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+.