WO1999006337A1

WO1999006337A1 - Composite materials

Info

Publication number: WO1999006337A1
Application number: PCT/EP1998/004990
Authority: WO
Inventors: Johan Human
Original assignee: Construction Research 2000 Ltd.
Priority date: 1997-07-30
Filing date: 1998-07-28
Publication date: 1999-02-11
Also published as: SE9702833D0; AU9072398A; SE9702833L; GB9907252D0; SE509781C2

Abstract

A construction element formed of a composite material comprising a binding component including cement and/or plaster and plant fibres treated by an abrasive process to improve their adhesion to the cement and/or plaster. Use of a drum mixer for such a treatment.

Description

COMPOSITE MATERIALS

This invention relates to composite materials and articles made from such materials. One use for such materials is in the manufacture of construction elements such as composite boards.

The interior living environment in many places around the world can be unpleasant: often too hot or cold and in many cases too humid. One cause of such a poor interior climate is the inferior insulation properties of many walls and roofs.

Different techniques and materials have been tried to achieve better insulation, such as wood-wool panels, cement wood-wool and mud/straw composites. A report from United Nations Expert Working Group Meeting in Vienna 1970 (UNIDO 1972) describes surplus organic fibre products that, in combination with cement, were tested as construction materials; examples were straw, rice, cotton stems, corn stems and hemp. However, there have been problems of curing with such materials, as will be described below.

Plant straw, for example wheat straw, is normally protected by a thin outer layer of waxy carbohydrate material approximately 0.01 mm thick. The wax is typically structured around a large molecule of 60 carbon-12 atoms. Without special cleansing methods, the wax prevents the straw from adhering to cement. Additionally, this thick layer of carbohydrate can impair the curing of cement slurry containing such straw. Therefore, it has been recognised that to overcome the curing problem measures must be taken to remove the waxy coating.

These measures have in the past involved chemical processes. In the manufacturing of straw-based cement products, the straw is traditionally pre-processed by soaking in a CaCI₂ solution of approximately 5% (by mass), for 24 to 48 hours, in order to reduce the carbohydrates' impairment to adhesion with the cement mixture. The proportion by mass of straw to solution is usually around 1 to 10. The CaCI₂ solution can be re-used 2 to 3 times. There are several other techniques used for the removal of the carbohydrate coating, for example: a) boiling water with 1% citric acid; b) boiling water with NaCI or CaCI₂. The disadvantage with the use of acids is that the plant fibre must be rinsed with water and the citric acid must be neutralised with lime. If one uses NaCI or CaCk, the boiling time is 2 to 10 minutes. It is a costly and inefficient procedure to use chemical substances as the cleansing agents for this purpose.

Some examples of prior methods for forming composite materials will now be described.

Rice is the most common crop in the world. The chaff of rice has been developed for use as an insulation material in cement mixtures at the Instituto E. Torroja de la Construccion y del Cemento, Madrid, Spain (Salas et al. 1986). The best result was achieved when the chaff was soaked in a 5% lime solution for 24 hours.

Wheat is the second most common crop in the world after rice. Wheat straw has been processed with CaCI₂ as described above and then mixed with cement and water. Modules have then been filled with cement and straw mix; these take 24 hours to cure. The modules may be stored horizontally or vertically. Straw of wheat has also been utilised to form a building material in a manner where it was initially impregnated to prevent it from rotting and then cast in a cement slurry. There are examples in the USA of structures formed with such materials. Straw has also been used for other related purposes, such as raw material for the manufacturing of boards, an example being the Swedish product STRAMIT, that was manufactured from cleansed straw which was impregnated to resist decay and then pressed to boards under a temperature of 280° centigrade.

Wood is used to produce wood-wool board. The wood is submerged in a CaCI₂ solution before being mixed with cement and water under gentle stirring. The mixture is continually filled into modules manufactured of Masonite. These modules are stacked on top of each other, in groups of approximately 20, and are pressed together under a weight on top of the module. After around 24 hours, the wood-wool board is removed from its module. Lumber in many countries is not a suitable building material due to the climate and parasites. Also, in the manufacturing of cement-based construction elements containing plant fibres, e.g. wood-wool, with present technology the production time for curing is unacceptably long, which results in low productivity. However, materials made from wood products such as chipboard or particle boards are simple to use because it is possible to use simple hand tools to assemble and tailor the material to the building construction.

Another building material that is used is gypsum board, but this is difficult to handle, is moisture absorbent and loses its strength when wet.

