Most of the economic activity that took place subsequent to the Industrial Revolution followed an 'open-ended' approach as regards flow of materials and energy in all production processes. The approach involved transformation of natural resources into useful products and returning the worn-out products and wastes / by-products of the production process back to the Mother Nature. It had no concern whatsoever for conservation of natural resources and environmental quality and hence, led us to a situation where 'sustainable waste management' has become our highest priority today.

Although the high consuming societies of the developed world need to take lion's share of the blame and responsibility for the environmental damage, we in India cannot remain to be silent observers to the worsening ecology around. To resolve this apparent conflict between development and environment, hereafter, all governments, businesses and individuals will be required to move towards a framework where development (or growth) becomes environmentally sustainable. This is the basis of the 'industrial ecology' concept. It aims to transform the 'open' system of production into one where material and energy flows are 'closed', i.e. all wastes and by-products get reused within the 'system' as a result of transfers among 'symbiotic' participating industries.

Utilization of waste materials in brickmaking is one good example of industrial ecology at work.

2.0 'Sustainable Waste Management' and Brickmaking :

Sustainable waste management is often based on waste hierarchy - Reduce, Reuse, Recovery and (safe) Riddance - in the descending order of priority. Utilization of industrial, urban and agricultural wastes in brickmaking achieves all these objectives simultaneously. Not only does it solve the disposal problem of the generating agencies and minimizes the requirements of primarily extracted clays and fossil fuels, but it also has many positive effects on the brickmaking process and the brick quality. Therefore, all wastes have been categorized below under 4 groups, depending upon their principal effect on the manufacturing process / product quality.

1. Fuel containing wastes
2. Plasticity reducing / enhancing wastes
3. Fluxing wastes and
4. Flyash

Of the above, although flyash can be included under category '1' or '2' - and in some instances, under category '3' - considering the volume of its production / availability and severity of its environmental impact, flyash has been classified separately.

3.0 Non-Flyash Wastes :

3.1 Fuel containing wastes are 'Potential Heat Carriers', in the sense they contain high percentage of organic and / or carbonaceous matter. They are therefore extensively used in the production of fired clay bricks. This category includes residues from urban waste treatment plants and wastes of coal processing, textile, tanning, sugar, liquor, petroleum, paper, wood-processing and host of other agro / food processing industries. The calorific value of these wastes ranges from 1,200 to 33,000 KJ/kg . The addition of these wastes to brick bodies is almost never more than 10 % by mass and their main effects include energy saving during firing, increase in porosity of fired goods (making them more lightweight, thereby improving their heat / acoustic insulating properties) and reduction in shrinkage and mechanical strength.

3.2 Plasticity reducing wastes (i.e. 'opening' or 'filler' materials) and plasticity enhancing wastes (i.e. 'binders') play an important role in the production of both fired as well as cured bricks. They chiefly include wastes from mining, metallurgical and chemical industries viz. blast furnace slag, red mud, stone dust, burnt foundry sand, drilling muds, phosphogypsum / gypsum, coal washery sludges, ore processing wastes, etc.

Coarse-grained wastes generally reduce plasticity and their content in brick bodies varies between 10 and 60 % by mass. They effect slight reduction in shrinkage and mechanical strength and in some instances, increase in efflorescence. Fine-grained wastes generally enhance plasticity and they are added in quantities ranging from 5 to 15 % by mass. Chemically reactive substances such as pozzolanic materials, cement wastes, lime sludges, red mud, phosphogypsum / gypsum, etc. are ideally used in the production of 'cured' bricks.

3.3 Fluxing means 'glass formation' and therefore it essentially involves firing. Fluxing reduces the firing temperature (consequently saving energy) and porosity (consequently increasing fired strength). Fluxing wastes originate from glazing lines of ceramic tile manufacturing units, plating plants and a number of metallurgical / engineering industries. They chiefly contain alumino-silicates
(> 50 % by mass) and variable contents of alkalies / alkaline-earth elements - which contribute to fluxing - and heavy metals. Glazing sludge can be added upto 20 % by mass, and other wastes upto 5 % by mass to the brick body to improve the brickmaking process / product.

3.4 Manufacturing technologies and Indian experiences :

So far, hand-moulding and soft extrusion technologies have been predominently employed in India for manufacture of fired bricks from non-flyash wastes. In case of cured / autoclaved bricks, semi-dry pressing technology - incorporating either small vibro-compaction / toggle-type machines - has been utilized. Indian experience with small capacity plants (3 to 10,000 bricks / day / shift) is much better as compared to large capacity plants.

