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CLOSE THIS BOOKStabilizers and Mortars ( for compressed earth blocks) (GTZ, 1994, 20 p.)
VIEW THE DOCUMENT(introduction...)
VIEW THE DOCUMENTAcknowledgements
VIEW THE DOCUMENTNatural products
VIEW THE DOCUMENTPortland cement
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Portland cement



Portland cement is obtained by firing at 1500°C a precise mix of ground calcareous rock and clayey materials. The mix which emerges as lumps at the end of the firing period is known as clinker; this clinker is then cooled and finely ground, and a small quantity of gypsum - which delays setting time - is added, to give Portland cement itself. Depending on the nature of the raw materials, the proportions used and the way the process is carried out, the final characteristics of the cement and, above all, its mechanical performances will vary.

Various products can be added to the clinker obtained at the end of the firing stage. The most common of these is slag, which is a by-product of the metallurgical industry. Slag is the gangue of iron ore and is obtained by separation at the moment of fusion of the ore. The various products obtained by mixing clinker and ore each have their own strength characteristics and also the capacity to withstand chemical corrosion especially sulphates.

Other cements are produced for special purposes: cements with a very high sulphate content (supersulphated cements) which are highly resistant to sulphates in sea-water, aluminium cements obtained by firing calcium carbonate and bauxite which have very slow setting times; pozzolanic Portland cements, obtained by mixing clinker and pozzolana - a volcanic rock - in order to prepare durable mortars; white cement prepared with raw materials containing no iron oxide.

Conditions of use

Portland cement without retardants begins to set fairly quickly (1 hour). Reworking a mortar which has started to set merely breaks down the links which have begun to be formed; a mortar containing Portland cement must therefore be used without fail within an hour of being prepared. Hardening begins 3 to 12 hours after setting has started and takes 4 weeks. While it is setting, the cement takes up a certain amount of water (hydration); the mortar must therefore retain sufficient humidity, especially during the beginning of the hardening stage. When using a cement mortar on a dry, porous surface, this will rapidly absorb part of the water contained in the mortar and the cement will no longer have enough moisture to harden. This phenomenon is known as "grilling" the mortar and should be avoided by wetting the surface before applying the mortar.

Since cement goes off in damp conditions, dry storage is essential.

Cement shrinks as it hardens; using it neat would result m cracks.

Cement lends itself well to mixing with other binders, notably limes, but on the other hand it cannot be mixed with gypsum.


Historical background

The first attempts to use cement as a stabilizer were made for road-building purposes in the USA in 1915. Cement stabilization developed independently in Germany in the Since about 1935, cement stabilization has been increasingly used in the USA for both road and runway construction. The applications of cement-stabilized material have multiplied enormously since then and it is used world-wide for infrastructure works as much as for building construction. Today, our understanding of cement stabilized soils is very comprehensive.

How stabilization occurs

Hydrated cement reacts in two different ways in a soil:

a. By conventional hardening of the cement by hydration and bonding with the sandy "skeleton" of the soil.

b. By undergoing a 3-phase reaction with clay:

1 Hydration triggers the formation of cement gels on the surface of the clay aggregates. The lime, which is released during the hydration of the cement, tends to react with the clay. The lime is quickly used up and the clay starts to change its character.

2 Hydration proceeds and encourages the clay aggregates to break down. The latter are deeply penetrated by the cement gels.

3 The cement gels and the clay aggregates become intimately interlinked. Hydration continues, but more slowly.

In effect, three combined structures are obtained:

an inert sandy matrix bound with clay;
a matrix of stabilized clay
a matrix of unstabilized soil.

Stabilization does not affect all the aggregate. A stabilized matrix covers the composite aggregations of sand and clay.

Suitability of different cements

Ordinary Portland cement or similar cements are very suitable. There is nothing to be gained by using very high-strength cements, as these bring no particular benefits and are moreover very expensive. Higher-strength cements require very specific curing conditions and are liable to deteriorate, making them unsuitable for use on work sites far removed from the factory producing them. Portland cements of class 250 or 350 is preferred. Cements containing other materials such as slag, fly-ash and other pozzolanas can also be used, although these will only be available close to steel plants power stations and similar localities. In contrast, cements with high contents of other materials should not be used, because they need a precisely controlled curing environment. These include iron Portland cement, blast-furnace cement, mixed metallurgical cements and slag clinker cements.

Proportions of cements to be used

Excellent results can be obtained with between 3 and 16% by weight. Some soils require only 3%, while the same proportion in others makes them perform less well than if no cement at all had been used. Generally speaking, depending on the composition of the soil (determined by testing), at least 6 to 8% cement is required to get satisfactory results.

Soils that can be used

Nearly all soils, except those containing an excessive amount of organic material, can be treated with cement, which significantly improves their properties. Salt-rich soils are also difficult to stabilize with cement, but increasing the proportion of cement can often yield good results. Soils with a high clay content mix only with difficulty and require large amounts of cement. When the mixing process is very closely controlled under laboratory conditions, good results can be achieved with clayey soils. In practice, however, cement is not used for stabilizing clay when the liquid limit is greater than 50% and the clay content is higher than 30%. Preliminary treatment of these extremely clayey soils with lime may improve the chances of obtaining good results when cement is added later on. Numerous tests give indications about the suitability and proportion of cement.

