Making cement production less polluting

PORTLAND cement may acquire anew, environmentally friendly,chemical composition. Researchersare focussing on how to reduce emissions from cement manufacture andalso use less energy. Another area ofresearch that has emerged recentlyexamines the use of bauxite (aluminiuni oxide) and wastes such as fly ash,scrap iron and blast furnace slag incement manufacture.

Large amounts of carbon dioxideand nitrous oxides are produced inmanufacture of cement. Della M Royof the Materials Research Laboratoryof Pennsylvania State University estimates 30 per cent of limestone usedin cement production becomes carbon dioxide. Emissions can bereduced by lowering temperatures atwhich different cement varieties aremanufactured, but this requires altering the composition of cement.

Belite cement, for example, usesgypsum and bauxite to replace 10 percent of the. limestone. This hasreduced energy use by 16 per centand carbon dioxide emissions by 10per cent. But a disadvantage is that ittakes longer to set than Portlandcement. This can be solved by addingsuperplasticizers that reduce thequantity of water needed to mixbelite.cement into concrete.

Alinite cement, developed in theformer Soviet Union in 1977, alsocontains less limestone and uses calcium chloride, which makes constituents react at a lower temperature. Its drawback is that the chlorinefrom calcium chloride corrodes steeland other metals used in reinforcedconcrete. 16 use is, therefore, limitedto unreinforced concrete or foundations.
Changing processEmissions as a percentage of cementproduction fall when productionrises. However, this has its limitations After a point, says HiroshiUchikawa, R&D director of OnodaCement Co Ltd, one of japan'sleading cement manufacturers.Significant emissionreductions can comeonly from changing themanufacturing process,mainly in two areas.One is the type of reactor us e*d and the other,the type of grindingtechnique.

Portland cement ismanufactured in arotary kiln. If the kilnis replaced by a fluidised - bed reactor,energy use would becut by more than one-third and constructioncosts by one-fourth in aplant producing 330tonnes of cement perday. Although fluidised bed reactors arenot new, they havebeen adapted by thecement industry onlyrecently.

Whether producedin a rotary kiln or a flu- Ulmidised bed reactor,cement has Ito beground to a fine powder. Grinding can beimproved if the conventional ball mill iscomplemented with aroller mill as the latter has a greatercontact area on the grinding surfacethan a ball mill. Therefore, smallerball-and-roller mills that use lesselectricity can be used.

The disposal of fly ash is a majorproblem in India. Industries andpower plants in the country produce40 million tonnes of this fine powderevery year. While some companiesuse fly ash as landfill material, disposal remains a vexatious process.Fly ash does not require to be groundand can be used instead of clay in themanufacture of cement. Fly ash ranalso be used in concrete - a mixtureof cement, stones, sand and water.Says Roy, "Fly ash can replace up to50 per cent of the cement in concretewithout any noticeable fall instrength. Such a mixture does, however, take longer to set than concretewithout fly ash."

Blast furnace slag is used to manufacture another type of cementcalled belite-sulphoaluminatecement. "This type of cement doesnot save energy but has desirablecharacteristics such as no shrinkageon drying, no efflorescenceand a short setting time. It is used forglass fibre, reinforced concrete andprecast concrete in Japaii," saysUchikawa.

Indian cement manufacturers,who have yet to introduce new varieties of cement developed abroad,use the dry process in which the constituents of cement react in a drykiln. While this process uses muchless energy than those in Which theconstituents are moistened and madeto react, it is more polluting as it isfar dustier.