FR2719053A1 - Fly ash and lime compsns. for stabilising soils - Google Patents

Abstract

Compsns. for stabilising soils esp. road foundations, contain 25-80 wt.% fly ash and 20-40 wt.% lime. Also claimed is a process for stabilising soils by one-step treatment with a compsn. as above.

Description

Composition for soil stabilization as well as pro
assigned soil stabilization with such composi
tion
The present invention relates to a composition for the stabilization of soils, in particular pavements of roadways, especially motorways.

 In the field of road engineering, the first phase of pavement construction typically consists of earthworks carried out using special equipment consisting of stripping certain parts in relief and recovering cuttings to fill the holes (embankments) so as to obtain a relatively flat surface.

 In the particular case of fine, wet soils, especially sandy loam soils, it is necessary, prior to the realization of the upper layers of the roadway, to provide a stabilization treatment allowing in particular to give the ground an immediate lift allowing the traffic of the building site.

This stabilization is conventionally obtained thanks to the use of lime which acts essentially at three levels.
- by supply of dry product,
- by consumption of the quantity of water necessary for the chemical hydration of quicklime in slaked lime,
by evaporation under the action of the heat released as a result of the exothermic nature of this reaction.

 In addition, the lime acts by flocculating the clays and thus making the soil more friable, and by moisturizing the silicates and calcium aluminates it can, if necessary, contain.

 Such treatment, however, is not free of drawbacks and must in particular often be followed by a complementary treatment, in particular with cement so as to obtain an improvement over time in the mechanical characteristics of the soil.

 To overcome this drawback, it has been sought in accordance with the invention, to replace this conventional treatment with another, more satisfactory treatment.

 This has led to the idea of using fly ash which corresponds to a product available in large quantities, in particular a by-product often discarded from the operation of coal or oil-fired power plants. activity as back-up plants seems set to grow in the coming years.

 The chemical composition of fly ash depends on their origin as an example, these may contain potassium carbonate, calcium and magnesium carbonate and phosphates, silica, aluminum silicate or oxides iron and manganese.

 Fly ash from thermal power plants is, however, generally stored in the wet state to prevent them from flying off and contain, on average, a proportion of moisture of the order of 20 to 40 a and can even sometimes raise up to about 55%.

 This feature makes it necessary to provide a drying operation prior to tcute implementation.

 According to the invention, the idea was to carry out this operation, thanks to the addition of quicklime.

 The invention therefore relates to a composition for the stabilization of soils, in particular road surfaces, especially highways, characterized in that it contains in proportion by weight between 25 and 80% of fly ash and between 20 and 40% of lime.

 The fly ash used in accordance with the invention is preferably constituted by a product from thermal power stations, in particular with coal or fuel oil, having a moisture content of between 20 and 55%.

 Experience has shown that ashes from coal-fired power plants such as, for example, those from the Pont sur Sambre thermal power plant, whose main constituent is aluminum silicate, make it possible to obtain particularly satisfactory performance.

 It should be noted that lime is added beforehand to fly ash in the form of quicklime; however, part of it is consumed for drying wet fly ash in accordance with the well-known chemical hydration reaction of lime-lime (CaO + HO <Ca (OH)>) strongly exothermic which occurs from almost immediately to give a mixture with pozzolanic properties that can cutter assimilated to cement. Consequently, when it is used for the treatment of soil, the lime contained in the composition is in the form of a mixture of quicklime and slaked lime.

 The composition according to the invention is particularly advantageous for the stabilization of soils particularly sandy-loam or clay with a high moisture content, of the order of 22 to 24.

 Indeed, in the case of such soils, it is necessary to provide a double treatment first lime to obtain, in addition to the necessary lift, a homogeneous and optimal water content, and then cement.

 This conventional stabilization process, however, has the disadvantage of being able to be implemented in two steps, and therefore to require two successive passes, which increases to a large extent its cost price.

 However, the composition according to the invention, allows, thanks to the application of lime and fly ash, to replace this double treatment by a stabilization in one step and therefore can be implemented in a single pass, this for a result at least the same.

 It is, however, not totally excluded to carry out a lime pretreatment on this type of soil if the latter does not yet have the characteristics necessary to be treated with lime / ash mixture.

 According to a preferred feature of the invention, the composition contains, in a proportion by weight, between 25 and 45% of fly ash, between 20 and 40% of a mixture of quicklime and slaked lime, and between 25 and 45 a fine limestone with a particle size less than 80 p.

 As already indicated, the lime is in fact in the form of a mixture of quicklime and of slaked lime having already reacted with the moisture possibly present in the starting fly ash.

 However, it is imperative that the proportion of quicklime be sufficient to treat soil moisture.

 By way of nonlimiting example, a composition containing in weight proportions about 35% of fly ash, about 30% of lime and about 35% of fines has been found to be particularly satisfactory.

 According to this preferred feature, part of the fly ash is therefore replaced by fine limestone (girl) which generally correspond to a dusting product that is present in suspension and stored in silo.

 The addition of this product makes it possible to obtain a specific surface close to that of the cement.

 The invention also relates to a method of stabilizing soils, in particular pavements of roads, especially highways by which the soil to be stabilized, in a single step, with the composition described above.

 According to another characteristic of this process, the soil to be stabilized is treated by spreading the composition on the ground and kneading it to a height of about 30 cm.

Claims (3)

 10) Composition for the stabilization of soils, in particular road surfaces, especially highways, characterized in that it contains, in a proportion by weight, between 25 and 80% of fly ash and between 20 and 40% of lime.  20) Composition according to claim 1, characterized in that the fly ash has a moisture content of between 20 and 55 t.  3) Composition according to any one of claims 1 and 2, characterized in that the fly ash from thermal power plants, including coal or fuel.  40) Composition according to any one of claims 1 to 3, characterized in that it contains in proportion by weight between 25 and 45% of fly ash, between 20 and 40 k of a mixture of quicklime and slaked lime, and between 25 and 45 a fine limestone with a particle size less than 80.  50) Composition according to claim 4, characterized in that it contains in weight proportion about 35 a fly ash, about 30 a lime and about 35% of fines.  6) Method for stabilizing soils, in particular road surfaces, in particular highways, characterized in that the soil to be stabilized, in a single step, is treated with a composition according to any one of claims 1 to 5.  7) Process according to claim 6, characterized in that the soil to be stabilized is treated to a height of about 30 cm.