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This study focused on the synthesis of aragonite whiskers through a synergistic wet carbonation technology utilizing waste magnesia slag (MS) and magnesium sulfate (MgSO₄), aiming to improve the microstructure and mechanical properties of ordinary Portland cement (OPC) paste. The influence of MgSO₄ concentration on the wet carbonation process, phase composition, and microstructure of MS was investigated. Furthermore, the effect of incorporating carbonated MS (C-MS) on the mechanical properties and microstructure of Portland cement paste was evaluated. Results showed that appropriate MgSO₄ concentrations favored aragonite whisker formation. A concentration of 0.075 M MgSO₄ yielded 86.6% aragonite with high aspect ratio nanofibers. Incorporating 5% of this C-MS into OPC increased the seven-day compressive strength by 37.5% compared to the control OPC paste. The improvement was attributed to accelerated hydration and reduced porosity by the filling effect and microfiber reinforcement of aragonite whiskers. MS demonstrated good CO₂ sequestration capacity during carbonation. This study provides an effective method to synthesize aragonite whiskers from waste MS and use it to enhance cementitious materials while reducing CO₂ emissions, which is valuable for the development of a sustainable cement industry.