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Malawi: The use of rice-husk and bagasse ash as building material*

* By Uffe Leinum, Architect/planner, building materials and low-cost

In Malawi, about 45,000 tons of rice are milled every season but only a small proportion of the remaining husks is used by some of the large industrial concerns as fuel energy, and, thereafter, the rice-husk ash is usually discarded as a waste product. It is already common knowledge that rice-husk ash contains the highest proportion of silica compared with that derived from any other ashed natural plant material, and it is thus, an immediately available asset and under-developed source of wealth.

Approximately 8500 tons of high-silica rice husks are obtainable each year from three ricemills, namely Blantyre, processing 13,000 tons of milled rice, Karonga, 6000 tons, and Nkhotakota, a further 6000 tons. Yet this potential source, for use in building-materials industry, has received very little attention within the economy. In Malawi, a continuing study of this naturally occurring waste material has been carried out over several years, and a low-cost plant capable of manufacturing sodium silicate (waterglass) solution from rice-husk ash was designed and built in the laboratory. One of the prototype installations is at present being used in conjunction with work on a large low-cost housing project financially supported by the Danchurchaid and the German Development Assistance Association for Social Housing (DESWOS). Sodium silicate (waterglass) solution can be shown to have a considerable number of advantages when applied to building materials, and may eventually help to increase durability and lower the costs of low-cost housing construction throughout the country.

Cement

Cement has proved to be a very expensive commodity for use in many recent low-cost housing programmes in Malawi, 50 kg is priced at MK 29.00 (1992 price). It is estimated that 1800 tons of ash could be produced annually in relation to the present milling capacity of approximately 45,000 tons of rice, as 1 ton of rice yields about 40 kg of ash as a by-product. However, because of the limited quantities which would be readily available, it was decided that any ash that was produced should be used for the manufacture of sodium-silicate (waterglass) solution rather than dealt with as an extender for cement, owing to the fact that the former product could have a much more extensive application in the construction industry.

Waterproofing liquid

liquid coating of sodium-silicate (waterglass) solution dries into a hard vitreous film with a “varnished” type finish that is both protective of the outward appearance and water-resistant. An application to the surface of porous concrete, burnt bricks, compressed soil and sun-dried building blocks produces an additional hardness and renders the material waterproof. In addition, this material is suitable for preserving thatched roofs and also gives considerable protection against fire in existing buildings.

Bricks

It has proved practicable to mix rice husks with clay in order to produce strong and more resistant burnt bricks or blocks, and bricks and blocks containing this mixture have been found to be very suitable for use in the construction of foundations and floors. As the quality of clay differs considerably from place to place, a test of the mixture should be carried out at the specific locality prior to manufacture, although reasonable results have usually been obtained when incorporating 10 per cent rice husk by volume. Both burnt bricks and sun-dried blocks are easily manufactured locally, but it must always be borne in mind that a considerable amount of firewood is used for firing - in fact, approximately 1 cubic metre for every 1000 bricks produced, which puts an additional stress on the economy of a rural community, as wood is inevitably used extensively for cooking. Because of the urgent need to conserve these finite resources, the use of burnt bricks is recommended mainly for foundation works and where the external walls tend to suffer from extreme exposure to rain. However, they may be considered in small quantities for decorative purposes, such as below windows, a detail which has been incorporated in existing designs for rural low-cost houses recently built in Malawi.

Figure 1. When applied as a paint coat, the sodium-silicate sludge has been found to he extremely useful for foundation plinths (Habitat)

Paint and adhesive

A sodium-silicate (waterglass) solution can be used as a protective coating or paint for solid walls, with the addition of extenders, such as limestone, clay or iron oxide. The decorative finish achieved by this process has proved to be considerably cheaper than any surface treatments obtained by using imported paints or waterproofing. The liquid also has the properties of a general adhesive and can be used as a gluing agent for metal, wood and concrete products, such as roofing tiles, as well as being capable of fixing various glass and ceramic materials.

Varnish

Where the groundwater table is very high, the application of a sodium-silicate (waterglass) solution on bricks or concrete will quite satisfactorily substitute for a waterproof membrane and inhibit rising damp. At the same time, it provides a decorative surface finish which is both hard-wearing and easy to keep clean.

