Silica fume is a by product in the reduction of high purity quartz with coke in electric arc furnace in the production of silicon and ferrosilicon alloys. Micro silicon is available in the following forms-Dry and Wet forms
1.3.2 PROPERTIES OF SILICA FUMES
Ø Undefined forms with bulk density of 200-300kg per m3.
Ø Densified forms with bulk density of 500-600kg per m3.
Ø Micro-pelletized forms with bulk density of 600-800kg per m3.
Ø Slurry forms with density 1400kg per m3.
Ø Slurry is produced by mixing undensified micro silica powder and water in equal proportions by weight. Slurry is the easiest and most practical way to introduce silica into the concrete mix.
Ø Surface area 15-20m2/kg.
Ø Standard grade slurry Ph value 4.7, specific gravity 1.3 to 1.4.
Ø Dry content of Micro silica 48 to 52%.
ADVANTAGES OF SILICA FUMES
Because of its extreme fineness and silica content, silica fume is very effective pozzolanic material. Standard specification for silica fume used in cementitious mixtures are ASTM C1240, EN 13263.
Silica fume is added to Portland cement concrete to improves properties, in particular its compressive strength, bond strength and abrasion resistance. These improvements stem from both the mechanical improvements resulting from addition of a very fine powder to the cement paste mix as well as from the pozzolanic reactions between the silica fume and the free calcium hydroxide in the paste.
Addition of silica fume is also reduces the permeability of concrete to chloride ions, which protects the reinforcing steel of concrete from corrosion, Especially in chloride –rich environments such as coastal regions and those of humid continental roadways and railways (because of the use of deicing salt) and salt water bridges.
Prior to the mid- 1970s, nearly all silica fume was discharged into the atmosphere. After environmental concerns necessitated the collection and landfilling of silica fume, it became economically viable to use silica fume in various applications, in particular high- performance concrete.
Silica fume significantly improves the properties of fresh and hardened concrete made with silica fume is more cohesive and therefore less phone to segregation than concrete without silica fume,
In addition to improved durability, the lack of bleeding allows a more efficient finishing process to be used with silica fume concrete flatwork.
Microsilica is much more reactive than fly ash or natural pozzolona. The reactively of a pozzolona can be quantified by measuring the amount of Ca(OH)2 in the cement paste at different times. In one case 15% of Micro silica reduced the Ca(OH)2 of two samples of cement from 24% to 12% at 90 days and from 25% to 11% in 180 days. Most research workers agree that the C-S-H formed by the reaction between Micro silica and Ca appears dense and amorphous.
Influence on fresh concrete
Water demand increase in proportion to the amount of microsilica added. The increase in demand of concrete in water demand of concrete containing microsilica will be about 1% for every 1% of cement substituted. Therefore, 20mm max size aggregate concrete, containing 10% microsilica, will have an increase water content of about 20 l/m3. Measure can be taken to avoid this increase by adjusting the aggregate grating and using superplasticizers.
The addition of micro silica will lead to lower but more slump but more cohesive mix. The micro silica make the fresh concrete sticky in nature and hard to handle. It was also found that there was huge reduction in bleeding and concrete with micro silica could be handled and transported without segregation.
It is reported that concrete containing micro silica is vulnerable to plastic shrinkage cracking and therefore sheet or mat curing should be considered. Micro silica concrete produces more heat of hydration at the initially stage of the hydration. However, the total generation of heat will be less than that of reference concrete.
The first national standard for use Silica Fume (“micro silica”) in concrete was adopted by AASHTO in 1990(AASHTO designation M307-90). The AASHTO and AASTM C1240 covers micro silica for use as a mineral admixture in PCC and mortar to fill voids and in cases which pozzolanic action is desired. It provides the physical and chemical requirements, specific acceptance tests, and package marking.
CHARACTERISTICS OF SILICA FUME
Proper introduction of silica fume in concrete improves both the mechanical and durability characteristics of the concrete. This paper presents the results of research efforts conduced at the American University in Cairo using Egyptian silica fume in
Concrete. The program investigated various characteristics of silica fumes concrete.
It emphasized the effect of silica fume on workability level and its maintenance of fresh concrete; the strength development, strength optimization and elastic modulus of hardened concrete; and chemical and mechanical durability of mortar. The experimental comprised six levels of silica-fume contents (as partial replacements of cement by weight) at 0% (control mix), 5%,10%,15%,20%, and 25% without plasticizer. It also included two mixes with 15% silica fume added to cement in normal concrete.
Durability of silica-fume mortar was tested in chemical environments of sulphate components, ammonium nitrate, calcium chloride, and various kinds of acids. It was found that there was on optimal value of silica- fume content at which concrete strength improved significantly. Due to the slow development of pozzolanic effect, there was a drop in early strength up to seven days and late significant gains up to 56days upon introducing silica fume in concrete.
Elastic modulus, toughness and steel-concrete bond increased at the optimum silica fume content in concrete. Silica fume mortar exhibited significant improvement in durability against chemicals attacks of most salts and acids. The improvements was moderated in the case sulphate compounds. Mechanical erosion resistance increased moderately in silica fume concrete.