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In our project, the demanded high strength structure necessitates the usage of durable concrete. In order to attain durability, concrete is mixed with microsilica. The later particles is very smaller in size than those of concrete, thus allowing it to react with the cement particles to strengthen the bond within each other, and thus prohibiting the passage of water particles through the voids that may generate in concrete. The induced strength in concrete provide additional ability of it to resist sulfate attacks subsequently after the voids are filled with microsilica.

Therefore we found that microsilica is an eligible subject to be discussed in the following section.

Microsilica is a mineral admixture composed of very fine solid glassy spheres of silicon

dioxide (SiO2). Most microsilica particles are less than 1 micron (0.00004 inch) in diameter, generally 50 to 100 times finer than average cement or fly ash particles.

Frequently called condensed silica fume, microsilica is a by- product of the industrial manufacture of ferrosilicon and metallic silicon in high-temperature electric arc furnaces. The ferrosilicon or silicon product is drawn off as a liquid from the bottom of the furnace. Vapor rising from the 2000- degree-C furnace bed is oxidized, and as it cools condenses into particles which are trapped in huge cloth bags. Processing the condensed fume to remove impurities and control particle size yields microsilica.

A- Work of microsilica in concrete

Microsilica in concrete contributes to strength and durability two ways:

-As a pozzolan, microsilica provides a more uniform distribution and a greater volume of hydration products.

-As a filler, microsilica decreases the average size of pores in the cement paste.

Microsilca’s effectiveness as a pozzolan and a filler depends largely on its composition and particle siz which in turn depend on the design of the furnace and the composition of the raw materials with which the furnace is charged. At present there are no U.S. standard specifications for the material or its applications.

Dosages of microsilica used in concrete have typically been in the range of 5 to 20 percent by weight of cement, but percentages as high as 40 have been reported.

Used as an admixture, microsilica can improve the properties of both fresh and hardened concrete. Used as a partial replacement for cement, microsilica can substitute for energy-consuming cement without sacrifice of quality.

A.1-Pozzolanic action

Addition of microsilica to a concrete mix alters the cement paste structure. The resulting paste contains more of the strong calcium-silicate hydrates and less of the weak and easily soluble calcium hydroxides than do ordinary cement pastes. Because the microsilica particles are so small—their average diameter is about 1⁄100 that of cement particles—they disperse among and separate the cement particles. The resulting fine, uniform matrix can give markedly higher compressive, flexural, and bond strength. Compressive strengths as high as 15,000 psi with ordinary aggregates and 30,000 psi or more with special aggregates have been reported.

Relation ship between strength and water-cement ratio for

two microsilica concretes and a reference concrete. Curves are similar in shape, but microsilica concretes reach significantly higher levels, up to nearly 14,000 psi .

A.2-Freeze-thaw durability

The small microsilica particles are very good at infiltrating and plugging capillary pores in

concrete— making pores smaller and fewer and concrete more dense. This gives the concrete good resistance to freezing and thawing. Air entrainment improves the resistance of microsilica

concrete in the same way it does ordinary concrete. However, microsilica concrete even with relatively low cement content can reportedly be compounded to be frost resistant without air- entraining agents.

Comparison of compressive strengths of a proprietary microsilica concrete and a low-slump dense concrete without microsilica, both compounded for bridge deck overlays. Early strength of the microsilica concrete is lower. But after two days, values are about equal. After 28 days, microsilica concrete is about 40 percent stronger and after 56 days, 50 percent stronger.

A.3-Protection of reinforcement

Concrete’s ability to protect embedded steel against corrosion depends mainly on the alkalinity of the pore water. As long as the water is highly alkaline, a passive oxide film on the steel protects it. If the passivity is destroyed by aggressive ions, either carbonates or chloride ions, the steel will corrode at a rate depending on the concrete’s electrical resistivity and rate of oxygen transport through water- saturated concrete.

Fortunately, microsilica thanks to its pore-filling capabilities reduces (in some if not all cases) the rate of carbonation, decreases permeability to chloride ions, imparts high electrical resistivity, and has little effect on oxygen transport. Therefore, microsilica concrete can be expected to be strongly protective of reinforcement and embedments.

A.4-Sulfate resistance, reduced aggregate reactivity

Probably because it has a finer pore structure and less calcium hydroxide, microsilica concrete has improved resistance to sulfate attack . In addition, microsilica binds the potassium and sodium oxide alkalies present in cement, thus reducing detrimental effects with alkali-reactive aggregates.

A.5-Aids strength gain of fly ash concretes

Preliminary indications suggest that microsilica may be useful in controlling heat generation in mass concrete. It has also been found useful in combination with fly ash. Early-age strength

development of concrete in which fly ash replaces cement tends to be slow because fly ash is relatively inert during this period of hydration. Adding microsilica, which is more reactive in early hydration, can speed the strength development.

B-Mixing and placing considerations

B.1-Handling the microsilica

Because of its extreme fineness, microsilica presents handling problems. A cement tanker that could ordinarily haul 35 metric tons of cement accommodates only 7 to 9 tons of dry microsilica and requires 20 to 50 percent more time for discharging. Some producers mix microsilica with water on a pound-for-pound basis to form a slurry that is transportable in tank trailers designed to handle liquids. The water of the slurry replaces part of that ordinarily added to the mix.

One supplier prepares a slurry which, used at the rate of 1 gallon per 100 pounds of cement, will provide about 5 percent microsilica by weight of cement. In 1984, that supplier was quoting a price of $1.70 per gallon at a plant in West Virginia. In Canada, patented methods have been used to densify the microsilica for shipment to ready mix producers. Some concrete producers also use the loose microsilica just as it is collected.

B.2-Water requirements of the mix

When no water reducing agent is used, the addition of microsilica to a concrete mix calls for more water to maintain a given slump. Water content can be held the same by using a water reducer or superplasticizer along with the microsilica. Water reducing agents appear to have a greater effect on microsilica concrete than on normal concrete. Thus water demand for a given microsilica concrete can be controlled to be either greater or smaller than for the reference concrete.

B.3-Placing and finishing, curing

The gel that forms during the first minutes of mixing microsilica concrete takes up water and stiffens the mixture, necessitating adjustment of the timing of charging and placing. Scandinavian researchers have concluded that microsilica concretes often require 1 to 2 inches more slump than conventional concrete for equal workability. When cement content and microsilica dosage are relatively high, the mixture is so cohesive that there is virtually no segregation of aggregates and little bleeding. This may cause problems for floors or slabs cast in hot, windy weather because there is no water film at the surface to compensate for evaporation.

Plastic shrinkage cracking can readily develop unless precautions are taken. It is important to finish the concrete promptly and apply a curing compound or cover immediately. With lean concrete mixes or mixes containing fly ash replacement of cement, different effects

have been reported. For example, Reference 4 reports that mixes with less than 380 pounds of cement per cubic yard plus 10 percent microsilica are both more cohesive and more plastic so no extra water is needed to maintain slump.

B.4-Concrete color effects

Freshly mixed concrete containing microsilica can be almost black, dark gray, or practically unchanged, depending on the dosage of microsilica and its carbon content. The more carbon and iron in the admixture, the darker the resulting concrete. Hardened concretes are not much darker than normal concretes when dry. Sometimes there is a faint bluish tinge, but when the microsilica concrete is wet, it looks darker than normal.