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CLOSE THIS BOOKStabilizers and Mortars ( for compressed earth blocks) (GTZ, 1994, 20 p.)
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There are there main types of lime: high calcium limes (which we will subsequently refer to simply as "limes"), hydraulic limes and dolomitic limes.

HYDRAULIC LIMES gain greater strength, and at a faster rate, than high calcium limes. They also set under water and produce a generally more durable product. Their behaviour, the way they work and their characteristics closely resemble those of a lower strength Portland cement. This material will therefore not be further explored here and the Portland cement sheet may usefully be consulted. Their use should not be considered unless other types of lime are not available. Natural hydraulic limes are more effective stabilizers than artificial hydraulic limes.

HIGH CALCIUM LIME is obtained by calcination at 900° to 1,000°C of a relatively pure, clay-free calcareous rock. A calcium carbonate content of 97% would normally be considered the minimum for production of high calcium limes. During baking, this carbonate dissociates and part of it escapes in the form of carbon dioxide CO2. This leaves only calcium oxide CaO, known as quicklime, the proportion of which determines the quality of the final product. Pure high calcium lime will contain more than 97% CaO; fat lime, more than 85%; and a lean lime, less than 85%. When it is placed once more in contact with the carbon dioxide present in the atmosphere, the lime again becomes calcium carbonate.

DOLOMITIC LIME is similar to a high calcium lime, except that it contains a proportion of magnesium oxide (MgO) combined with calcium oxide (CaO). This lime needs more care in usage compared with a high calcium lime and it is particularly important to ensure that it has been properly burned and slaked, chat is, it is burned at a lower part of the temperature range for lime production and a longer time is allowed for slaking it. A good proportion of the world's limestone is dolomitic limestone.

There are two main forms of lime: quicklime and hydrated lime.

QUICKLIME (CaO): Quicklime is produced directly by firing the stone in kilns. The sensitive conditions of storage and maintenance it requires can limit its use. Quicklime is extremely hygroscopic (ie it attracts water) and must be protected from moisture. It is a caustic material, becoming very hot during the hydration stage (up to 150°C), and must be handled with great care. On an equal weight basis, it is more effective than hydrated lime, because it can supply greater quantities of calcium ions.

HYDRATED LIME (CaOH2): Since quicklime is a chemically unstable and slightly hazardous product, it is normally hydrated, becoming not only more stable but also easier and safer to handle. Hydrated lime is produced by adding water to quicklime in a process called "hydration" or "slaking", where the calcium oxide and water combine chemically to form calcium hydroxide. Fat slaked limes do not have to be very finely crushed to be effective. Industrial qualities contain between 90 and 99% of "active lime’ while artisanal lime may only contain between 55 and 75%, the rest consisting of unburnt or excessively burnt material.

Conditions of use

The setting of lime, which is in fact the recarbonation of the product when exposed to the carbon dioxide present in the air, implies that prolonged storage of the product after manufacturing is harmful, since setting begins naturally. In addition, while it is being used, if the thickness of the product is too great, the carbon dioxide in the air will penetrate to the bottom of the product slowly and incompletely, and the lime will therefore set only in part.

In the case of artisanal production of lime, the quality varies widely and can differ very significantly from theoretical limits (less carefully chosen raw material poorly monitored firing, partial extinction, etc). In this event, a few preliminary tests are always recommended to gauge the quality of the product and the proportions used should be adapted in the light of their results.

After mixing lime with soil, it is preferable to leave the mix for a while before use, to make best use of the plasticising qualities of the lime.

For stabilizing compressed earth blocks, lime is always used in powder form. For mortars, however, the use of lime in the form of a paste presents certain advantages, but makes calculations of amounts to be used more inaccurate, as the exact proportion of lime in the paste is not well-known beforehand; the mix of sand or soil for the mortar must then be carried out well to ensure that it is uniform.


