Use of the Methylene blue stain test in preliminary analyses of lime-treted clayey soils

The construction of road, railways and airport embankments requires the use of materials with good physical and mechanical properties, the supplying of which often requires the opening of quarries located far from the construction site; this make arise evident problems both on the environmental and on the economical point of view. There is therefore a strong interest for the techniques able to improve soils characteristics and allowing to use materials which would otherwise be rejected, such as clayey soils. The most popular method to stabilize clayey soils is the lime stabilization, which is widely used due to its simplicity and cheapness. When lime is added to a clayey soil it first reacts by exchanging calcium ions with the interlayer cations of clay and inducing the formation of flocks; such a process is known as modification. The modification leads to a more open framework and reduces dry density and plasticity, making the soil more workable. Once the affinity for lime of the clayey part of a soil is satisfied, if a further amount of lime is available, and there are suitable conditions, the stabilization can occur. This process improve the soil workability, reduces plasticity, compressibility and swelling and improves the main geotechnical parameters. The reactions leading to stabilization are known as pozzolanic and the amount of lime necessary to induce them strongly depends on the cation exchange capacity of the treated soil. Most of the methods normally used to estimate the amount of lime needed to pass from modification to stabilization are affected by uncertainty, since they establish such amount on the basis of the plasticity index, which is not always representative of cation exchange capacity. On the contrary a good estimation of the cation exchange capacity can be obtained by evaluating the specific surface. Thus, introducing in the preliminary analyses for lime treatment tests able to estimate the specific surface of clayey soils, can be useful to have a sound indication of the amount of lime necessary to pass from modification to stabilization. Furthermore, since the specific surface of clayey soils is known to decrease while adding lime, the use of such tests can help in verifying the efficiency of modification. In this work the Methylene blue stain test was used to study the efficiency of lime treatment, in the field of modification, in soils with clayey fraction made by Kaolinite and Na-Montmorillonite, mixed in different proportions. The results have shown that this test is able to determine whether a soil is prone to be efficiently modified. The Methylene blue test is able to follow the evolution of clayey minerals in the lime-modification field, since it detects the gradual reduction of specific surface which takes place as lime is added, up to a lime weight% which satisfies the affinity of clayey minerals for lime, and which depends on the clay nature. The analysis showed that the higher the soil activity the higher the specific surface decrease and the higher the lime percentage above which specific surface does no longer reduces; such percentage corresponds to the lime quantity which separates the modification and stabilization fields. The Methylene blue test can therefore be used both to preliminarily estimate how efficient the modification process of a soil would be, and to define the lime quantity allowing to enter the stabilization field. The mineralogical investigations (X-ray powder diffraction analysis of glycolated and untreated sample to evaluate the swelling properties evolution) carried out on the soil more rich in montmorillonite strengthened the results obtained by the Methylene blue test. In this soil, the addition of about 4% of lime reduces interlayer distance, mass loss and swelling properties. Moreover this lime addition changes the microtexture of clay soils, increasing the pore index and the workability of the material, as evidenced from SEM analysis.