Environment-friendly Blended cement – Eases pressure on Industrial waste disposal

Environment-friendly Blended cement – Eases pressure on Industrial waste disposal and reducing emissions per unit weight of cement manufactured

Portland Cement, an essential component of concrete, hardens and become strongly adhesive after application; used for various engineering and building construction activities. It is a mixture of Calcium silicate (CaO, SiO2), Calcium aluminate (CaO, Al2O3) in varying proportion, with small amount of magnesium, iron compounds etc. To slowdown the hardening process of cement, sometime Gypsum is also used.

Production of Portland cement is highly gas pollutant; emits considerable amount of Carbon dioxide (CO2) in the atmosphere, polluting air. CO2 is well known for its ill-effects towards Global warming as greenhouse gas. In cement manufacturing plant alone, CO2 produces (a) during de-carbonation of limestone, (b) during kiln fuel combustion, (c) during transportation by vehicles in the cement plant & for cement distribution.

Considering CO2 associated with electric energy consumed by a cement plant, we have observed, for production of 1 tonne of Portland cement 1 tonne of CO2 is produced and emitted to the atmosphere. This is a quite large quantity of CO2 which a Portland cement unit pollutes. To mitigate this extent of pollution menace, many of the steps have been taken by cement industries - production and use of environment-friendly blended cement, i.e., use of alternate or supplementary cementitious materials such as fly ash and slag, is among such steps taken to bring down emission per unit weight of cement manufactured. In addition to reduction of emission per unit weight of cement manufactured, it eases problem of Industrial waste disposal; as both fly-ash and slag are the waste from thermal power and steel making industry respectively, available abundantly almost at free of cost.

Fly-ash blended cement: Fly ash is the waste generated by thermal power units, resulting from combustion of powdered coal. Fly ash consists of mostly Silicon dioxide (SiO2), Aluminum oxide (Al2O3) and Iron compounds. It is pozzolanic, i.e., it reacts with calcium hydroxide and alkali to form cementitious materials.

Activated Fly ash can replace up to 50% by mass in blended cement. It is less expensive and it is advantageous in a host of applications. Fly ash can be used to improve workability and pumpability of concrete. Due to its generally slower rate of hydration, fly ash also lowers the heat of hydration and is important in mass concrete structures, such as large foundations, bridges, and piers. High fly ash concrete shows less bleeding and shrinkage than straight cement mixes. Fly ash is also used as a component in the production of flowable fill, which is used as self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill.

Due above advantages of fly ash blended cement, more and more fly ash is being used beneficially as a recycled material.

Slag blended cement: Slag is the waste product of iron making industry. These are the impurities present in iron ore and obtained from blast furnace while processing for pig iron. It was found that ground granulated slag reacts with water to produce cementitious properties. Therefore it is used in concrete in combination with Portland cement as part of blended cement. Concrete containing ground granulated slag develops strength over a longer period, leading to reduced permeability and better durability properties. Slag cement concrete is less vulnerable to alkali-silica and sulfate attack.

As with fly ash, processing blast furnace slag into slag cement or slag aggregate eases the burden on environment. Apart from reducing the burden of waste disposal, it reduces the air emissions at the cement kiln as well as the material in landfills. Most significantly, slag decreases Portland cement usage by as much as 50 percent, thereby diminishing CO2 emissions, the amount of energy required to produce concrete, and the quantity of virgin land extraction through mining raw materials for Portland cement.

Concrete made with blended cement has generally higher density than concrete made with Ordinary Portland Cement. Hence it improves the impermeability and durability. All these make for better long-term strength, dense and impermeable concrete and improved corrosion resistance and longer service life with reduced cost for repairs and maintenance.

Thus, the Blending cement is not only environment-friendly, but also make improved concrete performance.

