Comparative assessment of thermal methods for quantifying CO2 sequestration in alkaline industrial residues

Reliable quantification of CO 2 sequestration in alkaline industrial residues remains challenging due to their heterogeneous composition and overlapping thermal decomposition reactions. Co-combustion ash (CCA), steel slag (SS), and stone wool (SW) exhibit high alkalinity and reactive mineral phases, making them promising candidates for mineral carbonation. This study comparatively evaluates the applicability and limitations of the loss on ignition (LOI), thermogravimetric analysis (TGA), thermogravimetry coupled with mass spectrometry (TGA-MS), and calcimetry for assessing the CO 2 sequestration capacity in these materials. CCA exhibited the highest CO 2 sequestration capacity, with values of 144.2, 212.6, and 188.7 g CO 2 kg −1 determined by TGA, TGA-MS, and calcimetry, respectively. The corresponding values for SS were 78.7, 87.0, and 91.7 g CO 2 kg −1 , whereas SW showed no measurable CO 2 uptake under the applied carbonation conditions. The larger method-dependent differences observed for CCA resulted primarily from CO 2 evolution below the fixed temperature interval conventionally used for TGA evaluation. TGA-MS directly identified this low-temperature CO 2 release, while quantitative X-ray diffraction with Rietveld refinement showed that crystalline carbonate phases did not account for all CO 2 measured by the thermal and chemical methods. Both CCA and SS had undergone partial natural carbonation before treatment, demonstrating the importance of characterising residues in their as-received state. The results show that CO 2 quantification is strongly material-dependent and that complementary thermal, mineralogical, and chemical analyses are needed to identify sources of discrepancy; the study therefore provides a comparative assessment rather than establishing a universally applicable reference method.