Study on Carbonization Performance of Metakaolin on Tailings- and Recycled-Concrete-Based Green Concrete

As a highly active mineral admixture, metakaolin is often used as an efficient performance-enhancing material for concrete, but its performance in long-term complex service environments still needs to be verified. This article presents a composite green concrete with a substitution rate of 30% for recycled coarse aggregates (RCAs) and iron tailings (IOTs) and a waste polypropylene fiber (WPF) content of 0.6%. Concrete with different mixing conditions of metakaolin was prepared, and its carbonization resistance was studied with macro- and micro-experimental methods. The results indicated that when the content of metakaolin was less than 10%, its mechanical and deformation properties gradually improved, resulting in a maximum increase of 70% in its cubic compressive strength. Overall, carbonization improved the corresponding mechanical properties. For example, when carbonized for 14 days, the compressive strength growth rate increased by nearly 30%, and the elastic modulus did not change significantly. Through microscopic testing, it could be seen that when the content of metakaolin reached 10%, an appropriate amount of metakaolin (10%) promoted the hydration of cement, with the least number of pores and the best compaction performance, resulting in the best overall performance. But when added excessively, the coupling effect of secondary hydration and carbonization reactions could lead to the emergence of new harmful pores in the matrix structure. In future engineering applications of metakaolin, it is recommended that its mixing ratio be less than 10% to achieve better concrete performance.