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Supplementary cementing materials for green concrete – A view ABSTRACT The objective of this paper is to discuss various supplementary materials used in the production of concrete. Use of these materials is compulsory in today’s RMC and high performance concrete. Proper concrete mix design is compulsory for ensuring adequate strength, workability and other properties. Introduction Use of high strength and high performance concrete is increasing day by day in our country due to many inherent advantages. Today we are seeing such concretes in many applications in mega cities including Mysore. Such high strength and high-performance concrete mixtures use one or more supplementary cementitious materials generally called as SCMs to enhance workability, strength, durability, service life, and other performance characteristics. While there are a number of potential SCMs that have been used in concrete (including metakaolin and rice husk ash), the three most common are fly ash, silica fume, and ground granulated blast furnace slag. These materials react chemically with calcium hydroxide released from the hydration of portland cement to form cement compounds. These materials are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties. SEMs and their Properties The process that produces these materials influences their properties, so a basic introduction to each is given below. i) Fly ash. The most commonly used pozzolan in concrete, is a finely divided residue that results from the combustion of pulverized coal and is carried from the combustion chamber of the furnace by exhaust gases. Commercially available fly ash is a by-product of thermal power generating stations (RTPC, Raichur, Karnataka). Fly ash contains some heavy metal (normally more than silica fume), so the heavy metal content of the concrete will increase. Fly ash refines the pore structure of the concrete, making it more resistant to chloride penetration. Not all fly ash is suitable for use in concrete. Fly ash shall conform to the requirements of Indian standard and shall be specified in accordance with BIS requirements. Normally 10% to 20% of the cement can be replaced with fly ash to reduce the environmental burden of the concrete. Substitution with fly ash at levels exceeding 25% is considered to be a high volume SCM application. Appropriate testing to ensure desired performance should be carried out. The use of fly ash can increase setting times. This may be an economic factor in precast concrete manufacturing if casting cannot be maintained on a daily cycle. Although fly ash offers environmental advantages, it also improves the performance and quality of concrete. Fly ash affects the plastic properties of concrete by improving workability, reducing water demand, reducing segregation and bleeding, and lowering heat of hydration. Fly ash increases strength, reduces permeability, reduces corrosion of reinforcing steel, increases sulphate resistance, and reduces alkali-aggregate reaction. Fly ash reaches its maximum strength more slowly than concrete made with only Portland cement. The techniques for working with this type of concrete are standard for the industry and will not impact the budget of a job. ii) Blast-furnace slag. Slag or iron blast-furnace slag, is a non-metallic product consisting essentially of silicates, aluminosilicates of calcium, and other compounds that are developed in a molten condition simultaneously with the iron in the blast-furnace. Substitution of slag at levels exceeding 35% for the Portland cement in precast concrete is considered a high volume SCM application, and its suitability for intended use must be prequalified. The addition of slag cement usually results in reduced need for water, faster setting time, improved pumpability and finishability, higher 28-day strength, lower permeability, resistance to sulfate attack and alkali-silica reactivity (ASR), and lighter color. Blast-furnace slag shall conform to the requirements of BIS. iii) Silica fume. Silica fume or fumed silica or silica dust, also called condensed silica fume and microsilica, is a finely divided residue resulting from the production of elemental silicon or ferro-silicon alloys that is carried from the furnace by the exhaust gases. Silica fume, with or without fly ash or slag, is often used to make high-strength concrete. Silica fume improves the quality, strength and durability of concrete by making the concrete much less permeable and more resistant to corrosion of the steel reinforcement. Silica fume shall conform to the requirements of BIS. Advantages Supplementary Cementing Materials can significantly reduce the embodied energy of concrete and its products significantly. Their judicious use in concrete production is desirable both for environmental and energy conservation as well as for the technical benefits they can provide. SCMs are added to concrete as part of the total cementitious system, either as an addition or partial replacement of Portland cement. When properly used, the SCMs can enhance the following properties of concrete: Generally improve the workability and finishing of fresh concrete Reduce bleeding and segregation of fresh concrete Lower the heat of hydration beneficial in mass pours Improve the pumpability of fresh concrete Generally improve the long term strength gain Reduce permeability and absorption Reduce alkali-aggregate reactivity Conclusion The use of SEMs in concrete is picking up in India in all application. It is highly advantages to use maximum SEMs and minimum cement in concrete from point of view of many advantages. References 1. Nataraja, M. C, “Mixture proportioning and properties of slag concrete’, The Master Builder, India, Vol., 2005, pp.23-31 2. Nataraja, M. C. and Lelin Das, ‘ A simplified mix proportioning for cement based composites with crushed tile waste aggregate’, Journal of Scientific and Industrial Research, Vol. 70, No. 5, May 2011, pp. 385-390. 3. Nataraja, M. C. and Lelin Das, ‘Concrete mix proportioning as per IS:10262-2009-Comparision with IS:10262-1982 and ACI 211.1-91’, Point of view, Indian Concrete Journal, Vol. 84, No. 9, September 2010, pp. 64-70.

