Effects of CO₂-mineralized aragonite recycled concrete powder (ARCP) on the early-age strength and microstructure of limestone calcined clay cement (LC3)

Limestone calcined clay cement (LC3) is considered the most promising strategy for reducing CO₂ emissions, owing to its ability to substitute over 50 % cement clinker. However, the low reactivity of limestone limits early-age strength development in LC3. This study employed crystal phase regulation to prepare aragonite recycled concrete powder (ARCP) as a limestone substitute in producing LC3-ARCP, aiming to overcome the drawback of the low early-age strength in conventional LC3. The effects of ARCP on macroscopic (particularly early-age compressive strength) and microscopic properties of LC3 were systematically investigated, with comparative analysis against conventional LC3 and CO₂-mineralized recycled concrete powder system (LC3-CRCP). The results indicate that LC3-ARCP significantly enhances early compressive strength compared to LC3, particularly at 10 % ARCP replacement (LC3-ARCP10). The compressive strength of LC3-ARCP10 increased by 23.52 % at 3 days and 26.64 % at 7 days compared to LC3. This improvement is attributed to the highly reactive aragonite and Si-Al gel in ARCP, which accelerate the formation of calcium silicate hydrate (C-S-H) and monocarboaluminate (Mc). These hydration phases promote the formation of a more compact microstructure. Furthermore, ARCP exhibited a higher specific surface area than limestone and CRCP, providing abundant nucleation sites for the formation of C-A-S-H, thereby accelerating cement hydration and enhancing the early-age strength. Additionally, the 28-day splitting strength of LC3-ARCP10 increased by 57.51 % compared to LC3. This increase was attributed to the needle-like structure of ARCP, which enhances the toughness of the matrix. Grey correlation analysis shows that the grey correlation degree of LC3-ARCP10 was improved by 61.66 % and 71.22 % compared to LC3 and LC3-CRCP10 (with 10 % CRCP replacement), respectively. These increases demonstrate that ARCP improves the comprehensive performance of LC3 and shows superior performance to CRCP. As a novel low-carbon cementitious material, LC3-ARCP offers significant potential, not only by facilitating the resource utilization of solid waste but also by providing a new pathway for the wide application of LC3 systems.