Innovative use of solid CO2 as an eco-friendly additive in alkali-activated concrete for enhancing its performance

To date, carbon utilization within the concrete industry is predominantly revolved around gaseous and liquid carbon dioxide (CO2), with relatively limited research dedicated to the application of solid CO2. This study was the first to use solid CO2 (dry ice) as an additive in alkali-activated slag and fly ash (AASF) to investigate its influences on the alkali-activation process and various property improvements. Two study systems were designed: one with alkali-activated slag (AAS) as the control group and fly ash (FA) added at substitution rates of 10%, 30%, and 50%; and the other with 50% FA alkali-activated materials (AAM) as the control group and dry ice added at substitution rates of 4%, 8%, and 12%. Several tests were conducted on AASF, encompassing assessments of setting time, compressive strength, and durability (electrical resistivity and chloride ion diffusion coefficient). Furthermore, a comprehensive analysis of its microstructure was undertaken, employing analytical techniques, such as X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), and thermogravimetric analysis (TGA). The results showed that FA substitution effectively mitigated the short setting time of AAS; however, the mechanical properties and durability decreased by approximately 50%. When dry ice was added to AAM concrete, it induced a series of physical and chemical effects that altered the kinetics of the hydration reaction of AAM. Specifically, adding dry ice significantly prolonged the setting time of AAMs. Moreover, this addition did not damage the mechanical properties or durability of AAS(F).