The term "natural products" covers a wide range of stabilizers of animal, vegetable and mineral origin. The products on which we will concentrate are chose obtained directly from nature or those subject to a simple preparation process, rather than chose which are the product of involved processing, which are more like synthetic products, such as agricultural wastes. Accurate scientific information about such natural stabilizers, the use of which depends largely on traditional skills, is sparse and research into them virtually non-existent.
The main advantage of these stabilizers is their local availability. Despite this, they are rarely available in sufficient quantities to be used on anything more than an individual basis. On the other hand, because of their rarity and the fact that they are produced by very simple processes, their market value is higher than chat of industrial products. Often, they have other uses, such as for agricultural processes or even as food, so chat their use as stabilizers is not always a priority.
Conditions of use
These products are often only effective in very specific conditions of preparation and environment. Even so, they have certain advantages, for example by increasing water resistance and slowing deterioration. The water absorption of a treated wall will be slower than that of an untreated wall, allowing deterioration to be remedied (between two rainy seasons, for example), while an untreated wall would deteriorate very rapidly. It should be remembered chat natural products are not as effective as industrially manufactured products such as cement, lime, bitumen, and so on.
II. FIELD OF APPLICATION
Origin and uses of products
Some types of sands, silts and clays belong to this category, as a variety of special mineral deposits found only in certain regions. They are excavated in quarries or even directly on the building site.
These products can be produced by animals drawn from animals, or be produced by simple natural or artificial processes from their corpses.
These products can be drawn from plants or result from simple transformation processes.
Natural products, with the exception of a few products of geological origin, are in most cases not available to be used as a stabilizer within the mass of compressed earth blocks. In general their use therefore remains restricted to use as stabilizers for plasters and renders.
Their role varies widely: some are used to correct the grain size distribution of soils (for example, sands can be added to clays and vice versa); some give an attractive pigmentation and are sought after as decorative renders. Others help to stabilize earth mortars by making them water resistant, and are used to improve the durability of external renders. Some display considerable adhesion and prevent internal plasters from powdering.
Nature and applications of products
BENTONITE: This is added in small quantities to the soil being treated. This type of clay has powerful degreasing properties and expands in the presence of water, thus preventing the passage of water.
POZZOLONAS: Some soils (volcanic sands in particular) have natural pozzolanic properties. They can be used for stabilizing clayey soils but often require the addition of at least 25% lime or 50% cement to make them an effective binder which in turn will stabilize clay when added at a rate of about 8%. Pozzolana-based renders are, however, far more flexible than cement-based ones. They are often used for finishing flat earth-brickwork roofs and for vaulting.
EXCREMENT: All sorts of excrement are used. Cow dung is undoubtedly the most widely used, although it is better used as manure or fuel. Other traditions make use of horse or camel dung, or of pigeon droppings. The action of these kinds of excrement is probably due to the presence of fibre (mixed straw) phosphoric acid and potassium. The use of animal urine can also be found. Horse urine used to replace mixing water effectively eliminates cracking and results in a marked improvement in the ability of the soil to withstand erosion. Surprisingly good results can be obtained when it is combined with lime.
ANIMAL BLOOD: The use of bull's blood has been known since Roman times. When combined with lime or polyphenols, stabilization with cow's blood is effective. The blood must be fresh and not in powder form.
ANIMAL FUR AND HAIR: Animal hair and fur play much the same role as some vegetable fibres.
CASEIN: Proteinic casein (the middle fraction of milk protides) is sometimes used in stabilization in the form of whey combined with bull's blood. Certain milk powders have been tried and give good results. "Poulh's soap" is also used. Prior to being added to the soil, this diluted casein is beaten to a paste after first being mixed with brick dust.
LIME: Lime can be prepared from shells or coral. This is skill done in some countries such as Somalia and Senegal.
ANIMAL GLUES: These can be used for stabilization, particularly for renders. Animal glues are produced from horn, bone, hooves and hides.
TERMITE HILLS: Termites secrete an active substance, which appears to be a non-ionic cellulose polymer of the polysaccharide type. Termite hills stand up well to rain and their soil can be mixed with another for the production of blocks. The substance has been synthesized by research workers in South Africa, but costs three times as much as cement.
