What is silica fume?

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silica fume

Silica fume (also called microsilica), is made that during smelting silicon iron and industrial silicon, after the oxidation of silicon vapor discharged from the flue, the amorphous and powdered silicon dioxide (Sio2) is collected by specially designed dust collector. The average particle size of the silica fume is 0.1-0.15 μm, which is one of the several hundred points of the average particle size of the cement. The surface area of the surface is 15-27 m2/g, and has very strong surface activity.

Technical parameters of physical and chemical properties of silica fume:

1, micro silica fume powder appearance: gray white, refractoriness >1600℃. Bulk density: 200-250 kg/m3.

2, the fineness of silica fume: the fineness of the micro silicon powder is over 90% of the fineness of 0.1μm, and the average particle size is 0.1 – 0.2μm, and the specific surface area is 20-28 m2/g. The fineness and specific surface area are about 80-100 times of the cement and 50-70 times of the fly ash.

3. The morphology of silica fume particles and the structure of the mineral facies:

During the formation of micro silica fume, the amorphous phase is formed by the action of surface tension due to the effect of surface tension during the formation of phase change, and the surface is smoother. Some are aggregates with multiple spherical particles adhered together. It is a very high surface area and highly active volcanic ash. With micro silicon fume powder materials, small spheroids can be lubricated.

Silica fume properties: http://www.microsilica-fume.com/silica-fume-properties.html

Silica Fume Physicochemical index:

Value 75-95 % 0.60 % 0.51 % 0.7 % 2.69 % 0.9 % Neutral

Product application:

High strength concrete, unshaped refractory material, dry mixed mortar (premixed), high strength non shrinkage grouting material, wear-resistant industrial floor, oil well cementing slurry, mortar, polymer mortar, thermal insulation mortar, impermeable concrete, self leveling concrete, concrete, concrete preservatives, compacting agent of cement based polymer waterproofing agent, rubber; plastics, unsaturated polyester, paint, coatiing and other polymer material reinforcement, and modified ceramic products.

Working principle of micro silica fume in cement concrete

Because of the fine particle size and high activity of microsilica fume powder, the micro silica fume has pozzolanic reaction in concrete. The strength of silica gel formed by hydration of micro silica powder is higher than Ca (OH) 2 crystal, and it works together with cement hydration gel C-S-H.

Silica fume is mainly filled with granular void in concrete, which improves volume density, reduces porosity and reduces the cement additive amount. Finally, it plays a role of reducing water.

Silica fume use method and notes:

1. Applied to refractory material;
orthogonal design is carried out according to the different properties of refractory material, and the best formula is selected through comparison test. Undensified silica fume is usually used in refractories.

2. Applied to the mix design of high performance concrete:

1) compatibility: the compatibility of micro silicon fume powder with water reducing agent, fly ash, mineral powder, fiber and other is good.

2) slumps: due to the interstitial effect, concrete viscosity is higher, so the silica fume concrete slumps design requirement is higher 2 – 3 cm than the ordinary concrete;

3) the amount of silica fume added (added amount): making high strength and shotcrete, added microsilica amount is 6-12% of cement; for underwater engineering, its amount is higher; When the microsilica fume is used as pump aid, the mixing amount is reduced to 2%. The final mix ratio is determined by the mixing ratio test.
Micro silica fume and cement, aggregate are stirred at the same time, strictly according to standard construction, and maintenance must be strengthened at the same time.

3, feeding and mixing;

1) usually the concrete industry uses densified microsilica, so artificial feeding or automatic feeding can be selected.

2) due to the unique ultrafine characteristics of silica fume, so requires to immediate put input after aggregate feeding, and the powder silica fume should not be added to the ready mixed concrete.

3) the mixing time of the concrete with microsilica is longer 30 – 40s than the ordinary concrete, so as to obtain good uniformity.

4) silica fume concrete transport has no special requirement, and the same as ordinary concrete.

4, silica fume concrete pouring and curing;

1) the vibration of the micro silica fume concrete must be compacted, and it should not be leaked, lack vibration and no vibration. The general vibration time is 10 – 30s.

2) require continuous supply of concrete, to prevent cold noodles, Cold noodles time is no more than 100min;

3) it is very important to prevent the premature evaporation of water in the maintenance process of silica fume concrete. Therefore, the pouring is finished, the laminating is immediately covered, the sacks are laid and the water is cured.

The mixing amount of silica fume

It is generally 5-10% of the amount of cementitious material.

The micro silica fume must be used together with the water reducing agent. It is suggested that the fly ash and the grinding slag be added to improve its construction.

When the concrete is made with microsilica, the weight ratio of the cementitious material is generally as follows:

High performance concrete: 5-10%;

Hydraulic concrete: 5-10%;

Shotcrete: 5-10%;

Pump agent: 2-3%;

Wear-resistant industrial floor: 6-8%;

Polymer mortar, thermal insulation mortar: 10-15%,

Amorphous refractory castable: 6-8%.

Before use, please choose reasonable and economical dosage according to the actual needs.