4.0 Flyash :

With a present share of about 40 %, coal happens to be the world's most extensively used fossil fuel for generating power. Its worldwide 'reserves to production ratio' is 4 times that for the oil and gas taken together. At the 1994 production level, economically accessible global coal reserves are expected to last at least another 230 years. This clearly shows the eminent position coal is expected to enjoy during the foreseeable future as a power-generation fuel.

India is the 3rd largest producer of coal in the world after China and U.S.A. However, Indian coals have high ash content ( 35 to 48 % as compared to 8 to 10 % in developed countries like U.S.A., Japan, Germany, France, etc. ) and low calorific value (3,500 to 4,000 KCal/kg as compared to 6,000 to 7,000 Kcal /kg in the developed countries). About 72 % of India's present power generation capacity is coal-based. This situation is expected to continue at least during the early part of 21st century. Use of coal as fuel brings in its wake its own share of problems, the most important being that of flyash generation. When pulverized coal (70 % passing through 200 mesh) is burned in a boiler furnace within 900-1500 0C temperature range, non-combustible part of the coal gets converted into ash. Nearly 80 % of this ash 'flies off' or gets carried away from the furnace by flue gases, which is then separated out by use of one or more Electro Static Precipitators (ESP's) and is called 'flyash'. The balance 20 % ash 'agglomerates' to larger particle sizes, ranging from 0.02 to 75 mm, and falls down to the bottom of the furnace through its grate as 'bottom-ash'. Both these ashes are mixed with water and sent to ash pond for storage and further disposal.

About 1,000 million tonnes of flyash has already accumulated in Indian ash ponds, which are increasing at the alarming rate of about 80 million tonnes every year. It is estimated that about 140,000 MW of additional power generation capacity would be required by the end of the 10th Five Year Plan, i.e. the year 2007, to meet the growing indigenous demand arising out of rapid industrialization, farm mechanization and changing individual lifestyles. This will then lead to a staggering figure of 175 million tonnes of flyash generated per year, which in turn would engage about 40,000 hectares of land for construction of ash ponds.

Storage of flyash in ash ponds requires very large quantities of water ( average ash : water ratio is 1 : 12 ) and it involves huge capital and operating costs in setting up and running the mixing, pumping and transport facilities, respectively. It not only blocks large tracts of land permanently but also results into serious groundwater contamination and deterioration of the surrounding eco-system. This brings to the fore the dire need of the hour to utilize as much quantity of flyash as possible and to solve its critical disposal problem.

4.1 Characterisation of Indian Flyashes :

The chemical and physical properties of flyash depend upon many parameters such as coal quality, type of coal pulverization and combustion process followed, nature of ash collection and disposal technique adopted, etc. Flyash is generally 'pozzolanic' in nature while bottom-flyash is generally not. Pozzolanicity of a material is its capacity to react with CaO or Ca(OH)2 in presence of water at room temperature to form solid and water-isoluble cementitious compounds. The pozzolanicity of flyash mainly stems from the presence of various silicates and aluminates in amorphous form.

Chemical Composition :
Except Neyveli flyash, which is high in CaO (5.0 - 16.0 %) and MgO (1.5 - 5.0 %) contents and low in SiO2 (45.0 - 59.0 %) content, the range of chemical composition of Indian flyashes is given in the following Table. Corresponding data for American and German flyashes is also given for comparative purpose.

Composition ( % by w/w ) :
Component Indianflyash American flyash Germanflyash
SiO2Al2O3Fe2O3CaOMgONa2OK2OSO3LOI 50.0 - 65.016.0 - 25.05.5 - 15.2 1.5 - 2.50.8 - 1.00.5 - 0.90.6 - 1.00.5 - 0.82.0 - 15.0 40.0 - 51.0 17.0 - 28.0 8.5 - 19.0 1.2 - 7.0 0.8 - 1.1 0.4 - 1.8 1.8 - 3.0 0.3 - 2.8 1.2 - 18.0 42.0 - 56.024.0 - 33.0 5.4 - 13.0 0.6 - 8.3 0.6 - 4.3 0.2 - 1.3 1.1 - 5.6 0.1 - 1.9 0.8 - 5.8

From the above data it can be seen that Indian flyashes are more silicious and contain higher percentage of unburnt carbon as compared to American / German flyashes.

Physical Properties :
Flyash is generally grey in colour, abrasive, acidic and refractory in nature. Its specific surface area varies between 4,000 and 10,000 cm2/g , which is lesser than cement which has a specific surface area of about 3,000 to 3,500 cm2/g. Morphologically, flyash consists of 3 types of particles - irregularly shaped particles, solid spheres and cenospheres.

