Although the term "Wastes" is in common use, it may be misleading. Not all wastes are useless rubbish and freely available. It is also mainly a matter of definition: from one point of view a material can be of no use, while it is a valuable resource from another.
In this context, wastes can be defined as by-products (of agricultural, forestry, industrial or even household processes), which do not essentially have anything to do with building, but which, with special processing and treatment, or in conjunction with other materials, can economically substitute (or even improve the quality of) conventional building materials. Exceptions to these wastes are recycled materials from demolished buildings, which continue to serve as building materials, though perhaps in a different way.
Discarded consumer goods (such as bottles, tins, car tyres), which have been experimented with in several industrialized countries, are of less significance in developing countries, as such materials already have numerous other uses (eg household articles, musical instruments, shoes).
The materials referred to in this section are extremely diverse, but are basically of two types: organic and inorganic wastes. As a further sub-division, organic wastes are generally agricultural or forestry by-products and also household and urban wastes, while inorganic wastes are mainly obtained from industrial processes and demolition of old buildings, but there are several exceptions.
• The outer skin of rice grains can be used in the dry state, chemically treated, or in the form of ash.
• Full or crushed husks mixed with clay in brick production, help to burn the brick uniformly, creating voids, and thus producing lightweight bricks.
• Water glass (sodium silicate), a useful binder, can be manufactured from rice husks. This can be used in the bonding of full or crushed husks to produce particle boards. Other binders can also be used.
• Rice husk ash (RHA) is a useful pozzolana, which can be mixed with lime to produce a cementitious binder. (Details are given in the section on Pozzolanas).
• RHA mixed with soil, nodulized and sintered in a kiln, makes lightweight aggregates for concrete.
• These include fresh husks, coconut shells and waste from the coir industry.
• The husks consist of 15 - 35 cm long fibres (about 60 % of husk), with high tensile strength, which is affected by moisture. The fibres, and more so the pith (soft cork-like material), are chemically reactive, as long as they are kept dry. During the resting process (softening by soaking in water) they become inert. The difference in reactivity between rested and fresh husks necessitates different methods of conversion into building materials.
• Unretted husks, hot-pressed (at 150° C, 1 MPa pressure for 15 to 25 minutes) without any additives, produce strong particle boards.
• Unretted pith, obtained by defibrating mature husks, hot-pressed without additives, produce strong, moisture resistant boards. Lighter, resilient boards are made in the same way, but with addition of rested pith (low density, highly elastic granular material).
• Retted pith mixed with cashew nut shell liquid resin (rubbery substance) produces an expansion joint filler, which is resistant to temperature and moisture fluctuations and to insect and fungal attack.
• Retted pith granules as an aggregate in concrete are useful for thermal insulation.
• Unretted fibres, mixed with paraffin wax and hot-pressed, make strong and flexible hardboards (fibre boards).
• Coir shearing waste, containing fibre, pith and dust, bonded with an adhesive, produces particle boards with an attractive mottled appearance.
• Coir waste, mixed with portland cement and moulded under compression, produces large corrugated roofing sheets (see section on Fibre concrete).
• Coconut shell chips and conventional adhesives make good quality particle boards.
• Coconut shell tar, obtained during the destructive distillation of the shells, is a slightly viscous liquid with anti-microbial properties.
• Sawdust, woodchips, wood shavings and other wood residues from sawmills can be used in the conventional ways to produce particle, fibre and woodwool boards.
• With sawdust as aggregate in concrete, preferably with magnesium oxychloride cement, precast lightweight concrete components (eg door and window frames) can be made.
• Wood waste, mixed with inorganic materials (cement, trass, lime, pozzolana) in a mixer/ pulper machine, produce pulp cement boards for various non-loadbearing uses.
• Tannin is extracted from the bark of various timber species (obtained in timber processing) to produce tannin-based adhesives for the manufacture of particle board.
Reeds and straw
• Straw from wheat, barley, rice and other plants are hot-pressed, without any binders, to produce rigid boards, faced with paper on both sides (Stramit process).