The table overleaf gives an idea of the cement requirements of various soils.

Effects of cement stabllization

Some of the effects of cement stabilization on soils are the following:

DRY DENSITY: This is lower for soils which compact well and higher for soils which compact less well.

WET COMPRESSIVE STRENGTH: Under good conditions, the wet compressive strength equals or attains values over 50% of dry compressive strength.

TENSILE STRENGTH: This varies from 1/5 to 1/10 of compressive strength values.

The following tests can be performed to evaluate the effects of cement stabilization.

ABRASION: The proportion of cement should reduce material lost to 3% or less after 50 cycles of brushing, which is an excellent performance.

EROSION: The proportion of cement should reduce the mean depth of pitting to 15mm or less-an excellent performance as this is an extremely severe test.

WETTING-DRYING: An optimum proportion should reduce material losses to 10% or less - an excellent performance as this is an extremely severe test.

FREEZE-THAW: An optimum proportion should reduce material losses to 10% or less an excellent performance as this is an extremely severe test.

Effects of certain products on cement-stabilization

ORGANIC MATTER: This is generally recognized as being harmful, particularly if it contains nucleic acid, tartaric acid, or glucose. Its effect is to slow down the setting of cement and to lower its strength. As a general rule, an organic matter content in excess of 1% represents a hazard and soil containing more than 2% should not be used

Some organic products (amine acetate, melamine, aniline) and certain inorganic products (ferrous chloride! reduce the sensitivity of some soils to water.

Lime (2%) can reduce the harmful effects of organic matter, as does calcium chloride (0.3 to 2%), which also accelerates setting. Lime also serves to modify the plasticity of the soil and to limit the formation of nodules.

When the pH value is > 7 (alkaline or neutral): calcareous soils, brown alkaline soils, and some gley soils can be stabilized with 10% cement, and rates of between 1 and 2% of organic matter are in general not a problem.

When the pH value < 7 (acid): gley soils can be successfully stabilized with 10% cement if the organic matter content is less than 1%. Podsols and acidic brown soils can sometimes be stabilized successfully if they contain less than 1% organic matter. If anomalies are found to exist, preliminary treatment with calcium chloride ( 1 to 2%) may bring about a certain improvement.

SULPHATES: These have very harmful side-effects, calcium sulphate (anhydrite and gypsum) in particular, and are often encountered. They result in the destruction of the hardened cement from the inside of the cement-soil, an increase in the sensitivity of the clay to moisture. For soils with a sulphate content of more than 2 to 3%, a special study is essential.

OXIDES AND METALLIC HYDROXIDES: These are basically iron and aluminium oxides. They rarely exceed 5% of the soil, and thus have little effect.

WATER: In principle, water containing organic matter and salts should not be used as these may cause efflorescence. Water with a high sulphate content may also have harmful effects.

BITUMEN: Between 2 and 4% bitumen added as an emulsion or cutback makes the soil waterproof.


Bonding mortar

The texture of a good bonding mortar is generally more sandy than that of compressed earth blocks, with a maximum particle diameter of 2 to 5 mm Cement-stabilized mortar must always be used with cement-stabilized compressed earth blocks. In this event, the proportion of cement used should be increased by a factor of 1.5 or 2 to achieve the same strength as that of the earth blocks.

Plasters and renders

Cement mortars are too rigid and suffer from the defect of not adhering well to compressed earth block walls. Cracking, blistering and falling off in sheets are frequently observed symptoms .Their use is not recommended and should at best be a temporary solution, with Proportions of the order of 1 part cement to 5 or 10 parts soil or sand. Preferably a little lime should be added: 1 part lime to 1 part cement, or 1 part lime to 2 parts cement if at all possible. Cement renders should be applied on a wire netting. This reduces cracking and falling off in slabs, but does not improve adhesion, Usually this solution is fairly expensive and not very satisfactory.

Cement-stabilization of plasters and renders is only really effective if the soil is very sandy. Proportions may vary from 2 to 15% cement, depending on whether a mild improvement or genuine stabilization is required. Cement-stabilized renders should preferably be applied to stabilized walls. It is also possible to add between 2 and 4% bitumen. This mixture tends to darken the dressing without spoiling the colour, but greatly improves water resistance.

The Unified Soil Classification System (USCS) is based on the size of the soil particles, the amounts of the various sizes and the plastic characteristics of the very fine grains. This system takes into account the engineering properties of the soils. It is descriptive and easy to associate with actual soils, and it has the flexibility of being adaptable both to the field and to the laboratory. Probably its greatest advantage is that the soil can be classified readily by visual and manual examination based on the results of simplified grain size distribution tests and plasticity tests.

The table can be found at the back of the leaflet on "Gypsum", in this Product Information folder.






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