Evaluation of the use of eucalyptus chip ash in the treatment of clayey and silty soils

Eucalyptus Chip Ash (ECA) is a calcium-rich biomass combustion by-product generated in significant quantities in Brazil, yet its geotechnical use remains insufficiently documented when compared with other biomass ashes and conventional stabilizers. This study investigates the behavior of clayey and silty soils treated with ECA, focusing on compaction, California Bearing Ratio (CBR), expansion, flexural tensile strength, and microstructural and mineralogical changes. Soil-ash mixtures containing 0, 5, 10, 15, and 20% ECA by dry mass were prepared and characterized by X-ray fluorescence, X-ray diffraction, scanning electron microscopy, and laser granulometry. The results showed an optimum mechanical response to near 15% ECA for both soils. At 15% ECA, CBR approximately doubled in the clay under all compaction energies, increasing from 17.1 to 21.7 to 34.4–43.7, whereas in the silt it increased from 1.4 to 3.0 to 13.2–15.9, corresponding to gains of approximately 430–843% depending on compaction energy; long-term flexural tensile strength reached approximately 3.2 MPa for the clay and 3.3 MPa for the silt. In contrast, expansion remained negligible in the clay (< 0.03%) but persisted close to 3% in the silt, restricting the broader applicability of the latter. The mineralogical and microstructural results are consistent with calcium-mediated soil modification, partial pore filling, and carbonation-related transformation, although these mechanisms were not directly quantified. Under the investigated conditions, ECA behaved primarily as a soil-modifying additive rather than a conventional pozzolanic binder, demonstrating greater technical potential for clayey soils and for applications where volumetric stability can be adequately controlled.