The cement industry's carbon footprint necessitates sustainable supplementary cementitious materials (SCMs). This study evaluates a circular utilization pathway in which CO2 is first captured by halloysite-rich kaolin-based adsorbents and the CO2-laden spent solids are reused as SCMs in cement mortars. Raw halloysite-rich kaolin (Hal), its calcined counterpart (CHal), and biocomposites containing locust bean gum (Hal/LBG) or quillaja saponin (Hal/SPN) were evaluated in both CO2-loaded and non-loaded forms at 10% partial cement replacement and a fixed water-to-binder ratio of 0.47. Hal/SPN-containing mortars remained plastic after 24 h and were therefore excluded from hardened-state testing. Mortars were cured for 3, 7, 28, and 90 days and assessed for compressive and flexural strength, supported by X-ray diffraction, thermogravimetric analysis, Fourier transform infrared spectroscopy, and scanning electron microscopy at 28 and 90 days. The results showed that CO2 loading significantly enhanced 3-day compressive strength, with 10%CHal-CO2 reaching 31.18 MPa compared to 21.53 MPa for the control. At 28 and 90 days, 10%CHal-CO2 achieved the highest compressive strengths among the SCM-containing mortars, reaching 59.40 and 60.09 MPa, respectively, compared to 54.07 and 57.20 MPa for the control. The same mixture also achieved the highest 28-day flexural strength of 8.55 MPa, approximately 17% above that of the control. These improvements were accompanied by lower portlandite contents and greater hydrate-related phase development in the calcined systems. In contrast, raw Hal showed limited reactivity, while Hal/LBG exhibited reduced later-age compressive strength. Overall, the findings support coupling clay-based CO2 capture with partial cement replacement as a promising circular utilization pathway.
