In Situ Raman Spectroscopy of a Silica Gel‐Templated Hydration Pathway in CO ₂ ‐Activated Cement

This study investigates early‐age carbonate mineralization in cementitious systems using in situ Raman microspectroscopy. In the presence of dissolved CO 2 , clinker phases undergo accelerated dissolution, decomposing to form various calcium carbonate polymorphs and a transient amorphous silica gel network. Once the available CO 2 is consumed, clinker hydration resumes, leading to delayed calcium–silicate–hydrate (C–S–H) and portlandite (Ca(OH) 2 ) formation. The precipitation of portlandite through the pore network triggers a localized pozzolanic reaction at the silica gel–portlandite interface, yielding a distinct calcium–silicate–hydrate (C–S–H*). This templated mechanism produces a homogeneous and highly polymerized binder, leading to improved 24‐h compressive strength compared to reference samples. Correlation function analysis confirms that the evolution of silica gel, portlandite, and C–S–H follows a three‐stage sequence— mineralization , transition , and stabilization— quantitatively demonstrating a strong spatial anticorrelation between silica gel dissolution and portlandite precipitation. These findings establish a new chemomechanical framework for CO 2 mineralization in cement, highlighting transient silica gel as a critical intermediate for engineering sustainable, high‐performance concrete.