Effect of the interaction between calcium sulphate and mineral additives on shear strength parameters of clayey soils

In Algeria, inappropriate soils with high plasticity and low bearing capacity are commonly encountered. These soils can be improved using chemical stabilization technique in order to render them acceptable for construction projects. However, various forms of deterioration have been observed in these construction projects due to the presence of sulphates responsible for the formation of new expansive minerals such as mineral ettringite. For this raison, an experimental investigation was undertaken to study the effect of the presence of calcium sulphate (CaSO4 center dot 2H(2)O) on the behavior of the shear strength of two clayey soils stabilized with lime (L; P0L0, P0L4, and P0L8), natural pozzolana (NP; P10L0 and P20L0), and their combination (L-NP; P10L4, P20L4, P10L8, and P20L8). In this study, the mechanical property investigated is the shear strength on samples cured for 7 to 120 days. In addition, X-ray diffraction (XRD) test is also conducted in order to investigate the mineralogical changes of both clayey soils. The results show that both clayey soil samples without CaSO4 center dot 2H(2)O can be successfully stabilized with L or with a combination of lime-natural pozzolana (L-NP), which substantially increases their shear strength and produces high values of shear parameters. In addition, when combining a fraction of CaSO4 center dot 2H(2)O (especially with high content) to samples containing L alone or a combination of L-NP, a further increase in shear strength values is obtained and higher shear parameters values are recorded. The increase in strength values in the presence of CaSO4 center dot 2H(2)O is attributed to the formation of ettringite, which is observed in XRD diagrams. In general, it is shown that the chemical soil stabilization success depends on several factors: the CaSO4 center dot 2H(2)Ocontent, the type of additive and its content, the curing period, and the mineralogical composition of soil.