There is a huge demand for techniques to permit the manufacture of low-cost housing with simple technology, local labour and labour-intensive, rather than machine-intensive methods. There is a need for a material suitable for construction boards that is easily and efficiently manufactured.

Wheat straw and the like are practically-speaking free raw materials in many developing countries, but at present they are difficult to refine to a construction element of acceptable load bearing capacity.

According to a first aspect of the present invention there is provided a construction element formed of a composite material comprising: a binding component including cement and/or plaster; and plant fibres treated by an abrasive process to improve their adhesion to the cement and/or plaster.

According to a second aspect of the present invention there is provided a method for forming a construction element, comprising: treating plant fibres by an abrasive process to at least partially remove a layer from their outer surface; forming a mixture of the treated plant fibres with a binding component including cement and/or plaster; and curing the mixture to form the construction element.

The construction element may suitably take the form of, for example, a board, a pillar, a beam or a column. The construction element may be a structural element and/or an insulating element, for example for thermal or noise insulation. The abrasive process suitably comprises tumbling raw or part-processed plant fibres, preferably together with an abrasive material. The abrasive material may be a particulate material, such as sand, which is preferably loose during the tumbling process. A liquid such as water may also be present during the tumbling step. The tumbling may be performed in a rotating drum. The speed of rotation of the drum is preferably in the range from 1 to 100 revolutions per minute (preferably around 50 revolutions per minute), and the duration of the tumbling is suitably in the range from 1 to 50 minutes (preferably from 1 to 20 or from 5 to 20 or around 20 minutes). The tumbling step suitably cleanses or partially cleanses the plant fibres. The tumbling step is preferably such as to remove or substantially remove any waxy coating or membrane (which may principally comprise carbohydrates) from the surface of the plant fibres, to improve their adhesion to the binding component.

After the tumbling step the treated plant fibres are preferably separated out, e.g. from the abrasive material, for instance by draining. This draining step, where performed, preferably lasts for a period in the range from 1 to 20 minutes.

The treated plant fibres are suitably mixed with the binding component and one or more further components. One such further component of the mixture may be a liquid such as water. Another such further component may be an accelerator for accelerating curing of the mixture, such as calcium chloride. The plant fibres suitably constitute 5 to 50% by mass of the mixture, preferably 5 to 30% by mass. The accelerator, where present, preferably constitutes from 1 to 5% by mass of the mixture (preferably around 3% by mass).

The density of the composite material is suitably in the range from 75 to 600 kg/m³.

The plant fibres may include or be derived from husks and/or straw, of one or more of rice, wheat, hemp, jute and coconut. Other plant fibres could be used. The plant fibres preferably act to reinforce the composite material after curing.

After the mixture including the treated plant fibres has been formed it is preferably placed in a mould during at least part of the curing step. The mould is preferably shaped so as to form the composite material into the desired shape of the construction element. The mixture is preferably subjected to pressure during the curing step. The material is preferably not heated during the curing step.

The present invention will now be described by way of example, without limitation to the generality of the invention.

This example describes the formation of a building element from a composite material that comprises plant fibres. Examples of suitable plant fibres include husks and straw of plants such as rice, wheat, hemp, jute and coconut. In this example wheat straw is used.

The wheat straw is firstly processed by abrading its surface so as to partially or fully remove any detrimental outer coating. In this example the wheat straw is processed by mixing with water and quartz sand in a rotating or tombola mixer for 20 minutes at 50 rev/min. The rotating mixer is a cylindrical container with an openable hatch at one end. The mixer may be freely rotated about axles attached at either end. In this way, the wax-like coating or membrane on the surface of the straw fibres, which consists mainly of carbohydrates, is removed or partially removed by the abrasive polishing action of the sand, due to the tumbling motion in the mixer. Instead of sand, other materials (preferably hard and/or sharp particulate materials) may be used. Examples are aggregates, preferably minerals in powder form, or other material of sintered or molten products such as slag or glass. The material can be waste, especially waste from an industrial process. The mixing time and speed of rotation of the mixer may be varied widely to achieve the desired effect.

After mixing, the mixture is drained for 20 minutes to separate the straw from the water and sand. The draining time may be varied considerably to achieve the desired effect, and other separation processes may be used instead.

The processed straw is then mixed with cement, calcium chloride (as an accelerator) and water in a rotating mixer for 20 minutes at 50 rev/min to produce a slurry which contains 3% by mass of accelerator and between 5 and 50% by mass of straw. The cement/water ratio can be set as needed: typical ratios are in the range from 1:1 to 1 :2. The purpose of the accelerator is to reduce the curing time of the slurry.

