Soils that do not possess the desired characteristics for a particular construction can be improved by adding one or more stabilizers.
Each stabilizer can fulfil one (or at the most two) of the following functions:
• Increase the compressive strength and impact resistance of the soil construction, and also reduce its tendency to swell and shrink, by binding the particles of soil together.
• Reduce or completely exclude water absorption (causing swelling, shrinking and abrasion) by sealing all voids and pores, and covering the clay particles with a waterproofing film.
• Reduce cracking by imparting flexibility which allows the soil to expand and contract to some extent.
• Reduce excessive expansion and contraction by reinforcing the soil with fibrous material.
The effect of stabilization is usually increased when the soil is compacted. Sometimes compaction alone is sufficient to stabilize the soil, however, without an appropriate stabilizer, the effect may not be permanent, particularly in the case of increased exposure to water.
But, before considering the use of a stabilizer the following points must be investigated:
• Does the available soil satisfy the main requirements even without stabilization? This is largely dependent on the local climate, natural hazards and type of construction.
• Does the building design take into account the characteristics and limitations of the material? Building on a high level and incorporating damp-proof courses (to minimize damage by rising water) and providing wide roof overhangs (for protection against rain and solar radiation) are examples of appropriate design.
• Is the stabilization of the entire construction really necessary, or can a good surface protection (egg stabilized render) be sufficient?
By reducing the need for stabilization, considerable costs, time and effort can be saved.
Kinds of Stabilizers
A great number of substances may be used for soil stabilization, and much research is going on to find the most suitable stabilizer for each soil type. But, despite these research efforts, there is no "miracle" stabilizer that can be used in all cases. Stabilization is not an exact science, so that it is up to the builder to make trial blocks with various kinds and amounts of stabilizers which can be tested.
The most common naturally available stabilizers used in traditional constructions are:
• sand and clay
• straw, plant fibres
• plant juices (sap, latexes, oils)
• wood ashes (cinders)
• animal excrete (mainly cow dung, horse urine)
• other animal products (blood, hair, glues, termite hills).
The most common manufactured stabilizers, (ie products or by-products of local village industries or large industrial processes) are:
• lime and pozzolanas
• portland cement
• commercial soil stabilizers
• sodium silicate ("water glass")
• whey (casein)
The listed stabilizers are briefly described below. The choice of the most suitable stabilizer will mainly depend on local availability and costs, but also to some extent on social acceptance.
Sand and clay
• These are used to correct the quality of soil mix, that is, addition of sand to clayey soils or addition of clay to sandy soils.
• Mixing should be done in the dry state, otherwise it cannot be uniform.
• Dry clay is usually found in the form of hard lumps, which have to be well crushed before mixing.
Straw, plant fibres
• These act as reinforcements, especially to check cracking in soils with a high clay content.
• They also make the soil lighter, increase its insulating properties (good in arid and highland regions) and accelerate the drying process (by providing drainage channels).
• Straw is universally the most common soil reinforcement; almost any type is acceptable (wheat, rye, barley, etc.), also the chaff of most cereal crops.
• Other fibrous plant materials are sisal, hemp, elephant grass, coir (coconut fibre), bagasse (sugar cane waste), etc.
• To achieve satisfactory results, the minimum proportion of plant reinforcements is 4 % by volume; 20 to 30 kg per m3 of soil are common.
• Since plant reinforcements tend to weaken the end product and increase water absorption, excessive use should be avoided.
• The straw and fibres should be chopped to lengths of not more than 6 cm, and mixed thoroughly with the soil to avoid nests.
• The juice of banana leaves precipitated with lime improves erosion resistance and slows water absorption.
• Reduced permeability is also achieved by adding the latex of certain trees (eg euphorbia, hevea) or concentrated sisal juice in the form of organic glue.
• Vegetable oils and fats must dry quickly to be effective and provide water resistance. Coconut, cotton and linseed oils are examples; castor oil is very effective, but expensive.
• Kapok oil can also be effective. It is made by roasting kapok seeds, grinding them to a fine powder and mixing it with water (10 kg powder: 20 to 251 water).
• Ash from hardwood is usually rich in calcium carbonate and has stabilizing properties, but is not always suitable for clayey soils. Some ashes can even be harmful to the soil.
• The addition of 5 to 10 % (by volume) of fine, white ashes from fully burnt hardwood appears to be most effective, that is, improvement of the dry compressive strength.
• Ashes do not improve water resistance.
• These are mainly used to stabilize renderings.
• Cow dung is the most common stabilizer, which is valued mainly for its reinforcing effect (on account of the fibrous particles) and ability to repell insects. Water resistance is not significantly improved, while compressive strengths are reduced.
• Horse or camel dung are less common alternatives.
• Horse urine as a substitute for mixing water effectively eliminates cracking and improves resistance to erosion. Even better results are obtained by adding lime.
