In Situ Raman Spectroscopy of a Silica Gel‐Templated Hydration Pathway in CO2‐Activated Cement
This study investigates early‐age carbonate mineralization in cementitious systems using in situ Raman microspectroscopy. In the presence of dissolved CO2, clinker phases undergo accelerated dissolution, decomposing to form various calcium carbonate polymorphs and a transient amorphous silica gel network. Once the available CO2 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 CO2 mineralization in cement, highlighting transient silica gel as a critical intermediate for engineering sustainable, high‐performance concrete.