Harnessing solid CO₂ in hybrid alkaline cement: Dry ice as a pathway to high-performance and low-emission materials

The concrete industry urgently requires innovative carbon management strategies to mitigate its substantial CO₂ footprint. Conventional carbonation curing is often constrained by equipment requirements and limited applicability to precast components, highlighting the need for alternative solutions suitable for cast-in-place concretes. This study introduces a novel method of incorporating solid CO₂ (dry ice) into hybrid alkaline cement (HAC) systems, enabling simultaneous performance enhancement and carbon storage. HAC mixtures containing 0-15% dry ice were prepared and systematically investigated in terms of hydration kinetics, mechanical strength, durability, and phase evolution. Isothermal calorimetry, XRD, TG, FTIR, and SEM were employed to reveal the mechanisms underlying the observed changes. Results demonstrate that dry ice moderates system alkalinity, promotes clinker hydration, and induces early precipitation of carbonates that subsequently transform into carbonaluminate phases. At an optimal dosage of 10%, compressive strength increased by 37.45% and surface resistivity by 22.69% at 28 days, accompanied by significant microstructural densification. However, excessive addition (15%) led to early temperature drops and reduced slag activation, which impaired overall performance. Sustainability analysis considering two boundary scenarios of CO₂ escape revealed that incorporating 10% dry ice reduced unit strength CO₂ emissions to 3.87-7.8 kg·CO₂/MPa, representing reductions of 51.9-3.23% compared with the control. These findings demonstrate that dry ice addition provides a low-cost, simple, and scalable route to integrate carbon storage with HAC development. This strategy offers new opportunities for achieving carbon-neutral cementitious materials with enhanced durability and structural performance, particularly in field applications where conventional carbonation curing is impractical.