Use of Fly ash - in cement making etc.:

Use of Fly ash - in cement making etc.:

Fly ash is the mineral residue resulting from the combustion of powdered coal in power generating plants. Fly ash consists of mostly of silicon dioxide, aluminum oxide and iron oxide. It is pozzolanic in nature, meaning it reacts with calcium hydroxide and alkali to form cementitious compounds. In the coal fired power plant the fly ash is collected by using electrostatic precipitators or filter bags. Initially, this collected ash was disposed of in ash ponds or landfills. Once its pozzolanic properties were discovered however, it became useful as a replacement of Portland cement in concrete. Fly ash can replace up to 50% by mass of Portland cement. Generally less expensive than Portland cement, it also presents advantages in a host of applications.

• Fly ash can be used to improve workability and pumpability of concrete. Due to its generally slower rate of hydration, fly ash also lowers the heat of hydration and is important in mass concrete structures, such as large foundations, bridges, and piers.
  


• High fly ash concrete shows less bleeding and shrinkage than straight cement mixes.
  Fly ash is also used as a component in the production of flowable fill, which is used as self-leveling, self-compacting backfill material in lieu of compacted earth or granular fill.

More and more fly ash is being used beneficially as a recycled material, although it is still far from fully utilized as more than 75% of fly ash produced from coal power plants is disposed in landfills. This amounts to hundreds of megatons in India and China, creating environmental problems. Though it is costly to retrofit older coal-fired power plants to filter fly ash from the atmosphere, over time the economic incentive of selling the captured fly ash pays for the initial expense of installation.

Nuisances of fly-ash generated by coal-fired power plant:

Nuisances of fly-ash generated by coal-fired power plant:

Thermal Power plants using pulverized coal or lignite as fuel generate large quantities of fly-ash as a by-product. With the increase in commissioning of several super thermal power plants with large capacity and with the increasing use of low grade coal of high ash content, the amount of generation of ash from them is becoming very large. This poses serious ecological problems. Most of the ash generated from the power plants is disposed off in the vicinity of the plant as a waste material covering several hectares of valuable land. Looking into the nuisances poses by fly-ash; it has been well appreciated by everybody and various Government agencies, the need for the safe disposal and effective utilisation of fly-ash. The properties of fly-ash depend on several variables such as coal source, degree of pulverization, design of boiler unit, loading and firing conditions, handling and storage methods etc.

At present in the under-developed country like India, the focus is on demonstration of coal ash related technologies - these include fly-ash characterisation, Hydraulic Structures, Handling and Transportation, Agriculture related studies and Application, Ash Ponds and Dams, Reclamation of Ash Ponds for Human Settlement, Roads and Embankments, Underground Mine Fills etc. Only a small quantity of the total ash produced is utilized in concrete, brick making, soil-stabilization treatment etc. The bulk utilization of ash is yet to begin in full swing.

Fly-ash characterization: - The physical, geotechnical and chemical parameters to characterize fly-ash are the same as those for natural soils, e.g., specific gravity, grain size, Atterberg limits, compaction characteristics, permeability coefficient, shear strength parameters and consolidation parameters.

Fly-ash disposal in Ash Ponds: - Fly-ash is mostly disposed off using either Dry or Wet disposal procedure. (a) In the case of dry disposal method, the fly-ash is transported by truck, chute or conveyor from the power plant to the site of disposal where these are disposed off by constructing a dry embankment (dyke). (b) In wet disposal, the fly-ash is transported as slurry through pipe and disposed off in impoundment called "ash pond". Most of the power plants use wet disposal system.

Environmental concerns: - The ash produced in thermal power plants cause all three environmental risks, such as, air, surface water and groundwater pollution. . Directly related to these concerns is the additional environmental goal of aesthetically enhancing ash disposal facilities. Air pollution is caused by direct emissions of toxic gases from the power plants as well as wind-blown ash dust from ash mound/pond. The air-borne dust can fall in surface water system or soil and may contaminate the water/soil system. The wet system of disposal in most power plants causes discharge of particulate ash directly into the nearby surface water system. The long storage of ash in ponds under wet condition and humid climate can cause leaching of toxic metals from ash and contaminate the underlying soil and ultimately the groundwater system. The environmental aspects of ash disposal should be taken care of in order to minimize air and water pollution. Most of the above environmental problems can be minimized by incorporating engineering measures in the design of ash ponds and continuous monitoring of surface and groundwater water systems.