Elevating carbonation efficiency and CO₂ capture capacity of steel slag by sodium tripolyphosphate (STP): The role of CaCO₃ morphology modification

Carbonation degree significantly impacts the volume stability of steel slag powder. To elevate the carbonation efficiency of steel slag and thereby alleviate the volume unsoundness of steel slag powder, sodium tripolyphosphate (STP) was used to enhance the micro morphology of CaCO₃ and accelerate Ca leaching and CO₂ diffusion. Three methods for assessing CO₂ uptake were employed to reliably determine the degree of carbonation. Results indicate that steel slag added with 0.05 wt% STP obtained a promotion exceeded 80 % in CO₂ uptake. Additionally, STP increased the CO₂ capture capacity of steel slag by 24.84 %. Under the influence of STP, the expansion of steel slag paste was reduced by up to 31.45 % following an autoclaving test. The formed micro CaCO₃ particles in carbonated steel slag without STP were polyhedral grains and a dense carbonation layer was formed around calcium silicate. Contrastively, CaCO₃ formed in the carbonated STP incorporated steel slag were conical or acicular particles and consequently generated a porous layer. The porous microstructure provided “tunnels” for the continuous CO₂ diffusion and Ca²⁺ leaching and hence maintained relatively higher reaction speed throughout the carbonation process. Furthermore, STP also played the role as a chelator which accelerated the leaching of Ca²⁺.