Adobe plaster for external walls

As a result of tests, it has been proved that the addition of between 0.5 per cent and 1 per cent sodium-silicate (waterglass) solution in the mixing water used for the preparation of mud plaster produces a hard surface finish, but, in order to make the facing material totally resistant to driving rain, a further coat of solution is required. This also acts as a good undercoat, prior to decoration with a “home-made” product - calcium stearate/cement paint.

Adobe blocks and stabilized sun-dried block walls

For centuries, soil has been used as a basic material for the construction of walls throughout the civilized world, and these will often last indefinitely, provided they receive proper maintenance and preservation. In rural areas of Malawi, the use of adobe blocks for walls is quite common, and these structures, built out of soil in the shape of sun-dried building blocks, have adequate strength under any conditions and can be very durable, in spite of rain and/or a humid climate, if supported on a good foundation. An additional design factor that can be incorporated is the provision of a generous overhang to the roof. In order to withstand a humid atmosphere, an economical means of giving protection to the sun-dried block is to stabilize the soil content with a diluted sodium-silicate (waterglass) solution. This process enables an unskilled labour force to cast small and comparatively cheap sun-dried building components, without the conventional use of either cement or lime additives, both of which are expensive to purchase. The compressive strength of each block can be doubled by the addition of 0.5 per cent sodium-silicate (waterglass) solution to the mixing water. Walls can be either plastered with mud plaster which has had incorporated between 0.5 and 1 per cent sodium-silicate solution as a stabilizer, and/or then decorated by means of a protective coating consisting of calcium stearate/cement paint combined with a sodium-silicate (waterglass) solution.

Figure 2. Application of waterproof cement paint on adobe wall (Chikwawa, Malawi).

The recommended procedure to be adopted prior to decoration is as follows:

(a) One priming coat consisting of sodium-silicate (water-glass) solution to cover the mud-plaster surface;

(b) One finishing coat (coloured if required) of calcium stearate/cement paint. The cement paint is rendered much more durable if it is protected with further application of sodium silicate.

Calcium stearate/cement paint

Although materials such as waterproof paint are available for use on mud-brick/block walls, they are usually produced with imported elements. By using materials which are readily at hand in the locality and then made up in accordance with the formula developed by Uffe Leinum, consultant architect to the Christian Service Committee of the Churches in Malawi, overall costs can be greatly reduced. A programme consisting of the construction of five demonstration houses was undertaken, with a recommendation for on-site manufacture of waterproof cement paint. The water-repellent paint contains the following essential ingredients: Portland cement, hydrated lime, calcium stearate (a fat coated with lime) and rice-husk ash.

Calculations relating to the approximate cost of 10 kg of paint with an optimum covering capacity of 20 square metres are as follows:

A mixture of home-made waterproof paint weighing 1 kg would be expected to cover 2 square metres of wall surface at a nominal thickness of 0.5 mm. However, if an initial application of sodium-silicate (waterglass) solution is used as a priming coat, coverage of up to 4 square metres of wall surface should be achieved. A comparison with the cost of equivalent commercial products will show a net saving of 80 per cent even when a relatively expensive pigment is incorporated as a colouring agent.

Sugarcane (bagasse) as a waterproofing agent

Adjacent to one of the selected development areas in the Southern Region, the Sugar Corporation of Malawi (SUCOMA) has a large plant for the production of refined sugar and sugar by-products. Investigations have already confirmed that the wax obtained from sugarcane has excellent waterproofing properties and, if prepared, is suitable for external application to walls. Molasses is known to be an efficient binder for clay and soil. It also has a comparatively long-lasting waterproofing effect on these products, as well as endowing the clay/molasses mixture with additional compressive strength. Elsewhere, the sugarcane crop is already being converted into a wide range of products, from monosodium glutamate for the food industry to hardboard for furniture; but the possible diversification of sugarcane waste into an increased supply of bagasse (the cane fibre), which remains after the sugar-extraction process has been completed, appears to be particularly advantageous.

