To effectively face rising ecological and environmental threat from waste concrete and from some of the industrial wastes, recycling has become need of the modern civilization. In this article recycling of the following waste materials from constriction is discussed.
- Concrete Rubble
- Crusher Dust
- Fly Ash
- Silica Fume
- Spent Foundry Sand
- Blast Furnace Slag
- Red mud
Recycling Waste Concrete
Every year millions of tonnes of concrete rubble is generated in this world due to various reasons such as:
(a) Destructions of cities during the wars and earthquakes.
(b) Rejected precast concrete elements.
(c) Testing of concrete cubes.
(d) Concrete pavements reaching the end of their useful life
It is estimated that in USA itself approximately 150 million tonnes of concrete rubble are generated annually. Earlier disposal of concrete rubble was for filling landfills like borrow pits of soils or depleted quarries. But environmental consciousness, depletion of quarry aggregate and economical consideration have made it necessary to reuse concrete rubble. The research in this field started immediately after second world war in Europe. Nowadays from many concrete laboratories in India also encouraging results have been reported. Concrete rubble is mainly recycled as aggregate for new concrete.
The recycle aggregate has slightly less specific gravity and has more absorption value. Impact value and crushing values are also reduced. The strength of recycled aggregate concrete is about 10 to 15 per cent less as compared to concrete with fresh aggregate. However suitable mix designs may be made and reliable results obtained. The mix requires slightly higher quantity of cement or using admixtures to reduce water requirement. Recycled aggregate concrete can be safely used as plain concrete. With proper corrections in mix design, it can be used for R.C.C. works also.
Recycling Crusher Dust
Crusher dust is the by-product of crushing of rock to obtain coarse aggregates/jelly for concrete. The disposal of this dust is a serious environmental problem. If it is possible to use this crusher dust in production of concrete and mortar by partial or full replacement of natural sand, then this will not only save the cost of construction but at the same time it will solve the problem of disposal of the crushed dust. The researchers have reported that replacement of natural sand with crusher dust results into reduction in workability which can be compensated by using chemical admixtures. The test results reveal:
- Replacement of natural sand by crusher dust is not detrimental upto 50%.
- Concrete with 100% crusher dust can be used with greater precaution. In such concrete durability requirement is not met in stipulated 28 days. However, incorporating fly ash can enhance the durability characteristics.
- Combination of crusher dust and fly ash is beneficial in cost and durability aspects.
Use of Fly Ash
Fly ash is the by-product from thermal plants. The thermal plant owners take all care to prevent it from flying in the air and dump it in wet ponds. Till 15-20 years ago it was treated as a waste and disposal as a serious problem. This gave rise to research to utilise fly ash. Now it is found that it is a useful material in the following works:
- It is a dependable resource material for brick production. 40 to 50 per cent fly ash can be used with 25 to 30 per cent sand, 10 to 15 per cent lime and 10-15 per cent gypsum to produce strong and durable bricks.
- Adding 15 to 30 per cent fly ash in cement, blended cement is produced. The blended cement has got very good weather resisting capacity and hence it is more durable than ordinary Portland cement. Hence use of blended cement is becoming more popular than ordinary Portland cement.
- In recent years high-performance high-volume fly ash concrete has emerged that incorporates a large volume of fly ash (HVFA) into conventional Portland cement. Fly ash used is 40 to 50%. Use of fly ash reduces heat generation during setting of cement. Use of HVFA makes concrete pavements cost effective.
This concrete takes more time to gain full strength (120 days compared to 28 days by ordinary concrete) and needs small quantity of superplasticizer (chemical to improve workability). Nowadays demand for fly ash has increased so much that there is no problem of disposing but thermal power stations can sell it to nominal price.
Silica fume is a by-product from the silicon, ferro-silicon manufacturing process from quartz and carbon in electric furnace. It is in the form of extremely fine spherical particles. Before 1990 it was viewed as factory waste. But in 1987, silica fume concrete was used for Deepak Fertilizers It was discovered that it has beneficiary effect on concrete, including increase in strength and durability. Now nearly all major projects are using High Performance Silica Fume concrete. Some of the important projects where silica fume is used are Tehri Dam, Bandra Worli Sea Link, Nuclear Power Station, Kaiga, Karwar, Mumbai to Poona express way.
Replacing cement by 12.5 per cent silica fume has the maximum advantage in increasing the strength. As a result of increase in its demand, now silica fume is expensive. However, when analysed against the cost of an alternative concrete of similar performance it is found to be economical also. This conclusion has been drawn from the cost analysis of Mumbai-Poona express way.
Spent Foundry Sand
The most common type of casting process in the foundry industry is known as sand casting. The sand used for preparing the moulds is known as foundry sand. Molten metal is passed into the mould and allowed to cool. After cooling, the mould is broken away from the metal piece in a process called shake out. These broken pieces of mould which consists of slag, wastes etc. is called spent foundry sand. This spent foundry sand is posing a threat to the environment and needs safe disposal. To reduce this environmental pollution, this spent foundry sand can be used as part of concrete.
It has been found that if 10% of natural sand is replaced with spent foundry sand compressive strength of concrete increases by 3%. More than 10% replacement result into reduction in concrete strength.
Blast Furnace Slag
The blast furnace slag which is a by-product in the manufacture of pig iron is converted into foamed/expanded slag by tripping steam while cooling process is on. Industries have come up near the steel mills to manufacture readymade building blocks and partition wall panels using blast furnace slag. Ground Granulated Blast Furnace Slag (GGBFS) from cement industries are used as replacement of cement.
It is found that only 5% of strength is lost, if 40% of cement is replaced by GGBFS. However, in this case strength gained is slow. It takes about 60 days to get almost full strength compared to 28 days taken by ordinary cement concrete. However, it is the best means of recycling this industrial waste.
Red mud is a waste material obtained from aluminium plants causing environmental pollution and disposal problems. For every tonne of alumina produced equal quantity of hazardous, highly alkaline red mud is produced, which mainly consists of hydrous hydrated oxides of alumina, titanium, silica and alkalis’ process has been developed by CBRI Roorkee utilising red mud and fly ash to manufacture bricks of various colours and texture. Compressive strength achieved is 40 N/mm2 (minimum compressive strength required being 3.1 N/mm2).
Water absorption of such bricks is only 12%. It has been reported that in mortars 10% and in concrete 5% of cement may be replaced to get better strength. In Taiwan red mud is used with PVC to produce red mud plastics (RMP). The corrugated roofing sheets manufactured have shown highly improved properties over PVC in terms of weather resistance.
Polythene is the environmental hazardous materials, since it is not getting dissolved in nature. While chemical engineers are busy in finding methods of dissolving it using chemical technology and reusing the polythene, civil engineers are trying to recycle it as building materials. Some success has been reported in using polythene as damp proof material in building low-cost houses.
Major breakthrough is likely in using it as a useful material in road works. Polythene cleaned and cut into small pieces is mixed with tar and is used in about 600 km of roads in and around Bangalore. Results are encouraging. Roads laid in 2002 are showing good performance. It is expected that durability of such roads is twice that of tar roads without using polythene.