Recycled concrete aggregates modification by enzyme mediated CaCO3 precipitation

Please check whether enrolling in this project is possible by directly contacting the responsible supervisor(s) indicated below as soon as possible. Note that although we announce many topics, we won't be able to offer all of them simultaneously due to limited lab capacity.

The utilization of recycled concrete aggregate (RCA) derived from substantial volumes of global construction waste represents an environmentally and economically advantageous solution. However, the widespread application of RCA faces a hurdle in the form of poor material quality, because they are characterized by large internal pore volume and cracks in the aged mortar. An appealing approach to address this issue involves the modification of RCA through the precipitation of CaCO₃. The reaction between the Ca²⁺ ions present in RCA and atmospheric CO₂ leads to CaCO₃ precipitation, potentially mitigating the problems of poor material characteristics by reducing porosity and enhancing the interface between the old mortar and aggregates. Simultaneously, atmospheric CO₂ sequestration could be achieved within the large amount of RCA globally.

Nonetheless, the process of CaCO₃ precipitation may be impeded by various factors, with the hydration reaction of CO₂ serving as one of the limiting steps. To facilitate CaCO₃ precipitation and RCA modification, the application of carbonic anhydrase (CA)may be a potential solution. CA is a group of enzymes that act as a catalyzer for CO₂ hydration to H⁺ and HCO₃^- and has been reported to increase the reaction rate by about 10⁷ times compared to the process without CA. Therefore, CA can facilitate CaCO₃ precipitation by accelerating CO₂ hydration process. Additionally, CA not only plays a role in the rate of CaCO₃ precipitation but also the morphology of the CaCO₃ crystals. Studies have shown that CaCO₃ formed under different conditions presents different morphologies, sizes, and crystallographic structures. Therefore, CA could be used for mediating CaCO₃ precipitation that could fit the RCA materials better and provide more mechanical strength than naturally precipitated CaCO₃.

This project aims to achieve efficient modification of RCA by utilizing CA and to investigate the influence of CA on the engineering performance of the RCA. Various environmental conditions could be interesting to study, such as different levels of CO₂ concentrations, carbonation duration, and relative humidity (dry or wet conditions). The physical properties of modified RCA will be measured to evaluate modification. The pore structure and microstructure of RCA will be studied using selected methods.

The student is expected to carry out experiments in the lab, which involve working with RCA samples. The student will get an opportunity to work with characterization techniques such as thermogravimetric analysis (TGA), BET surface area analysis, and mechanical investigations.