Given the escalating costs of fuel and its scarcity, using any local asset, such as bagasse, is becoming increasingly relevant and important. The ash produced after burning the bagasse has a comparatively high silica content, although somewhat less than rice-husk ash, but it is equally suitable and can be used quite economically in producing sodium-silicate (waterglass) solution for general use in the building industry. Sugar production is increasing to an extent that an estimated 480,000 tons of bagasse might soon become available. This amount would yield between 16,800 and 24,000 tons of ash per annum after it had been used as fuel/energy.

Figure 3. Storage of bagasse for fuel (Sugoma, Malawi)

Figure 4. Diagram of sodium-silicate plant

Production of sodium silicate (waterglass)

Production of sodium-silicate (waterglass) solution is at present being carried out at a materials-production industries' centre attached to a large low-cost rural housing programme in the Chikwawa district of Malawi, where the basic raw materials are waste rice-husk ash and bagasse. Most of the rice-husk ash for the project has been obtained from a large industrial plant, after it had been used with other combustible materials, such as wood and cotton waste, for fuel/energy. This specific mixture of substances prior to firing has resulted, in the presence of black carbon, which settles as “sludge” after the manufacturing process is completed to provide an extremely useful material for applying as a paint coat to brick foundation plinths. It is a completely satisfactory and cheap alternative to the use of cement rendering or imported bituminous paint.

The whole operation maximizes the use of a surplus material to advantage, as well as eliminating all traces of waste, although if pure, non-contaminated rice-husk ash is used, the “sludge” left behind has proved to be minimal. Unfortunately, caustic soda is still required for the manufacturing process - an imported raw material costing currently about MK 6.50 per kg (1992 price).

In order to produce a marketable quality of sodium-silicate (waterglass) solution, the various ingredients are introduced into the reactor vessel (drum): caustic soda, rice-husk ash as industrial waste, and water, in predetermined and controlled proportions. The production time required for obtaining 200 litres of material from one reactor vessel, which includes preparation and lighting up the boiler, cooking and cooling periods, complete with tapping off, is estimated at approximately 24 hours. However, a small-scale plant combining several production vessels connected to one boiler, could yield up to 1000 litres in a day (see figure 4).

Basic design for processing plant

Either rice-husk or bagasse ash is fed into the reactor vessel, where steam from the boiler can be led into the container. After the correct amount of caustic soda and water has been added to the ash deposit, the reactor vessel is subjected to the boiling process for approximately one hour. The resultant liquid can then be cooled, filtered and placed in containers ready for use on site.

Estimated manufacturing cost of filtered waterglass

Therefore, the estimated costs for sodium-silicate (waterglass) solution will be:

Figure 5. Sodium-silicate (waterglass) plant (Chikwawa, Malawi). Patent application has been filed

These calculations are based on the use of a single production vessel of 200 litres capacity and, therefore, present the worst example financially, as normally a multiple unit would be engaged in this operation. Production levels can then quite easily be brought up to five times the former amount, at the same fixed costs and workforce level. The paint coverage that can be anticipated for concrete floors, sun-dried blocks or burnt brick walls is about 14 square metres per litre, but a two-coat application is recommended for all sun-dried block-work, at an estimated cost of MK 0.10 per square metre.

Safety factors

1. When burning rice husks in large quantities, it is very difficult to determine if combustion is occurring at the centre of the pile. Because of this particular factor, a safety fence should be erected around the bonfire site, in order to permanently restrict access by children playing in the area.

2. Rubber gloves should always be worn for weighing out the caustic soda, as any contact with the skin is harmful.

3. The reactor vessel must be fitted with a lid, as, during the initial 20 minutes, the substances are alkaline and should not be allowed contact with the eyes. When the final stages of the manufacturing process have been reached, the mixture is only as caustic as lime. Soap and water should be used for cleaning skin surfaces.

Figure 6. Boiling calcium strearate for paint (Chikwawa, Malawi).

Figure 7. Reactor vessel

Figure 8. Boiler unit

Figure 9. Construction of a chimney stack/boiler roof (Georgetown, Guyana).

Figure 10. Steam heater and boiler unit under construction (Georgetown, Guyana).

Note

The sodium-silicate solutions described in this report are inorganic, non-inflammable, non-explosive, non-toxic and are not regarded as hazardous chemical substances. It is partly the relatively low environmental risks, which are already associated with soluble silicates, that account for their ever-increasing world-wide application.