Hydraulically Bound Mixtures
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Hydraulically Bound Mixtures Chemical & Physical Characteristics


HBM is the result of combination between the material to be treated and a hydraulic binder, which alters and improves the strength, erosion resistance, durability and volume stability of the soil or aggregate (natural, recycled or artificial) being treated. Hydraulic binders, constituents and basic characteristics


Cement, here used to describe Portland cements, is a hydraulic binder or stabilising agent because, in the presence of water, the calcium silicates and aluminates in cement form hydrated compounds that subsequently produce a strong, hard matrix that glue the soil or aggregate particles together. The material that has been treated becomes part of this matrix. Cement can be used to stabilize many soils but is most frequently used to stabilise granular or non-cohesive soils where thorough dispersion of the cement is possible because of the non-sticky nature of the soil.


Where cohesion prevents proper mixing, such as with medium to high plasticity clays, stabilization is better carried using quick lime [CaO] or slaked or hydrated lime [Ca(OH)2], which initially makes the soil friable and more sand-like before reacting with the soil. In more detail, the lime produces a high pH environment, which dissolves the aluminates and silicates from clay making them available for combination with the lime. The result is calcium silicates and aluminates and thus hydrates capable of setting and hardening as with cement. If necessary, the strength of the structure can be enhanced by adding cement at a second mixing stage when the friability of the soil-lime mixture enables efficient mixing and dispersion of the cement.

Pozzolanic Materials

Some soils contain reactive silicates and/or aluminates. These are known as pozzolanic soils or pozzolans. Most clays are thus pozzolans. Volcanic ashes are also pozzolans.

Fly Ash

Fly ashes from coal-fired electricity generation plants, also known as coal fly ash or inthe UK as pulverised fuel ash, contain reactive silicates and aluminates. Thus they are also pozzolans. Pozzolans react chemically with lime and potentially respond better to lime than cement. It follows also that a lime/fly ash combination may be used as a binder and thus as an alternative to cement for the stabilization of granular materials.

Hydraulic Slags

The slag from iron production, known as blast furnace slag, is a versatile construction material since it can be used as an inert aggregate or as a cementitious material or constituent in its own right, depending on how it is conditioned when discharged from the blast furnace. If allowed to air-cool, an almost inert hard crystalline material is produced which can be crushed to produce aggregate. If quenched rapidly in water on the other hand, a vitrified (glassy) product is produced which has hydraulic potential. This latter product is known as granulated blast-furnace slag [gbs], which is naturally but slowly cementitious. Gbs ground to enhance its speed of reaction is known as ground granulated blast-furnace slag or ggbs.

Like fly ash above, ggbs is a well-known constituent of concrete, frequently used to enhance the properties of concrete as well as allowing reductions in cement content. Also like fly ash, gbs or ggbs can be used with lime as a stabilising combination for the treatment of granular materials.

Clays Treated with Slag and or Fly Ash
The lime/ggbs combination is also effective with clays, capable of matching the potential of the lime/cement combination. In this respect, ggbs is better suited than fly ash since it is effectively cement, whose hydraulicity or cementitious potential is enhanced by lime. Fly ash on the other hand, being a pozzolan, can only become hydraulic or cementitious in the presence of lime or a source of lime. Since clay is also a pozzolan, it should be apparent that the addition of another pozzolan i.e. fly ash, to a material that is also a pozzolan, such as clay, is unnecessary. However when the clay is of low plasticity or the natural silicates and aluminates in the clay are relatively unreactive, the addition of fly ash can be very beneficial.

Steel Slag

It is possible to utilize the lime content in the slag produced from steel production for stabilisation purposes. This slag, known as air-cooled steel slag [ASS], is particularly suited, when combined with gbs, to the treatment of sands or combinations of fine and coarse aggregate.

Speed of Reaction

Although the speed of reaction is slower than with cement stabilisation, the use of lime stabilization or lime with fly ash or gbs or ggbs stabilisation is a perfectly viable and proven option. More importantly however, the above discussion illustrates the wide range of possibilities with certain materials and that stabilisation has the flexibility to treat most available soils and materials.

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