Long-term chemical composition of clinker-free hydraulic binder from metakaolin, anhydrite II and lime: 27Al and 29Si MAS NMR, DTA-TG-MS, XRD and SEM investigation

• Calcium-sulfate activation of metakaolin with 1:1 CaSO 4 :Ca(OH) 2 optimal ratio. • Long-term structural stability for sustainable construction. • Thaumasite formation by laboratory temperature as secondary product of hydration. • 29 Si MAS NMR uniquely identifies thaumasite undetected by conventional XRD. • C-A-S-H phase in hydraulic binder confirmed by NMR. This study presents a comprehensive long-term investigation of a novel clinker-free hydraulic binder (3C binder) based on calcium-sulfate activation of metakaolin with slaked lime and anhydrite II. Unlike previous studies limited to short-term hydration (28 days), this work explores the phase evolution and microstructure of the binder over 2 years of storage in a humid environment using advanced analytical techniques: XRD, 27 Al and 29 Si MAS NMR, DTA-TG-MS, DSC, and SEM. XRD revealed a high initial amorphous phase content (up to 60 wt. %), ettringite and anhydrite as main crystalline phases. Over time, mixtures with higher anhydrite and lower lime content displayed ettringite decomposition into gypsum and amorphous phase, while the optimized 1:1 CaSO₄:Ca(OH)₂ ratio (3C-A sample) maintained phase stability. In addition, 29 Si MAS NMR confirmed the presence of thaumasite in all samples, which had not been attested with certainty by other methods. Compressive strength reached 104 MPa for 3C-A after 1 year, which is higher than values ​​reported for Portland cement. The findings demonstrate that optimized clinker-free binders can achieve high durability and strength, offering a viable alternative to Portland cement. These results support the development of eco-friendly binders with lower carbon footprints, contributing to sustainable construction practices and facilitating more ecologically responsible building materials.