4.2 Manufacturing Technologies and Indian Experiences :

a) Flyash-Sand-Lime Bricks : Flyash-sand-lime bricks are manufactured by mixing flyash sand and lime in 70-80 %, 0-10 % and 8-20 %, respectively, proportions which may be followed by addition of 0.2-0.3 % chemical accelerator during wet mixing. This mixture is moulded under 80-240 kg/cm2 pressure. The green bricks can be air cured for 24-48 hours and then hot water / steam cured or autoclaved at 1-14 kg/cm2 pressure and 100-1800 C temperature for 3-12 hours. Some of the organizations which offer this know-how are NCB, CBRI, CFRI, ACC, SAIL, etc.

b) Flyash-Lime-Gypsum Bricks : 55-75 % flyash is mixed with 15-30 % lime and 10-15 % gypsum. Chemical accelerator may or may not be added. This mix is moulded under pressure. Air / sun drying may be done. Green bricks are then water cured. Some of the organizations which offer this know-how are AEC, Ahmedabad; ITC, Bhadrachalam; NLC, Neyveli; INSWAREB, CBRI, CFRI, CPRI, RRL Bhopal, etc.

c) Flyash-Clay Bricks : Flyash-Clay bricks can be manufactured by mixing 20-60 % of flyash with clay and hand-moulding or extruding the mix. Firing can be done in fixed chimney / high-draught or any other continuous kiln. Some of the organizations which offer this know-how are CBRI, NML, CPRI, RRL Bhopal, Walter Craven Ceramic Projects India Ltd.,Calcutta, etc.

d) Flyash-Stonedust-Cement / Lime Bricks : 20-50 % flyash, 25-75 % stonedust, 5-25 % cement / lime and upto 10 % chemical binder are wet mixed and pressed between 200-300 kg/cm2. The bricks are either water cured for 28 days or autoclaved. This know-how is being offered by CPRI and Damle Clay Structurals (P) Ltd., Pune.

Indian experience with large capacity plants manufacturing fired flyash-clay or cured flyash bricks is far from being satisfactory. In this light, development of high capacity indigenous presses ( instead of imported ones ) and thorough techno-economic evaluation of projects need major attention. Small plants (of 10,000 bricks /day / shift capacity) employing hand-moulding, egg-laying / toggle-type machines or mechanical / hydraulic presses have been performing far better in comparison.

5.0 Future Scenario :

As mentioned earlier, the industrial ecology concept - involving reuse and recycling of wastes - holds much promise for the Indian Brick Industry. It not only provides a way to reconcile our developmental and environmental imperatives, but also throws open a host of opportunities to develop, implement and market environmentally sustainable technologies.

Perhaps a concrete example shall explain this point better. At Kalundborg Industrial Complex, Denmark - a small coastal industrial zone 75 miles west of Copenhagen - a web of energy and material exchanges among companies has emerged over the last 20 years. The Kalundborg system consists of five core partners: Asnaes Power Station (a 1,500 MW coal-fired power plant), Statoil Refinery (of 3.2 million tonnes/year capacity), Gyproc (a plasterboard factory making 14 million sq. metres of gypsum wallboard a year), Novo Nordisk (an international biotechnology company with annual sales in excess of $2 billion, whose Kalundborg plant manufacturers pharmaceuticals and industrial enzymes), and the City of Kalundborg which supplies residential heat and water to its 20,000 residents.

The power plant pipes residual steam to the refinery, and in exchange, receives refinery gas, which substitutes some of the coal. Excess steam is also supplied to Novo Nordisk and the City (for heating). This replaces almost 3,500 individual oil furnaces (a major air pollution source). The power plant's desulphurisation process also yields gypsum, which meets about 2/3rd of Gyproc's needs. Sludge from Novo Nordisk's processes is used as a fertilizer on nearby farms, and surplus yeast from its insulin production is sold to farmers as pig food.

Industrial Clusters, in the form of industrial estates or industry belts, are quite common in India. Therefore, planning and implementation of 'industrial recycling networks' at locations which are either near to the source(s) of wastes (e.g. Coal-based Thermal Power Plants) or markets, appears very much feasible. However, to ensure the success of these industrial eco-systems, 2 pre-requisites assume significance in the Indian context. First, the economic benefits of the exercise must be spelt out very clearly to the participating industries and second, our policy and regulatory approaches need drastic changes. Rather than being mere law-making, enforcing and monitoring agencies, the Ministry of Environment and Forests (MoEF) and Central / State Pollution Control Boards need to achieve positive environmental outcomes through co-operation with the industry and by adopting incentive-based approaches. The recent MoEF Regulation, which makes it compulsory for all brick manufacturers within 50 kilometre radius of Coal-based Thermal Power Plants to use at least 25 % flyash in their raw-mix by mass and which makes all Thermal Power Plants accountable for their actions, shall also need similar soft-landing in order to be effective.