<p>The reusing of waste concrete aggregate in geotechnical engineering has drawn significant attention due to its already high but still increasing annual production globally. Due to low particle strength and undesirable grading characteristics, recycled fine aggregates (RFA), comprising up to 40% of recycled aggregates, are often discarded as waste. Aiming to reuse RFA as a substitute of cement-stabilized aggregates in highway subbase/base construction, the effectiveness and mechanisms of the MgO carbonation technique, which enables the coupled carbonation of RFA and the bonding agent MgO, on strengthening of the RFA were systematically investigated. The influence of water-to-cement ratio, MgO content, compaction density, and carbonation time on the physical–mechanical properties was thoroughly investigated. The carbonation significantly enhanced the strength of the RFA–MgO specimens, with 10% of MgO inclusion leading to 12.29&#xa0;MPa unconfined compression strength. However, different from traditional cement stabilization, excessive MgO inclusion and high compaction density can be detrimental to the overall strength because of the possible negative effect on the CO₂ transport. Microstructure analysis exhibited that rodlike nesquehonite, together with the calcite produced by the old cement on the surface of the RFA, provided the main bonding strength. Composite products such as magnesium silicate were observed, manifesting the contribution of the reaction between the MgO and the siliceous material on the surface of RFA to the strength growth. The results demonstrate the immense application potential of reinforced RFA and provide promising and sustainable method for the strengthening and application of RFA in road engineering projects.</p>