Historical background

It appears chat the large scale use of lime for stabilizing soils is fairly recent, and was pioneered in the nineteen-twendes in the USA. Since then millions of m² of roads have been constructed in lime-stabilized soil and immense experience has been gained. The construction of the Dallas - Fort Worth airport, covering some 70 km², in 1974 is one of the most spectacular applications of this technique. Indeed more than 300,000 tonnes of lime were used for the stabilization works. Lime has also been, and still is, used for constructing buildings and there is an increasing interest in lime stabilization in this field.

How stabilization occurs

The theory of lime stabilization suggests that stabilization occurs in 5 basic ways.

WATER ABSORPTION: Quicklime undergoes a hydration reaction in the presence of water or in moist soil. This reaction is strongly exothermic with the release of about 300 kcal for every kg of quicklime.

CATION EXCHANGE: When lime is added to a moistened soil, the latter is flooded with calcium ions. Cation exchange then takes place, with calcium ions being replaced by exchangeable cations in the soil compounds, such as magnesium, sodium, potassium, and hydrogen. The volume of this exchange depends on the quantity of exchangeable cations present in the overall cation exchange capacity of the soil.

FLOCCULATION AND AGGREGATION: As a result of the cationic exchange and the increase in the quantity of electrolytes in the pore water, the soil grains flocculate and tend to accrete. The size of the accretions in the fine fraction increases. Both grain size distribution and structure are altered.

CARBONATION: The lime added to the soil reacts with carbon dioxide from the air to form weak carbonated cements. This reaction uses up part of the lime otherwise available for pozzolanic reactions.

POZOLANIC REACTION: This is by far the most important reaction involved in lime stabilization. The strength of the material results largely from the dissolution of clay minerals in an alkaline environment produced by the lime, and the recombination of the silica and alumina in the clays with the calcium to form complex aluminium and calcium silicates, thus cementing the grams together. The lime must be added to the soil in sufficient quantities in order to proceed and maintain a high pH, which is necessary for the dissolution of the clay minerals for long enough to allow an effective stabilization reaction.

Proportions of lime to be used

When 1 % quicklime is added to the soil the exothermic hydration reaction dries the soil, removing between 0.5 and 1% of water. The addition of 2 to 3% quicklime immediately causes a reduction of the plasticity of the soil and the breaking down of lumps. This reaction is called the fixing point of the lime. This reaction is not part of the stabilization process, but should rather be considered a "modification". For ordinary stabilization work, between 3 and 20% quicklime is used, the difference - in comparison with cement stabilization - being that, with lime, there is an optimum quantity for each soil. Generally speaking quantities of 8 to 10% lime are required in order to obtain satisfactory results.

Soils that can be used

Lime has only every limited effect on soils with a high organic matter content (content higher than 20%) and on soils short of clay. It is more effective and can be more effective than cement on clay-sand soils and especially on very clayey soils. The effects of lime are thus highly dependent on the nature of the soils involved but a comparison with the effects of cement can, in many cases, be attempted. It has been observed that lime reacts far more quickly with montmorillonite clays than with the kaolinites, reducing the plasticity of the montmorillonites and having only aslight effect on the plasticity of the kaolinites. Water content has a significant effect on clay soils which can be stabilized with lime, particularly in the pulverization and compaction stage. Natural pozzolanas react particularly well with lime.

Note that proportions of lime quoted are for industrial quality lime containing between 90 and 99% quick lime. For lime produced by less sophisticated methods, which may contain only 55% quick lime (the rest being made up of unfired or over-fired components), the proportion must be increased. The two main methods of improving the performance of soil with lime are summarized below:

Modification of the soil: the lime is added until a setting point is reached. This operation reduces the plasticity of the soil and improves its flocculation.

Stabilization of the soil: the proportions are much higher. Reference monograms on the suitability of soils and the proportion of lime must be interpreted with a great deal of care.

Effects of lime stabilization

Some of the effects of lime stabilization on the soils are:

DRY DENSITY: For a given compression, lime reduces the maximum dry density, and raises the optimum moisture contents, because of flocculation.

COMPRESSIVE STRENGTH: The optimum proportion of lime should be determined by preliminary tests. Compressive strength tends to increase dramatically with the age of the product; therefore tests should be carried out only after allowing a curing period of 3 months. Values between 2 end 5 MPa can easily be obtained and when sophisticated industrial procedures are employed values of between 20 and 40 MPa can be expected.

