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close this bookGlobal Overview of Construction Technology Trends: Energy Efficiency in Construction (HABITAT, 1995, 210 p.)
close this folder4. Innovative technologies related to recycling of materials
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View the document4.1. Organic wastes
View the document4.2. Inorganic wastes

4.1. Organic wastes

Rice husk

Rice is cultivated as a major agricultural food crop in 75 countries of the world. About 400 million tons of paddy rice are produced annually in these countries. Based on various studies and research work, it is established that 1 ton of rice husk is generated from every 5 tons of paddy; thus, there should be about 80 million tons of rice husk available annually worldwide of which 64 million tons are produced in the Far East countries (33)

The increasing demand for rice by the growing populations in the rice-eating regions of the world creates an upward trend in the annual production of paddy rice. Due to the improvement in the milling process, there is also an expected increase in the amount of rice husk to be generated. The disposal of this low value by-product-rice husk-will continue to pose as a problem to the 75 countries where rice is grown. Apart from the construction potentials of rice husk its conversion into ash cement is, therefore, a better alternative to the present-day dumping and burning methods of disposing it 33. Table 1 shows rice husk availability in some African Countries.

Table 1. Rice husk availability in some African countries

Countries with 10.000 tons/year
(or more)

Estimated quantity
(1.000 tons)

Countries with 10.000 tons/year
(or more)

Estimated quantity
(1.000 tons)

Egypt

470

Zaire

49

Madagascar

422

United Rep. of Tanzania

43

Nigeria

218

Mali

25

Sierra Leone

103

Mozambique

14

Cd’Ivoire

102

Senegal

12

Guinea

70

Ghana

12

Liberia

49

Malawi

8

Source: UNCHS (Habitat) and Commonwealth Science Council (CSC), Journal of the Network of African Countries on Local Building Materials and Technologies Vol. 1 No. 1, April 1989


Figure 1. Demonstration unit of incinerator installed in Dhaka, Bangladesh. Courtesy UNDP/UNIDO, RENAS-BMTCS, Manila

Apart from its potential use as low grade fuel, insulation material and filler, rice husks can be used (after having incinerated to produce reactive ash) as a pozzolana to replace, partially, cement and also to be used in block-making.

In view of its abundant availability, many research institutions have undertaken considerable research over the last two decades, and the results and findings have been well documented. Table 2 gives an example of results of the average compressive strengths of mortar cubes made of Portland rice-husk ash cements.

Table 2. Average compressive strength of the RHA/OPC mortar cubes

Sample No.

Composition of the cementitious material
(percentage)

Number of specimens tested

Age
(days)

Compressive strength

Remarks

C1

100 OPC

30

7

12.4

Control samples 100 per cent OPC

P/C/10

10 RHA
90 OPC

60

7

14.3

115.3 per cent of controlled samples strength

P/C/20

20 RHA
80 OPC

60

7

12.9

104.0 per cent of controlled samples strength

P/C/30

30 RHA
70 OPC

60

7

11.7

94.0 per cent of controlled sample strength

P/C/40

40 RHA
60 OPC

60

7

10.5

84.7 per cent of controlled strength

P/C/50

50 RHA
50 OPC

60

7

10.2

82.3 per cent of controlled strength

A/L/30

30 RHA
70 Lime

90

7

8.2

Samples were at 50°C for 4 days

A/L/40

40 RHS
60 Lime

90

7

10.2

Strength for such accelerated curing is equivalent to the strength of 28 days normal curing

C1

100 OPC

30

28

17.85

Control samples 100 per cent OPC

P/C/10

10 RHA
90 OPC

60

28

19.41

108.7 per cent of controlled samples strength

P/C/20

20 RHA
80 OPC

60

28

16.88

94.6 per cent controlled samples strength

P/C/30

30 RHA
70 OPC

60

28

15.33

81.1 per cent of controlled samples strength

P/C/40

40 RHA
60 OPC

60

28

14.29

80.1 per cent of controlled samples strength

P/C/50

50 RHA
50 OPC

60

28

12.24

68.6 per cent of controlled samples strength

Source: UNCHS (Habitat), Endogenous Capacity Building for the Production of Binding Materials in the Construction Industry - Selected Case Studies, reference No. 23