When the slurry is well mixed it is poured into a mould which has the form of the construction element required, which could for example be a board or a pillar, and left to cure, preferably under pressure. For example, to form a construction board the slurry can be poured or forced into a wooden mould with a flat bottom surface and side walls defining the same length and width as the board required. The upper surface of the slurry is then put under pressure, e.g. by means of a press or weights, as the slurry cures. A suitable pressure is approximately 5kN/m². The amount of cement in the slurry can be substantially reduced if the press used for the compression of the slurry is powerful enough. When the board reaches a strength at which it can be handled, which for the composition described above is after approximately 24 hours, the board produced from the cured slurry is removed from the wood mould and stored for a week (to cure and harden further and reach its full strength) before being used as a building component.

The pressure applied to the curing slurry can be varied depending on the exact composition of the slurry and the final thickness of board required. The pressure could be omitted and the slurry levelled to the height of the top of the wood mould so it is freely cast.

The mould can usefully be treated to reduce the adhesion of the contents of the mould to it. For example, a wooden mould can be oiled.

The board has good construction properties. The bond between the cement and the straw is strong as a result of the pre-processing of the straw.

The accelerator may be omitted but the curing time is then increased.

Instead of cement (or other cementitious materials), plaster or a mixture of two or more such materials may be used in the slurry. The cement and/or plaster composite material can be used for manufacturing numerous different forms of construction parts for numerous different uses. The exact composition of the slurry can be varied depending on the intended use and form of the construction part, so as to tailor the properties of the composite to the intended function.

In load bearing structures such as beams and columns, the plant fibres suitably comprise 5-15% by mass of the composite slurry.

In facade elements, the plant fibres suitably comprise 15-30% by mass of the composite slurry.

In building elements where the main use is for heat- insulation or sound-absorption, the plant fibres suitably comprise 15-30% by mass of the composite slurry.

The present invention may include any feature or combination of features disclosed herein either implicitly or explicitly or any generalisation thereof, irrespective of whether it relates to the presently claimed invention. In view of the foregoing description it will be evident to a person skilled in the art that various modifications may be made within the scope of the invention.

Claims

1. A construction element formed of a composite material comprising: a binding component including cement and/or plaster; and plant fibres treated by an abrasive process to improve their adhesion to the cement and/or plaster.

2. A construction element as claimed in claim 1 , wherein the density of the composite material is in the range from 75 to 600 kg/m³.

3. A construction element as claimed in claim 1 or 2, wherein the construction element is in the form of a board.

4. A construction element as claimed in any preceding claim, wherein the abrasive process comprises tumbling plant fibres together with a particulate material.

5. A construction element as claimed in any preceding claim, wherein the plant fibres include husks and/or straw of one or more of rice, wheat, hemp, jute and coconut.

6. A method for forming a construction element, comprising: treating plant fibres by an abrasive process to at least partially remove a layer from their outer surface; forming a mixture of the treated plant fibres with a binding component including cement and/or plaster; and curing the mixture to form the construction element.

7. A method as claimed in claim 6, wherein the said mixture further includes water.

8. A method as claimed in claim 7, wherein the plant fibres constitute 5 to 50% by mass of the mixture.

9. A method as claimed in any of claims 6 to 8, wherein the mixture further comprises an accelerator for accelerating curing of the mixture.

10. A method as claimed in claim 9, wherein the accelerator constitutes from 1 to 5% by mass of the mixture.

11. A method as claimed in any of claims 6 to 10, comprising placing the mixture in a mould during at least part of the curing step to form the shape of the construction element

12. A method as claimed in any of claims 6 to 11 , wherein the abrasive process comprises tumbling plant fibres together with a particulate material.

13. A method as claimed in claim 12, wherein the particulate material is sand.

14. A method as claimed in any of claim 12 or 13, wherein the tumbling is performed in a rotating drum.

15. A method as claimed in any of claims 12 to 14, wherein the plant fibres are tumbled for a period of 1 to 20 minutes.

16. A kit for treating plant fibres by an abrasive process to at least partially remove a layer from their outer surface, the kit comprising a drum mixer.

17. A kit as claimed in claim 16, comprising instructions for treating the plant fibres by the abrasive process to at least partially remove a layer from their outer surface.

18. Use of a drum mixer for treating plant fibres by an abrasive process to at least partially remove a layer from their outer surface.

19. A construction element substantially as herein described with reference to the accompanying drawings.

20. A method for forming a construction element, substantially as herein described with reference to the accompanying drawings.