Low-carbon UHPC with carbonated blast furnace slag: Impact of mineral composition, carbonation degree, and CaCO₃ polymorphs

As a common supplementary cementitious material (SCM), blast furnace slag (slag) is widely used in ultra-high-performance concrete (UHPC) to enhance properties and reduce its carbon footprint. Slag can be carbonated before use to further improve resilience and sustainability of UHPC. However, using raw/carbonated slag and other SCMs in UHPC presents challenges: (1) Variations in supply and production cause fluctuations in chemical, mineral, and physical properties, resulting in significant variability in carbonated slag and UHPC properties; (2) The utilization rate of slag is limited due to its low reactivity; and (3) How variations in slag's mineral composition impact carbonation kinetics and UHPC properties remains unclear. This study addresses these fundamental limitations through comprehensive research. Four slags with unique mineral compositions were studied, replacing 40 % and 60 % of cement in UHPC. Both raw and carbonated slags were used to assess the effects of mineral composition, carbonation degree, and CaCO₃ polymorphs on UHPC properties. The findings revealed three insights: (1) slags with similar particle size and chemical composition showed varying impacts on UHPC properties, emphasizing mineral composition's role; (2) mineral composition significantly affects carbonation degree and CaCO₃ polymorph formation; and (3) slag carbonation notably enhances UHPC properties, potentially boosting slag utilization. Slags rich in alite, belite, and anhydrite show higher carbonation, while those with åkermanite and merwinite promote aragonite due to Mg²⁺ ions. Higher alite, belite, diopside, and gehlenite in raw slag increase UHPC compressive strength. Carbonated slag in UHPC increased compressive strength by 20 %, flexural strength by 30 %, and toughness by 45 %.