Besides the use of rice-husk ash for producing blended cements and for block-making, a recent research work has proved that rice-husk ash can be used in manufacturing sodium silicate (waterglass) solution. Waterglass can be used as water proofing agent if applied, as a paint to the external walls, foundation plinths, etc. It has also been used in the manufacture of adobe blocks to increase the durability of walls. For detailed treatment of this subject refer to the UNCHS (Habitat), Journal of the Network of African Countries on Local Building Materials and Technologies, Volume 1 No. 1, 1989 and Volume 2. No. 1 June 1992.

Natural fibres

Natural fibres such as asbestos, sisal, hemp, kenaf, coir, bamboo, and begasse are natural products, which, with no or very little processing, can be used in building materials. The most significant use of these fibres is in the production of roofing tiles or sheets - fibre concrete roofing (FCR). Among the above mentioned fibres, asbestos is the most attractive one, which until recently was extensively used for producing corrugated and flat roofing sheets and tiles, pipes etc.

However, the serious health risks (lung cancer) associated with mining and processing asbestos have led to the gradual abandoning of this material and replacing it with other fibres.

In the absence of asbestos, other fibres such as sisal, coir (coconut fibre), Jute etc. are attracting more attention in the FCR technology and among them the sisal has proved to be the most suitable one for small-scale FCR technology. Fibres are chopped to lengths of 15 to 40 mm and added to a mortar of cement sand. A 6 to 10 mm thick layer is laid onto a flat surface for being vibrated and then placed on a mould (corrugated) for drying. After drying, tiles are removed from the mould and water cured for at least 2 weeks.

A high proportion of energy required for the manufacture of FCR tiles is the amount of energy which is embodied in the cement used. The attractiveness of FCR-tiles, vis-a-vis concrete tiles, obviously lies in its thickness. An FCR-tile has an average thickness of 7 mm, whereas a concrete rooftile should be at least 12 mm thick. Table 3 shows some comparative energy requirements of four common roofing elements. As it can be seen, a standard concrete roof tile requires about 60 per cent more energy than FCR tile and a corrugated-iron sheet would require almost 13 times more energy than FCR-tiles to cover the same roof area.

Table 3. Comparative energy requirements of alternative roofing assemblies for a pitched roof

Roof assembly

Embodied energy requirements
(MJ/m2)

Corrugated-iron sheets on timber

605

Clay tiles on timber

158

Concrete tiles (12.5 mm)

72

Fibre-concrete tiles (7 mm)

46

Source: UNCHS (Habitat), Development of National Technological Capacity for Environmentally-Sound Construction, reference No. 34

Coconut wastes include fresh husks, coconut shells and waste from the coir industry. The husk consist of 15 - 35 cm long fibres (about 60 per cent 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 along as they are kept dry. During the retting process (softening by soaking in water) they become inert. The difference in reactivity between retted and fresh husks necessitates different methods of conversion into building materials.

Unretted husks, hot-pressed (at 150°C, 1 MP a pressure for 15 to 25 minutes) without any additives, produce strong particle boards. Unretted pith, obtained by defibrating mature husks, hot-pressed without additives, produces strong, moisture resistant boards. Lighter, resilient boards are made in the same way, but with addition of retted pith (low density, highly-elastic granular material)

The incineration of unretted pith produces an ash, which has pozzolanic characteristics, similar to those of rice husk ash, and can be used for making lime-pozzolana binders.

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. Its 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 (fibres 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. (This process, developed at CBRI, Roorkee, requires a hydraulic press and drag mould, together with a set of forms, in which the cast sheets are kept pressed for 4 hours, before demoulding and curing).

Coconut shell chips and conventional adhesives make good quality particle boards.

Coconut shell tar, obtained during the destructive distillation of the shell, is slightly viscous liquid with anti-microbial properties(35).