• Flexible boards are also made by placing reeds (or stiff varieties of straw) side by side and then stitching them across with ordinary galvanized wire.
• Straw and other dried fibrous material, chopped to lengths of 10 to 20 cm, softened in water, and mixed with wet clayey soil, can be compacted in formwork to make stiff, thermal insulating walls (straw clay construction).
• This is the fibrous residue from sugar cane processing. It is not suitable for reinforcement of cement based products, as the residual sugar retards the setting of cement.
• With a suitable organic adhesive, particle boards and fibre boards can be made from bagasse.
Banana stalks and leaves
• Banana fibres have been successfully used in fibre concrete.
• Stalks and leaves, chopped up and boiled in water, form a thick liquid, which is applied on soil walls and roofs for waterproofing and higher resistance to abrasion and cracking.
Cashew nut shell liquid
• A by-product from cashew nut processing is a viscous liquid extracted from the mesocarp. The CNSL severely blisters the skin of any person coming into contact with it, but is a useful anti-microbial and waterproofing agent. It is therefore used to protect materials which are susceptible to biological decay (eg thatch roofing), and is applied with a brush. It can also be sprayed if mixed with kerosene to reduce viscosity.
• This beautiful plant, originally found only in Brazil, has become a serious problem, clogging tropical waterways worldwide and invading paddy fields in Southeast Asia. It is now widely used to produce biogas, mulch for soil improvement and silage as animal feed.
• Research in India and Bangladesh has shown that tough, flexible hardboard can be made from a fibrous pulp of chopped water hyacinth stems.
Miscellaneous vegetable wastes
• A large variety of other agricultural wastes (eg jute and corn stalks, peanut shells) can be used in similar ways to those mentioned above. The most common uses are in the manufacture of particle board or fibre board.
• If used with cement as a binder, this is only possible if the waste material contains no cement "poison" (which retards setting), if the material has no cavities (which entrap and thus waste cement), and if the particles or fibres are long enough to provide strength by interlocking.
• Some non-edible grains are suitable for carbonization (conversion into carbon by slow burning) to produce particles of a fine cellular structure containing entrapped air. They are similar to, and used in the same way as, conventional lightweight aggregate (eg polystyrene beads), are biologically inert, fire resistant (up to 2000° C) and highly resistant to water and chemicals.
Waste paper and textiles
• While these are collected for other uses (such as recycled paper, packaging material, shoddy, bags, rag dusters, mats, etc.), shredded waste paper and cloth strips can serve as thermal insulations, for instance, in wall cavities and sandwich panels. Fire resistance can be achieved by soaking in a solution of borax, and drying.
• Asphalted corrugated sheets are produced by making a pulp out of washed and beaten paper and textile wastes, forming the pulp into sheets, drying in the sun or drying chamber, trimming, passing through an oven with corrugating rolls and finally dipping in a bath of hot asphalt.
• Sludge from wastewater treatment plants is normally dewatered and used for land-filling. This causing a serious disposal problem in the small island-state of Singapore led to research on utilization of the sludge as building materials (at Nanyang Technological Institute).
• Burnt bricks made of clay mixed with 40 % dried sludge or 50 % sludge ash showed better results with the ash, though higher percentages are not advisable.
• By adding pulverized sludge ash, to replace up to 20% of the cement in concrete, its workability improves, the setting time remains unaffected, but the compressive strength decreases with increasing proportions of sludge ash.
• The sludge ash can be partially crushed and used as graded aggregate in lightweight concrete, or as coarse aggregate in no-fines concrete, with satisfactory results.
• Coal is an organic material, but the wastes referred to here are largely inorganic, and can thus be ascribed to either group.
• Gangue is a by-product of coal production and is chiefly composed of silicon and aluminium with 75 % oxide. In China large amounts are used as building material: mainly as masonry blocks, aggregate in lightweight concrete, end es a cement replacement material.
