Geotechnical Performance of Alkali-Activated Uncalcined Clayey Soils with Hydroxide- and Aluminate-Based Activators

Abstract

This research evaluates the performance of two low- and high-plasticity clays as the sole precursors to develop alkaline activation at ambient temperature. NaOH solutions with different concentrations of 2 to 10 mol/L along with binary solutions with NaAlO2/NaOH mass ratios of 1 and 0.75 for CL and CH clays, respectively, have been used as alkaline activators. Unconfined compressive strength (UCS), indirect tensile strength (ITS), ultrasonic pulse velocity (UPV) and Atterberg limits tests have all been conducted to thoroughly assess the geotechnical properties of alkali-activated clays. The experimental results show that CL with the best performance when mixed with 8 M NaOH activator renders higher mechanical strength and stiffness compared to CH showing a peak at 4 M NaOH. This observation is primarily attributed to the higher amorphous content and a weak interlayer force in CL that in turn contributes to more reactive silicate phases and gel products. Moreover, binary solution considerably enhances the mechanical performance of parent clays by modifying the Si/Al ratio in the mixture. Another important observation is the vulnerability of the alkali-activated clays to cracks and expansion due to alkali–silica reactions, leading to a significant drop in their mechanical strength and stiffness. Microstructural analyses also reveal the increase in amorphous content and the formation of flocculated particles covered by aluminosilicate gels, especially in CL-based samples where the layer-like structure of clay changes to sponge-like with globular units. More importantly, uncalcined clay-based samples are observed to be prone to the curing time-dependent cracking associated with expanding phases due to alkali–silica reactions or flocculated particles. These observations could be useful in geotechnical engineering practice for a variety of field applications in terms of both safety and performance because not only natural clays have been used as the sole precursors but also a relatively low alkali concentration has contributed to the optimum improvement of the composite material.