Question

7) LIST AND EXPLAIN THE USE, ADVANTAGES and DISADVANTAGES OF:

a) Type "F" Fly-Ash

b) Slag Cement

c) Type "C" Fly-Ash

d) Silica Fume

Answer 1

TYPE F FLY ASH

Class F fly ash is designated in ASTM C 618 and originates from anthracite and bituminous coals. It consists mainly of alumina and silica and has a higher LOI than Class C fly ash. Class F fly ash also has a lower calcium content than Class C fly ash.

USES

When used in portland cement, Class F fly ash can be used as a portland cement replacement ranging from 20-30% of the mass of cementitious material.

ADVANTAGES

When used as a portland cement replacement, Class F fly ash offers the following advantages when compared to unmodified portland cement:

• Increased late compressive strengths (after 28 days)
• Increased resistance to alkali silica reaction (ASR)
• Increased resistance to sulfate attack
• Less heat generation during hydration
• Increased pore refinement
• Decreased permeability
• Decreased water demand
• Increased workability
• Decreased cost ($80/ton for portland cement vs. $30/ton for fly ash).

DISADVANTAGES

When using Class F fly ash as a portland cement replacement, it is important to know several precautions. The time of set may be slightly delayed, and the early compressive strengths (before 28 days) may be decreased slightly. Also, the fine aggregate fraction of the concrete will need to be modified because fly ash has a lower bulk specific gravity than does portland cement, and therefore occupies a greater volume for an equal mass. If using any organic admixtures such as air entrainment, the amount added must be modified since the carbon in the fly ash adsorbs organic compounds. Finally, if the fly ash has a high calcium content, it should not be used in hydraulic applications. When using this or any other alternative cementing material with portland cement, it is necessary to create trial mixtures to ensure proper proportioning for the desired properties.

SLAG CEMENT

Slag cement is a hydraulic cement formed when granulated blast furnace slag (GGBFS) is ground to suitable fineness and is used to replace a portion of portland cement. It is a recovered industrial by-product of an iron blast furnace. Molten slag diverted from the iron blast furnace is rapidly chilled, producing glassy granules that yield desired reactive cementitious characteristics when ground into cement fineness.

USES

Slag cement is commonly found in ready-mixed concrete, precast concrete, masonry, soil cement and high temperature resistant building products.The use of slag cement has demonstrated long-term performance enhancements allowing designers to reduce the environmental footprint of concrete while ensuring improved performance and increased durability. There are many different applications and uses of slag cement including mass concrete, flatwork, concrete paving, soil stabilization.

ADVANTAGES

• Improved workability
• Easier placeability and finishabiltiy
• Higher long-term compressive and flexural strengths
• Reduced permeability
• ASR mitigation properties
• Improved durability and resilience
• More consistent performance
• Lighter color

DISADVANTAGES

• Strength gain is slow
• ·In cold weather condition the low heat of hydration of slag cement coupled with moderately low rate of strength development, can lead to frost damage.

TYPE C FLY ASH

Class C fly ash is designated in ASTM C 618 and originates from subbituminous and lignite coals. Its composition consists mainly of calcium, alumina, and silica with a lower loss on ignition (LOI) than Class F fly ash.

USES

When used in portland cement, Class C fly ash can be used as a portland cement replacement ranging from 20-35% of the mass of cementitious material.

ADVANTAGES

When used as a portland cement replacement, Class C fly ash offers the following advantages when compared to unmodified portland cement:

• Increased early and late compressive strengths
• Increased resistance to alkali silica reaction (ASR) when >15% is added
• Less heat generation during hydration
• Increased pore refinement
• Decreased permeability
• Decreased water demand
• Increased workability
• Decreased cost ($80/ton for portland cement vs. $30/ton for fly ash)

DISADVANTAGES

When using Class C fly ash as a portland cement replacement, it is important to know several precautions. The time of set may be slightly delayed. Also, the fine aggregate fraction of the concrete will need to be modified because fly ash has a lower bulk specific gravity than does portland cement and therefore occupies more volume for the same mass. Class C fly ash must replace at least 25% of the portland cement to mitigate the effects of alkali silica reaction. If using any organic admixtures such as air entrainment, the amount added must be modified since the carbon (LOI) in the fly ash adsorbs organic compounds. Finally, if the fly ash has a high calcium content, it should not be used in sulfate exposure applications. When using this or any other alternative cementing material with portland cement, it is necessary to create trial mixtures to ensure proper proportioning for the desired properties.

SILICA FUME

Silica fume is a property enhancing material.Silica fume is a byproduct of producing silicon metal or ferrosilicon alloys. One of the most beneficial uses for silica fume is in concrete. Because of its chemical and physical properties, it is a very reactive pozzolan. Concrete containing silica fume can have very high strength and can be very durable.

USES

• Silica Fume is largely used for producing high strength and high-performance concrete.
• Silica Fume is broadly used for high strength structures.
• It is also used for high rise structures.
• Condensed silica fume is mainly used to produce concrete with higher resistance to chloride penetration for applications such as parking structures, bridges, and bridge decks.
• It also used for building decks, floors, overlays and marine structures etc.
• Micro silica is also used for shotcrete in rock stabilization; mine tunnel linings; and rehabilitation of deteriorating bridge and marine columns and piles.

ADVANTAGES

1.Very low chloride ion diffusion

2.Increased compressive strength

3.Reduced water permeability

4.Improved sulfate resistance

5.Improved abrasion resistance

6.Improved resistance to chemical attack

7.Improved stability in geothermal environments

8.Reduced efflorescence

DISADVANTAGES

• Silica fume concrete has a large shrinkage rate. In case the external temperature is high, it will lead to early dry shrinkage. Hence, it is more prone to cracks which may ultimately affect the overall strength.
• The construction with the silica fume concrete is difficult; as workability of silica fume concrete is poor. Therefore, it is difficult to place and compact. Moreover, it influences the smoothness of concrete and surface uniformity.
• Condensed silica fume has a large surface area. Hence the amount of water required in the concrete mix increases which makes it necessary to use a plasticizer or super-plasticizer along with it.
• Cost factor is also one of the disadvantages while using silica fume in cement.

Due to the filling effect of silica fume, the moisture in concrete decreases and local dry shrinkage cracks are caused.Therefore, strengthening concrete maintenance after construction can effectively alleviate the problem.

If the external temperature is too high, it will cause the dry shrinkage of concrete surface. Before concrete condensation, if the surface water evaporation rate is higher than the bleeding rate, the dry shrinkage crack will appear on the surface.

The excessive amount of silica powder will reduce the constructability, make the concrete become compacted, and make the surface of construction difficult.