• The burning of coal in thermal power plants produces basically two types of residues: cinder (or clinker), formed by burning lump coal, or pulverized coal which fuses to lumps and falls to the bottom of the furnace (also called "bottom ash"); fly ash (or pulverized-fuel ash) formed by burning pulverzed coal, producing a fine dust, which is carried upwards by the combustion gases. Coal ashes can contain unburnt carbon in varying proportions.
• Cinder and sintered fly ash are used as lightweight aggregate in concrete construction and blockmaking.
• Fly ash and/or crushed cinder can be used in making burnt clay brick, masonry mortars and aerated concrete. (For further details about fly ash see section on Pozzolanas.)
Blast furnace slag
• This is the molten material which settles above the pig iron at the bottom of the furnace. (Details are given in the section on Pozzolanas.)
• The washings of bauxite ore in the production of alumina are collected in ponds, which dry out leaving a residue called red mud.
• The red mud can be mixed with clay to make fired bricks and tiles, or pelletized and fired to produce lightweight aggregate for concrete. The fired pellets can also be finely ground to produce a high quality pozzolana.
• The sludge, in the form of finely precipitated calcium carbonate (with varying amounts of free lime), is obtained from fertilizer plants, sugar and paper factories, tanneries, soda-ash and calcium carbide industries.
• Lime sludges are used for the manufacture of portland cement and to produce sand-lime bricks.
• The lime sludge can also be moulded into bricks and fired in kilns to produce quicklime (calcium oxide).
• Dried lime sludge mixed with rice husks and fired in an open clamp produce a hydraulic binder (see section on Pozzolanas).
• Phosphogypsum (calcium sulphate, contaminated with phosphates) is produced as a slurry in the manufacture of fertilizers and phosphoric acid. It contains several impurities, which have to be removed by expensive washing, thermal or chemical treatments. It is also to some extent radioactive and thus not recommended for building.
• If the amount of impurities and radioactivity is sufficiently low, the purified gypsum can be used as a set-retarder in portland cement, or to produce gypsum plaster, fibrous gypsum plaster boards or gypsum blocks.
• Cements from phosphogypsum have delayed setting and slow rate of strength development at early ages, but strengths at later ages (28 days) are comparable with those of ordinary cements.
• Demolished buildings can provide a vast number of materials that can be recycled in new constructions. Careful dismantling and separation of various individual components (metal parts, timber boards and beams, windows, doors, tiles, pipes, etc.) help to conserve limited resources and save the immense costs and energy required to produce new components.
• Brick waste can be finely ground and used as a pozzolanic binder (see "Burnt clay" in section on Pozzolanas). It can also be crushed to a maximum size of 20 mm and used as coarse aggregate in concrete construction (especially important in countries, like Bangladesh, in which natural aggregates are scarce). Brick aggregate absorbs water, so that more water is required in preparing the concrete mix.
• Broken concrete serves well as aggregate in new concrete.
• The collection and reuse of metal scrap is one of the world's largest industries with regard to the number of companies, people employed, weight of material handled and value of equipment used. Metal scrap can be collected at construction sites (eg off-cuts of reinforcing steel and mesh, wire and nails), demolition sites, engineering workshops (off-cuts from lathes, drills, etc.), garages and factories (scrap cars, oil drums, disused machinery, etc.), households (tin cans, domestic appliances, broken tools, furniture, etc.) and refuse dumps.
• The collected and sorted metal scrap can be melted in small decentralized foundries to produce new metal components; reshaped on a forge; cut into suitable pieces; welded together to form new products; or reused without special processing.
• Discarded beverage cans, of which large quantities accumulate in industrialized countries, are less common in the Third World. In places where they are abundantly available, they have been successfully used as bricks to construct light, thermally insulating masonry walls.
• Swarf (metal off-cuts from lathes, drills, etc.), if it is not contaminated with oil, can be used as aggregate in concrete, especially where increased resistance to cracking, impact and abrasion is needed (eg road and pavement construction).
