Mechanistic role of Al in natural hydraulic lime: A mechanistic study on early hydration and carbonation

Natural hydraulic lime (NHL) is a promising carbon-sequestering binder owing to its low calcination temperature and inherent CO 2 uptake via carbonation. However, its engineering application is restricted by the low hydraulic reactivity of C 2 S and poor early mechanical performance. Here, a mechanistic model system of NHL was constructed using a sol-gel-derived Al-bearing precursor to clarify its role in improving early strength and carbonation reactivity. The results show that Al was incorporated into the SiO 2 network through Si-O-Al bonds, altering the local electronic environment, reducing the structural order of the silicate framework, and enhancing SiO 2 reactivity. During sintering, the activated SiO 2 reacted more readily with CaO, promoting the formation of hydraulically active β-C 2 S at lower temperatures while inhibiting its conversion to γ-C 2 S during cooling. Consequently, Al-modified NHL exhibited higher early hydration activity, as evidenced by earlier ion release, accelerated heat evolution, and lower residual β-C 2 S. Under accelerated carbonation, Al-modified samples showed earlier carbonation onset, faster CaCO 3 accumulation, and higher carbonation degree than undoped NHL. The enhanced hydration-carbonation coupling further accelerated strength development and improved CO 2 sequestration efficiency. This work demonstrates that Al regulates silicate activation, β-C 2 S formation, hydration kinetics, and carbonation reactivity, providing guidance for developing high-performance carbon-sequestering lime-based binders.