TENSILE STRENGTH: This is highly influenced by the quantity and quality of clays contained in the soil, which reacts with the lime.

DIMENSIONAL STABILITY: Just I to 2% Iime can reduce shrinkage from 8 or 10% to 1% and eliminate swelling.

ABRASION: The proportion of lime should reduce material lost to 3% after 50 cycles - an excellent performance;

EROSION: The proportion of lime should reduce the mean depth of holes to 15mm - an excellent performance for this extremely severe test.

WETTING-DRYING: The proportion should reduce material losses to 10% - an excellent performance for this extremely severe test.

FREEZE-THAW: The proportion of lime should reduce material losses to 10%: an excellent performance for this excessively severe test.

Effects of certain products on lime-stabilization

ORGANIC MATTER: These can block ionic exchange in clayey soils without, blocking the pozzolanic reaction. Soils containing up to 20% organic matter can be stabilized with lime, but care is essential.

SULPHATES: Calcium sulphates are less dangerous than magnesium sulphates, when dry. When moist, all sulphates are harmful.

PORTLAND CEMENT: In quantities of up to 100% of the lime content will increase compressive strength.

BITUMEN: Between 0 and 2% of bitumen added as emulsion or cutback makes the limestabilized soil waterproof. Potassium sulphate (K2SO4) can also be used.


Bonding mortar

High calcium lime is one of the most suitable binding agents for the preparation of mortar for earth buildings: its does not shrink too much, it is moisture-permeable, its mechanical performance is not too high and it lends a degree of elasticity to the mortar. It is plastic, and it is slow to set.

Depending on the mechanical characteristics of the background, mortars are prepared with greater or lesser proportions of lime. For surfaces which have fairly good mechanical strength, such as stabilized compressed earth blocks, a mixed lime-cement mortar - however, with a predominance of lime-can even be used. Combining high calcium lime with cement is also common practice, because lime sets very slowly and at least 6 months must be allowed before a fat lime reaches its final mechanical performance. Cement enables one to attain a minimum level of strength quickly.

There are some possibilities of additives to improve the basic qualities of lime: pozzolanic materials (rice husk ash, fly ash powdered brick, etc) to give a degree of hydraulicity, marble dust to increase hardness, and oil to quicklime to give a waterproof composite.

In an earth mortar, the effectiveness of lime as a stabilizer is debatable, as high calcium lime has an affect only on soils with a high clay content For mortars, therefore, one tends to use thinned soils which arc not very active. In fact, lime reinforces the binding action of clay a little and gives an earth mortar a plasticity and workability which improves its application.

Plasters and renders

The best results are obtained with hydrated limes, in the form of extremely fine-ground powder or a paste prepared be forehand. Theuse of hydrated lime as a surface rendering or plaster for soil structures is old and well established in many countries. It must be remembered that the hardening of a rendering based on slaked lime is the result of slow carbonation by carbon dioxide in the air, and as a result these dressings should not be too thick. The long hardening process makes these renders sensitive to atmospheric conditions. particularly frost and great heat. In many regions, lime renderings arc modified during preparation, with various additives which can improve their quality. For example, fresh bull's blood, leaving aside its importance in magical practice improves the waterproofing qualities of the renderings. Other practices include the addition of natural soft soap, which improves workability and waterproofing, and facilitates polishing. The addition of a little molasses helps to harden the render. When hydrated lime renders are exposed to considerable stressing, hydraulic lime or cement can be added. However, only a small proportion may be added to avoid excessive hardening or reduction of permeability. Experimentation has made it possible to specify proportions for lime or sand based multi-layer renders and mixed renders based on lime, cement and sand.

The lime-stabilization of soil renders and plaster has its greatest affect on clayey soils, when lime is used in large quantities, often over 10% A lime-stabilized render is best applied to astabilized wall. The addition of animal urine or dung can have truly astonishing effects on the render (less shrinkage, hardness and good permeability). The main drawback is the strong ammonia smell during mixing, which some people find disagreeable.





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