• Flattened cans, drums, car body material, serve as cheap jointing plates in timber constructions (eg for roof trusses).
• In most developing countries, clean, used bottles have a high resale value and will hardly be considered as material to build with. In more affluent countries, where the bottles have no value, they have been used for wall construction as bricks, permitting light to pass through and presenting an attractive appearance.
• Broken glass (cullet) can be recycled in glass manufacture, but also has some uses as building material.
• Waste glass, crushed to a fine powder and mixed with clay (7 parts powder: 3 parts clay), acts as a flux and reduces the temperature needed to fire the bricks by more than 50° C (saving nearly 50 % of the fuel). The bricks are tough and resistant to wind and rain. Very strong and resistant bricks are also made from 31% crushed glass, 6% clay, 7 % water and 56 % crushed old bricks.
• Crushed glass, with a continuous grading of about 3 mm to 2 micrometres can be used as aggregate in concrete, but certain types of glass (eg soda and pyrex glass) have been found to expand in the alkali environment of portland cement, causing cracks and ultimate disintegration of the concrete.
• Large amounts of sulphur are produced in the desulphurization of petroleum and natural gas. On account of its many applications as a building material, it has been dealt with in a separate section on Sulphur.
• Components, mainly boards, made with organic or inorganic binders, from rice husks, coconut wastes, wood residues, bagasse, banana fibres and other vegetable waste.
• Boards made by hot-pressing without binders from straw, coconut husks, wood fibres, water hyacinth.
• Thermal insulation material and lightweight aggregate in concrete from rice husk ash nodules, coconut pith, sawdust, straw, carbonized grains, paper and cloth strips, sewage sludge ash, cinder and sintered fly ash, blast furnace slag, sintered red mud pellets, foamed sulphur.
• Replacement of aggregate in concrete by brick waste and broken concrete (demolition waste), crushed glass.
• Materials for cement production and replacement (pozzolanas) from rice husks, fly ash, blast furnace slag, bauxite, lime sludge, phophogypsum, pulverized burnt clay.
• Additives in clay brick production from rice husks, wood residues, sewage sludge, cinder, bauxite waste, crushed glass.
• Corrugated roofing sheets using coir waste, woodwool, vegetable fibres, paper and textile waste.
• Adhesives and surface protection coating made from tannin, banana stalks and leaves, cashew nut shell liquid, lime sludge, sulphur.
• Conservation of scarce and expensive resources, and utilization of locally available materials, reducing costs and transportation.
• Reduction of pollution by the use of materials that are difficult to dispose of, and avoidance of excessive production of new materials in polluting industrial processes.
• Considerable saving of the energy required to produce new materials.
• Improvement of the quality of some materials (eg by using certain artificial pozzolanas in concrete).
• Handling of wastes can be dangerous, eg inhaling of fine particles; blisters, burns and illness from toxic substances; severe cuts from broken glass and metal scrap.
• Although the total amount of available waste is large, it may be produced in numerous decentralized units, making collection extremely difficult.
• Once a by-product becomes a useful building material, higher prices are charged, so that the benefit of using cheap materials is quickly lost.
• Not all building materials based on wastes provide the same strength and durability as the materials they were designed to substitute (but if the price is low, this drawback can be accepted).
• The concept of using wastes and the fear of future problems that may arise due to inferior qualities of materials makes builders reluctant to use them.
• Careful supervision and strict observance of safety precautions (eg use of gloves, goggles, protective clothing) in handling waste is of vital importance to reduce injuries and health problems.
• Producers of useful by-products need to be well instructed on appropriate methods of handling and storage of the material in order to facilitate collection.
• Especially in the case of lesser known but promising waste utilization, considerable efforts are needed to demonstrate the technology and its benefits. Prototype structures (preferably important public buildings) that are constantly used can convince most doubters.
• The use of wastes for building offers a wide field of research and should be given priority - even in the more affluent countries - as there is a great need to save resources, energy and costs, and at the same time provide more shelter for the homeless.
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