SHELLAC: Covering the twigs of trees in India and the Far East this is a resin secreted by the female lac-insects (Coccus lacca). It confers excellent strength on sandy soils, but the stabilized material does not stand up well to water.
OILS AND FATS: Fish oils and animal fats can serve as waterproofing agents. Similarly, the stearates contained in animal fats play the same role.
ASHES: Hardwood ash is rich in calcium carbonate and has stabilizing properties, but is not always suitable for soils which may be suited to lime stabilization. Ash additions is suggested in proportions of 5 to 10%. This improves dry compressive strength, but does not reduce sensitivity to water.
VEGETABLE OILS AND FATS: If these are to be effective, vegetable oils must dry quickly, so that they harden on contact with air and are insoluble in water. The use of castor oil is highly effective, but it is extremely expensive because it is also used in aviation. Coconut, cotton, and linseed oils are also used. Kapok oil prepared first by roasting kapok seeds, turning them into a flour which can then be transformed into a paste (20 to 25 1 of water to 10 kg of powder) can be effective. This depends on the quality of the seeds and the roasting process, which increases the yield, as well as the length of time it takes to prepare the paste (6 hours boiling). There is also palmitic oil which is obtained from saponified palm oil precipitated by 25% HCI. About 1 kg of palmitic acid is obtained per kg of palm soap in solution. Palmitic acid mixed with lime gives calcium palmitate, which is used in stabilizing renders. Shea (karite) oil or butter is also used for renders.
TANNINS: Tannins often act as dispersants and improve the coating of the grains by clays. They are also a good compaction acid (breaking up lumps) and reduce permeability. The amount of tannin required varies from a small percentage of the mixing water when tannin decoctions (solutions obtained by boiling) are used. The most commonly known are tannin from the bark of nere, oak, chestnut, and scorpioid acacia. The Hausa of Africa use the natural potash which accumulates in dying trenches, or an infusion of carob-bean husks, or even of mimosa, which the most wealthy import from Egypt.
HUMIC ACID OR POLYPHENOLS: These are derived from lignin (a by-product of the wood and paper industries) and form hard stable compounds, particularly in ferralitic soils.
SAP AND LATEXES: The juice of banana leaves precipitated with lime improves erosion resistance and slows water absorption. The latex of certain trees, such as euphorbia, reduces permeability slightly. The same applies to hevea rubber, and concentrated sisal juice in the form of organic glue. Latexes mix with acid soils (coagulation), but mix better with neutral soils.
GUM ARABIC: This is obtained from the acacia tree. Its ability to waterproof is low, because it is soluble in water. It acts primarily as a floccullent, helps to increase dry compressive strength, and slows the capillary absorption of water by acting on the kinetics of this phenomenon. When added to the soil, or even better to sand, gum arabic produces good protective coatings, which are hard, do not crack, and adhere well to earth walls. This product does not stand up well to water, hence it is better to use it on the inside of buildings. The colour obtained is a pastel red ochre. Gum arabic is used as a render chiefly in the Sudan, but is becoming increasingly expensive.
PALMO-COPAL: Copal is a resin obtained from certain tropical trees. Palmo-copal is a solution of copal obtained by heating palm oil. The amount required varies from 3 to 8% for sandy soils. Another variety, manilla copal, is the only copal resin which has a waterproofing capacity.
COLOPHANE This is obtained during the distillation of turpentine essences from oily pine resins. Soluble in organic solvents and in aqueous alkaline solutions, colophane resin forms a gel after reacting with certain metallic salts (iron and aluminium) and reduces the water absorption of soils.
VINSOL: This is also obtained during the production of turpentine. It is used in acid soils at critical control rates (+ 1%). Water repellent, it improves cohesion, but does not affect compressive strength.
LIGNIN: This is a by-product of the paper industry. It is a sort of alkaline resinous liquor with a waterproofing capacity. It is soluble and can become insoluble when reacting with chrome. Unfortunately, chromo-lignin is expensive.
MOLASSES: Sugar aldehydes from dehydrated molasses can be polymerized at high temperatures with phenolic catalysts. The resinous material obtained has characteristics similar to that of the naturally occurring asphalt and synthetic resins.