Micro Silica

Mixing and Placing Considerations

Handling Micro Silica

Owing to its extreme fineness, micro silica presents handling problems. A cement tanker that could ordinarily haul 35 metric tons of cement accommodates only 7 to 9 tons of dry micro silica and requires 20 to 50% more time for discharging.

Water Requirement of the Mix

When no water reducing agents are used, the addition of micro silica to a concrete mix requires more water to maintain a given slump. In order to avoid using excess water, a water reducer or super plasticizer is used along with the micro silica.

Placing, Finishing, and Curing

The gel that forms during the first few minutes of mixing micro silica stiffens the mixture. It is necessary to finish the concrete and apply a curing compound or cover instantly. Various effects have been reported differently with lean concrete mixes or mixes having fly ash replacement for cement.

Properties of Micro Silica

• The diameter of a silica fume particle ranges from 100 to 200nm.

• Its particles are spherical in shape.

• The density of condensed silica fume required to be used as an additive in concrete must be about 500 to 700 kg/m3.

• It contains 85 to 97% of silicon dioxide.

• The other constituents present in micro silica are carbon, sulphur along with the oxides of aluminium, iron, calcium, magnesium, sodium and potassium.

• Replacing the cement with about 10% silica fume subsequently results in the increase of compressive strength of the concrete.

• Reinforcement corrosion is greatly reduced due to increased resistivity, high tensile strength and less permeability.

Effects of Silica Fume on Concrete

01. Pozzolanic Effect

According to ‘Verma Ajay et al.’ (Published in Research Journal of Engineering Sciences), when water is added to ordinary Portland cement, hydration of concrete takes place resulting in the formation of the following products:

OPC + H2O  →  CSH + Ca (OH)2

In the presence of silica fume, the silicon dioxide reacts with the calcium hydroxide to form more aggregate binding CHS (Calcium silicate hydrate) which is as follows:

Ca (OH)2 + SiO2 + H2O  →  CSH

The reaction diminishes the amount of calcium hydroxide in the concrete.  This is helpful as a weaker calcium hydroxide does not impart any strength to the concrete. When it is combined with carbon dioxide, it forms a soluble salt which will leach through the concrete causing efflorescence, a familiar architectural problem. When high amount of calcium hydroxide is added to the concrete, the concrete becomes more vulnerable to sulphate attack, chemical attack as well as adverse alkali-aggregate reactions.

02. Micro Filler Effect

According to ‘Md Athar Kazmi et al.’ (Published in International Journal of Science and Research (IJSR)), micro silica is a finer material, approximately 100 times finer than cement in terms of average diameter. By adding nearly 8% of cement by weight, approximately 100,000 particles for each particle of cement will fill up the water spaces in fresh concrete. This eliminates bleeding and weak transition zone between aggregate and concrete paste. This micro filler effect greatly reduces permeability and improves paste-to aggregate bond in silica fume concrete compared to conventional concrete. The reaction of silica fumes is rapid, providing high strength and durability at an early stage. In terms of efficiency of silica fume, it is 3-5 times more than that of OPC and simultaneously performance of concrete can also be drastically improved.