• Despite their advantages, these materials face low social acceptance in most regions, while in others (mainly rural areas in Asia and Africa) they are well accepted traditional materials.
Other animal products
• Fresh bull's blood combined with lime can gready reduce cracking, however, here again low social acceptance.
• Animal hair or fur is often used to reinforce renders.
• Animal glues, made from horn, bone, hooves and hides, improve moisture resistance.
• Termite hills, which are known to resist rain, can be pulverized and used as a stabilizer for sandy soils.
Lime and pozzolanas
• Clayey soil (with liquid limits in the region of 40 % or more) can be stabilized only with lime, as it reacts with the clay particles in the soil to form a binder.
• For soils with a lower clay content, a suitable pozzolana (eg fly ash, rice hush ash) can be added to the lime, to produce a cementitious binder.
• Quicklime (CaO), produced by burning limestone, can be used for stabilizing, but has several drawbacks: it has to be well crushed before use; it becomes very hot (up to 150° C) and can burn the skin; the heat of hydration tends to dry the soil quickly, with the risk of delayed hydration after several months.
• Hydrated or slaked lime (Ca[OH]2), made by adding water to quicklime, has less drawbacks. It can be used as a dry powder (available in bags), as milk of lime (slaked lime with excess water) or as lime putty (a viscous mass).
• The correct proportion of lime (with or without a pozzolana) cannot be generalized and needs to be found by a series of tests. The required amount can range between 3 and 14 % by dry weight, depending largely on the clay content (more clay requires more lime).
• Dry soil must be crushed (as clayey soils usually contain hard lumps) and thoroughly mixed with the lime. Most soils can be dried and broken with quicklime.
• The wet soil-lime mix is best kept in that state under cover for a day or two, after which the lime will have broken the remaining clay lumps. The soil is mixed again (if necessary, with addition of a pozzolana) producing a homogenious mass, which can immediately be used in construction. (Proportion of lime: pozzolana can range between 1: 1 and 1: 3).
• The curing of lime-stabilized soil takes about six times that of cement-stabilized soil. High temperatures and humidity help to improve the ultimate compressive strength. This can be achieved by curing under a plastic sheet, or in an enclosed space built with corrugated iron sheets, for at least two weeks. Final strength is gained after two to six months.
• Curing can be accelerated by adding cement just before use in construction.
• Limestone with a high clay content produces a special type of lime, called hydraulic lime, which sets and hardens like cement. Soil stabilization with hydraulic limes reduces the period of curing, but may not achieve sufficient strengths.
• Soils with low clay contents are best stabilized with portland cement, which binds the sand particles and gravel in the same way as in concrete, that is, it reacts with the water in the soil mixture to produce a substance which fills the voids, forming a continuous film around each particle, binding them all together.
• The reaction of cement and water (known as hydration) liberates calcium hydroxide (slaked lime) which reacts with the clay particles to form a kind of pozzolanic binder. If the clay content is too low the lime remains free. This can tee remedied by replacing a proportion (15 to 40 % by weighs) of the cement with a pozzolana, which is usually cheaper than cement.
• Just as in cement-sand mortars, soil-cement mixes become more workable by adding lime. If the clay content is high, the additional lime reacts with it to further stabilize the soil.
• The appropriate cement content will vary according to the aspects mentioned above. A minimum of 5 % is recommended, while cement contents exceeding 10 % are considered unsuitable, because of the high cost of cement.
• Soil and cement must be mixed dry, and the water added and thoroughly mixed just before use, as the cement begins to react with water immediately.
• Once the cement has begun to harden, it becomes useless. Soil cement cannot be recycled.
• The more thoroughly the soil is mixed, the higher the ultimate strength, which is obtained by compaction (eg with a ramming device or block press).
• Portland cement is the stabilizer that provides the greatest strength as well as resistance to water penetration, swelling and shrinkage.
• Soil stabilization with gypsum is not common practice and information on its performance is very limited.
• Gypsum is abundantly available in many countries, either as natural gypsum or as an industrial by-product, and is cheaper than lime or cement (produced with less energy and equipment).
• Since gypsum mixed with water hardens rapidly, adobe blocks stabilized with gypsum require no lengthy curing period, but can be used for wall constructions soon after production. Gypsum contents around 10 % are best.
• The advantages of stabilization with gypsum are low shrinkage, smooth appearance and high mechanical strength. In addition, gypsum binds well with fibres (particularly sisal), is highly fire resistant and is not attacked by insects and rodents.
• The main disadvantage of gypsum is its solubility in water, which requires careful protective measures: protection from rain on outer walls by plastering, cladding or wide overhanging roofs; protection from indoor moisture development by avoiding steam (in kitchens) and condensation; protection against rising water by means of waterproof membranes.
