A comparison study between CaSO4·2H2O and Na2SO4 effects on geotechnical properties of clayey soils stabilised with mineral additives to recommend adequate mixtures as materials for road pavements

Abstract

This paper presents a comparison study between calcium (CaSO4·2H2O) and sodium (Na2SO4) sulphates effects on physico-mechanical properties (pH, plasticity index (PI), unconfined compressive strength (UCS) and shear strength parameters) of both grey and red clayey soils (GS and RS) stabilised with lime (L), natural pozzolana (NP) and their combination (L–NP) in order to recommend adequate mixtures which can be used as building materials for road pavements. In this study, Atterberg limits, pH, UCS and shear strength tests were carried out after different curing periods (1–120 days). Tests results showed that the addition of L as an additive produced a high decrease in the PI of both clayey soils but a considerable increase in UCS, cohesion (C), internal friction angle (φ) and pH was recorded, whereas the NP caused a slight effect. However, when combining both additives, a high decrease in the PI and a further increase in other properties were recorded. Moreover, it was found that the CaSO4·2H2O lowered the PI of both clayey soils whereas the Na2SO4 increased it. Thus, the increase in pH values of all mixtures with sulphate content was more pronounced with the Na2SO4 than with the CaSO4·2H2O. On the other hand, both CaSO4·2H2O and Na2SO4 accelerated the pozzolanic reactions responsible for strength gain. However, at a later stage, the degradation of RS specimens can be explained by the formation of ettringite observed in X-ray diffraction (XRD) patterns. In general, the CaSO4·2H2O developed better effects for soil improvement whereas the presence of Na2SO4 (with a high content) can be classified as a deleterious element for soil stabilisation. From these results, tow tables were given for choosing an adequate mixture for pavement subgrade soil stabilisation.