Use of biomass ashes for the treatment of expansive clayey soils

Clayey soils typically have the requirement to demonstrating undesirable engineering behavior due to their poor physical/mechanical characteristic. Clay soils are often treated by adding chemical agents able to improve their physical and mechanical characteristics. Lime/cement binders provide excellent results, but their production emits significant amounts of greenhouse gases. In this work, intending to create naturally activated soils and repurpose waste we used three different kind of biomass ash generated by pellet, olive and grapevine combustion as stabilizing agents for clayey soil. The mechanical effects of the treatment were evaluated using confined compressive tests at different curing times. When necessary, the ashes were activated by a sodium-based alkaline solution. The evolution of the clay ashes was evaluated through X-ray powder diffraction (XRPD), infrared spectroscopy (FT-IR) and scanning electron microscopy (Fe-SEM). The compressibility was determined utilizing edometric tests on raw and treated samples. Rietveld analysis of the XRPD results allowed a quantitative assessment of the amorphous phase. The XRPD results did not show relevant changes both in the mineralogical phases and amorphous content along curing time, with exception on the illite amount. The FT-IR spectroscopy shows an evolution of the Si-O vibrational mode positions and of their shapes. The EDS-SEM analysis shows an evolution of the Ca/Si distribution and grow of amorphous gels/nanocrystals of C-S-H. This evidence confirmed that the pozzolanic reaction occurs from the dissolution of the clay minerals with the growth of cement phases. These preliminary results indicate that selected biomass ashes could be implemented as an environmentally sustainable binder.