• For soil stabilization, bitumen can either be used as a cutback (ie mixed with a solvent such as gasoline, kerosene or naphtha), or as an emulsion (ie dispersed in water).
• After mixing a soil with bitumen cutback, it should be spread out to allow the solvent to evaporate before the material is used for blockmaking. It is best to mix the cutback with a small quantity of soil, which is then mixed with the remaining soil.
• Bitumen emulsions are usually very fluid and mix easily with moist soil. Excessive mixing must be avoided to prevent a premature break-down of the emulsion, leading to increased water absorption after drying. Emulsions should be diluted in the mixing water.
• Soil mixes required for compaction should not be too moist, hence a less quantity of stabilizer should be added.
• The bitumen content should be between 2 and 4 %. Higher proportions result in dangerously low compressive strengths.
• Bitumen stabilized soils should be cured in dry air at temperatures around 40° C.
• While bitumen stabilization does not improve the strength of the soil, it significantly reduces water absorption. In other words, while the dry strength of the soil is not very high, the strength is not reduced when wet.
• Bitumen stabilization is most effective with sandy or silty soils with a liquid limit between 25 and 35 % and plasticity index between 2.5 and 13 %.
• The presence of acid organic matter, sulphates and mineral salts can be very harmful. The addition of 1 % cement is a possible remedy.
Commercial soil stabilizers
• These are mainly industrially produced chemical products, which were developed primarily to stabilize the soil used in road construction.
• These chemical stabilizers work mainly as a waterproofer. In general, they do not improve the compressive strength of the soil.
• The required quantities of these stabilizers range between 0.01 and 1 % by weight, hence very thorough mixing is required to achieve a uniform distribution.
• A long list of commercial stabilizers is given in Bibl. 02.19.
• Sodium silicate, known as "water-glass", is cheaply available in many parts of the world.
• It works best with sandy soils, like clayey sands and silty sands, but is not suitable for clay soils.
• Sodium silicate works as a waterproofer, and also prevents fungal growth.
• If it is mixed with the soil, the usual quantity is 5 %.
• However, it is best to use it as a surface coating, made of 1: 3 parts of commercial sodium silicate: clean water.
• Soil blocks are dipped into the solution for about a minute, after which the solution is applied with a stiff brush. The procedure is repeated a second time and the blocks are left to dry in a protected place for at least 7 days.
• Deeper penetration of the solution is achieved by adding a very small amount of a surfactant (surface active agent).
• Resins are either processed plant extracts, such as sap from trees, or by-products of various industrial processes.
• Much research work is being undertaken on these materials and extraordinary results have been obtained with resin stabilization.
• The main advantages are water resistance (though not in all cases), rapid setting and solidification of very moist soils.
• The main drawbacks, however, are high cost, sophisticated production technology and the need for larger quantities than conventional stabilizers. Resins are often toxic and degradable by biological agents.
• Whey (casein) is the protein-rich liquid formed by making curd. Its use for building will be very limited in most developing countries, on account of its nourishing value. However, in regions where a surplus of whey is produced, its use as a surface stabilizer for soil constructions is well worth considering.
• By adding whey to a soil-lime plaster or to a limewash, a weather-proof surface protection is achieved, without forfeiting the capability of the soil to breathe.
• In order to achieve good adhesion and avoid cracks, the limewash should be applied in two or three thin coats. The use of whey as a primer can also give good results.
• Molasses are a by-product of the sugar industry.
• Adding molasses to the soil improves its compressive strength and reduces the capillarity of the soil.
• They work well with silty and sandy soils. In the case of clayey soils, small quantities of lime should be added to the molasses.
• The quantity of molasses normally added to the soil is about 5 % by weight of soil.
How to Use Stabilizers
Although the use of each stabilizer is mentioned above, some general rules are summarized here:
• The full benefit of using a stabilizer is achieved only if it makes contact with each particle of soil, hence, thorough mixing is necessary.
• Much preparation and testing is required to find the best combination and proportions of stabilizers for a given soil. It is certainly worth the time and effort, even if it takes one or two months of preparation.
• The only way to determine the correct proportion of stabilizer is to make 5 to 7 trial blocks from each mix and subject them to a series of tests, such as compression strength tests after different periods of drying, prolonged wetting and drying tests, and immersion in water.
• Portland cement and lime stabilized blocks need to be moist cured for at least 7 days to gain strength.
• Testing programs should take into account the local climatic conditions, the possible occurance of frost, and the like. The choice of stabilizer will also differ between arid and humid regions.
• It should be remembered that trial blocks need only a small amount of soil, which is easy to mix. During the actual construction or mass block production, the mixing of large quantities of the soil is more difficult, so that a slightly higher proportion of stabilizer should be added (except in the case of cement).
• The aim of the tests should always be to find the lowest amount of stabilizer to satisfy the requirements. Very often the specified requirements are unjustifiably high, leading to unnecessarily high costs.
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