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.