<p>Directly carbonating MgO (magnesia) through gaseous CO₂ is a sustainable method in the treatment of sandy and silty soils. However, it is challenging to use this method to stabilize clayey soils as the low permeability of such soils may block the CO₂ flow. This study proposed a new method (named as ‘indirect MgO carbonation’) that used NaHCO₃ as a CO₂ carrier and then mixed with MgO and clayey soils to achieve MgO carbonation and stabilization of clayey soils. To clarify the effectiveness of the indirect MgO carbonation, small-scale and large-scale carbonation tests in the laboratory were conducted to compare direct and indirect carbonation methods in treating clayey soils. The physical, mechanical, microstructural, and mineralogical properties of the soils subjected to direct and indirect carbonation were investigated. The results showed that indirect MgO carbonation achieved higher unconfined compressive strength (UCS) values and water content in clayey soils compared to direct carbonation at MgO/NaHCO₃ ratios of 1:0.25 to 1:0.5. Large-scale laboratory carbonation tests demonstrated that indirect carbonation produced more uniform strength and moisture distribution, as evidenced by lower coefficient of variation values. Independent of CO₂ flow conditions, indirect MgO carbonation presents a viable solution for in situ clayey soil stabilization.</p>