Supplementary cementing materials for green concrete – A view

ABSTRACT

The objective of this paper is to discuss various supplementary materials used in the production of concrete. Use of these materials is compulsory in today’s RMC and high performance concrete. Proper concrete mix design is compulsory for ensuring adequate strength, workability and other properties.

Introduction

Use of high strength and high performance concrete is increasing day by day in our country due to many inherent advantages. Today we are seeing such concretes in many applications in mega cities including Mysore. Such high strength and high-performance concrete mixtures use one or more supplementary cementitious materials generally called as SCMs to enhance workability, strength, durability, service life, and other performance characteristics. While there are a number of potential SCMs that have been used in concrete (including metakaolin and rice husk ash), the three most common are fly ash, silica fume, and ground granulated blast furnace slag. These materials react chemically with calcium hydroxide released from the hydration of portland cement to form cement compounds. These materials are often added to concrete to make concrete mixtures more economical, reduce permeability, increase strength, or influence other concrete properties.

SEMs and their Properties

The process that produces these materials influences their properties, so a basic introduction to each is given below.

i) Fly ash.

The most commonly used pozzolan in concrete, is a finely divided residue that results from the combustion of pulverized coal and is carried from the combustion chamber of the furnace by exhaust gases. Commercially available fly ash is a by-product of thermal power generating stations (RTPC, Raichur, Karnataka). Fly ash contains some heavy metal (normally more than silica fume), so the heavy metal content of the concrete will increase. Fly ash refines the pore structure of the concrete, making it more resistant to chloride penetration. Not all fly ash is suitable for use in concrete. Fly ash shall conform to the requirements of Indian standard and shall be specified in accordance with BIS requirements. Normally 10% to 20% of the cement can be replaced with fly ash to reduce the environmental burden of the concrete. Substitution with fly ash at levels exceeding 25% is considered to be a high volume SCM application. Appropriate testing to ensure desired performance should be carried out. The use of fly ash can increase setting times. This may be an economic factor in precast concrete manufacturing if casting cannot be maintained on a daily cycle.

Although fly ash offers environmental advantages, it also improves the performance and quality of concrete. Fly ash affects the plastic properties of concrete by improving workability, reducing water demand, reducing segregation and bleeding, and lowering heat of hydration. Fly ash increases strength, reduces permeability, reduces corrosion of reinforcing steel, increases sulphate resistance, and reduces alkali-aggregate reaction. Fly ash reaches its maximum strength more slowly than concrete made with only Portland cement. The techniques for working with this type of concrete are standard for the industry and will not impact the budget of a job.

ii) Blast-furnace slag.

Slag or iron blast-furnace slag, is a non-metallic product consisting essentially of silicates, aluminosilicates of calcium, and other compounds that are developed in a molten condition simultaneously with the iron in the blast-furnace. Substitution of slag at levels exceeding 35% for the Portland cement in precast concrete is considered a high volume SCM application, and its suitability for intended use must be prequalified. The addition of slag cement usually results in reduced need for water, faster setting time, improved pumpability and finishability, higher 28-day strength, lower permeability, resistance to sulfate attack and alkali-silica reactivity (ASR), and lighter color. Blast-furnace slag shall conform to the requirements of BIS.

iii) Silica fume.

Silica fume or fumed silica or silica dust, also called condensed silica fume and microsilica, is a finely divided residue resulting from the production of elemental silicon or ferro-silicon alloys that is carried from the furnace by the exhaust gases. Silica fume, with or without fly ash or slag, is often used to make high-strength concrete. Silica fume improves the quality, strength and durability of concrete by making the concrete much less permeable and more resistant to corrosion of the steel reinforcement. Silica fume shall conform to the requirements of BIS.

Advantages

Supplementary Cementing Materials can significantly reduce the embodied energy of concrete and its products significantly. Their judicious use in concrete production is desirable both for environmental and energy conservation as well as for the technical benefits they can provide. SCMs are added to concrete as part of the total cementitious system, either as an addition or partial replacement of Portland cement. When properly used, the SCMs can enhance the following properties of concrete:

Generally improve the workability and finishing of fresh concrete
Reduce bleeding and segregation of fresh concrete
Lower the heat of hydration beneficial in mass pours
Improve the pumpability of fresh concrete
Generally improve the long term strength gain
Reduce permeability and absorption
Reduce alkali-aggregate reactivity

Conclusion

The use of SEMs in concrete is picking up in India in all application. It is highly advantages to use maximum SEMs and minimum cement in concrete from point of view of many advantages.

References

1. Nataraja, M. C, “Mixture proportioning and properties of slag concrete’, The Master Builder, India, Vol., 2005, pp.23-31

2. Nataraja, M. C. and Lelin Das, ‘ A simplified mix proportioning for cement based composites with crushed tile waste aggregate’, Journal of Scientific and Industrial Research, Vol. 70, No. 5, May 2011, pp. 385-390.

3. Nataraja, M. C. and Lelin Das, ‘Concrete mix proportioning as per IS:10262-2009-Comparision with IS:10262-1982 and ACI 211.1-91’, Point of view, Indian Concrete Journal, Vol. 84, No. 9, September 2010, pp. 64-70.