpa have been achieved in some countries. Applying microsilica in concrete fertilizer storage silos effectively reduced calcium nitrate attack. Condensed microsilica (CSF) has been used in repairing a dam stilling basin to improve abrasion erosion resistance; it has also been employed as an essential additive to prevent alkali-silica reaction.
Though condensed microsilica is much easier to handle and transport, uncondensed microsilica (normally in the form of a slurry) is more effective. The smaller, already wetted particles mix much easier and distribute better, hence reactivity is better, too!
Chemical composition of microsilica
Chemical composition of SF varies depending on the nature of the the manufacture process from which the SF is collected. The main constituent material in SF is silica (SiO2), the content of which is normally over 90%. The following table lists a chemical analysis of a commercially available microsilica.Commercially Available
microsilicaSubstance
Percentage %
SiO2
92.85
A12O3
.61
Fe2O3
.94
CaO
.39
MgO
1.58
K2O
.87
Na2O
.50
Properties of (problems with) fresh concrete with microsilica:
Workability, water demand: Use of microsilica in concrete usually increases water demand. The increased water demand causes an increase in water to cement ratio and could negate the benefits of adding silica fume. For this reason, microsilica concrete (SFC) normally incorporates a water reducing agent or superplasticiser.
Stability: SFC is more cohesive than conventional concrete. This is true for SFCs both with and without superplasticiser. Increased cohesiveness reduces the likelihood of bleeding and segregation. This increased cohesiveness could however increase the required compaction energy.
Plastic shrinkage: Increased cohesiveness of SFC encourages the potentiality of plastic shrinkage and cracking that appears when the bleeding water cannot compensate for the water loss on the surface, due to evaporation. Under conditions of fast evaporation, curing measures should be taken immediately after placing the concrete.
It should be noted that to overcome the above shortcomings, sometimes FA and/ or RHA are also added to the concrete, together with SF.
Properties of hardened concrete with SF
Combining SF with the appropriate aggregates and water-reducing agent can produce high-strength concrete with a cube compressive strength of around 100Mpa, in extreme cases up to 300Mpa.
The impermeability of SFC is higher than that of similar concrete without SF.
Tests have proven that one part of microsilica can replace up to 3-4 parts of cement without any loss of strength. Replacing 10% by weight of cement with SF is a good starting point for experiments.
Unfortunately, some types of SF cannot be used in concrete. The combination of Si and FeSi-75% condensed microsilica has proven to work effectively, while mixtures of FeSi-75% with FeSi-50% and FeSi-75% with CaSi have proven to be ineffective.
Physical CharacteristicsThe microsilica particle consists mainly of ³vitreous² silica particles. It has a specific gravity of about 2.20, which happens to be the accepted value for the specific gravity of any vitreous silica. Nevertheless, it has been proven that the higher the amount of impurities in microsilica, the higher the specific value. Certain impurities such as iron, magnesium, and calcium (note: but not CaSi) have shown to increase this value.
"Condensed microsilica in Concrete", Malhotra, Ramachandra, Feldman, Aitcin, p. 9-10
As stated before, microsilica was first looked at as a replacement for cement, but today only a portion of the cement is replaced with a much smaller amount of condensed microsilica in the concrete mix. Besides that, microsilica - just like any other pozzolan - is not an inert filler, but plays an active role in the performance of the concrete.
-
Microsilica Silica Fume in Oil Well Grouting
2017-3-3 10:23:09 Oil Well Grouting
Whether used for primary (placement of grout as a hydraulic seal in the wellbore) or secondary applications (remedial operations including leak repairs, splits, closing of depleted zones), the addition of microsilica enables a well to achieve full production potential. Besides producing a blocking effect in the oil well grout that prevents gas migration, it provides these advantages:
Improved flow, for easier, more effective application
Dramatically decrease permeability, for better control of gas leakage
Increased durability and compressive strength
Lightweight -
Microsilica in Refactory and Ceramics
2017-3-3 10:22:02 Refactory and Ceramics
The use of microsilica in refractory castables provides better particle packing. It allows for less water to be used while maintaining the same flow characteristics. It also promotes low temperature sintering and the formation of mullite in the matrix of the castable. This produces a castable that has a low permeability to avoid gas, slag and metal penetration. Castables incorporating microsilica are stronger than non-microsilica containing castables especially at high temperatures with higher density they attain lower porosity and are more volume stable.