<p>Stabilization of an expansive soil containing sulfate with lime fails to effectively mitigate the swelling behavior of these soils as a result of the occurrence of ettringite, a highly expansive mineral. This study evaluated the effectiveness of utilizing 15% Soma fly ash (SFA) as an alternative to 4% lime (L) in enhancing the stability of expansive soil containing sulfate (SO₄) against swelling. Maximum allowable SO₄ content for the stabilization of the expansive sulfate clay with SFA was investigated to achieve the greatest reduction in swell potential and simultaneously inhibit the formation of ettringite. Index, swelling, and unconfined compressive strength (UCS) tests, together with scanning electron microscope (SEM) views and X-ray diffraction (XRD) analysis, were used to assess the impact of fly ash on the behavior of sulfate-bearing expansive soil. SO₄ concentration exhibited varying effects on the swell potential of treated specimens. A noticeable increase in the swell potential of expansive sulfate clay was observed when treated with 4% L, but no significant increase was observed when treated with 15% SFA. The UCS of the expansive sulfate clay specimens treated with both additives generally decreased. A significant reduction in strength was recorded for expansive sulfate clay treated with 15% SFA and exposed to 27,000&#xa0;ppm SO₄ when cured at 10&#xa0;°C, primarily due to ettringite/thaumasite and thenardite formation. The results indicated that 15% SFA can be safely used for stabilizing expansive soils containing SO₄ without causing heave or durability problems, provided that the SO₄ concentration of the expansive soil is below the threshold level of 6750&#xa0;ppm.</p>