Understanding the synergistic effects of CO2-impregnated zeolite in cement pastes blended with 70% BOFS

This study investigates the synergistic effects of integrating CO₂-impregnated 13X zeolite into cement pastes blended with 70 % basic oxygen furnace slag (BOFS). Replacing 10–20 wt% of BOFS with either the as-received or CO₂-impregnated zeolite, enabled a systematic investigation into the mechanisms affecting both fresh and hardened properties. The results revealed that the as-received zeolite initiated early-stage pozzolanic reactions, reducing flowability by 1–5 % and shortening setting times by 23–72 %, while enhancing compressive strength at 1–3 days by 35–123 %. However, at later stages (7–28 days), the fragile interfacial transition zone (ITZ) between zeolite and the BOFS-OPC hardened pastes, coupled with the inherently low strength of zeolite, resulted in a 5–16 % reduction in compressive strength. The incorporation of CO₂-impregnated zeolite triggered internal carbonation, decreasing the availability of Ca(OH)₂ from BOFS and forming CaCO₃ that initially inhibited pozzolanic activity. This led to a 34 % reduction in flowability and lower early strength compared to as-received zeolite pastes. At later stages, CaCO₃ densified the matrix, improving interfacial bonding and compensating for zeolite’s inherent weaknesses, ultimately improving strength. The 20 wt% CO₂-impregnated zeolite blend demonstrated the optimal performance, balancing early strength enhancement with long-term performance due to the strength-enhancing effect of CaCO₃.