Influence of salt-forming anions on the geotechnical properties of a soil stabilised with hydraulic binders: effect of cation type

Abstract

A treatment with hydraulic binders confers to soils enhanced physical and mechanical characteristics that allows their integration in a road section. To gain acceptance of this technique, there is a clear requirement of studies that provide information on success and failures. The aim of this study was to investigate the influence of the salts (i.e. anions and cations) which are present in the soil, on the physical and the mechanical properties of a stabilised soil. Four anions (sulphate, chloride, nitrate, phosphate), associated with sodium or potassium, were added to one soil, using different chemical and concentrations combinations, before treatment. Tests in accelerated curing conditions were used because they are the first realised by the earthworks companies before they undertake further advanced study. The influence of the cationic counter ion and/or salt was also assessed, contrary to the usual recommendations (technical guides) that focused on the anions. The synergistic effects of anions and cations were investigated. For the considered soil, the chloride, nitrate or phosphate ions were less disruptive (important losses of strength or/and volumetric swelling) when associated with sodium than with potassium, while a reverse effect was noticed for sulphate ions. The simultaneous presence of sodium and potassium salts, for each type of anions, except nitrate, induced a significant swelling (up to 24%) and a loss of strength (down to - 75%). Considering stabilisation disturbances, sulphate salts appeared as the most disruptive, the significant swellings associated with ettringite formation causing structural damages. Some technical guides attributed the unsuitability for a soil to a treatment formulation to the presence of some anionic species in soils. This study demonstrated that the unsuitability has to be attributed to a salt, i.e. to the specific combinations of anion and cation. The tests that use simultaneously different salts were innovative and increase the reliability of the usual standard tests. That's why it seems important to reconsider the treatment study methodology used by the companies.