Recycled concrete aggregates (RCA) as a carbon sink: CaCO3 precipitation mediated by bio-based materials

Introduction:

Using 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 poor material quality, 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 old mortar 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 globally within a large amount of RCA.

Nonetheless, the process of CaCO₃ precipitation may be impeded by various factors. One of the rate-limiting steps is the hydration of CO₂ (CO₂ + H₂O –> H⁺ + HCO₃^-), the other rate-limiting step could be Ca²⁺ supply from the solid phase to the liquid phase. This study will investigate factors that could play a role in CaCO₃ precipitation. To facilitate CaCO₃ precipitation, applying carbonic anhydrase (CA) may be a promising method. CA is a group of enzymes that exist in all living kingdoms, and CA acts 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, with the presence of Ca²⁺ ions in RCA, CA can facilitate CaCO₃ precipitation by accelerating the CO₂ hydration process. Additionally, CA plays a role in the morphology of the CaCO₃ crystals formed. Our preliminary results have shown that CaCO₃ precipitation mediated by CA shows faster calcite formation. Therefore, CA could mediate CaCO₃ precipitation in RCA and provide more mechanical strength than naturally precipitated CaCO₃.

Project aim:

This project aims to achieve CO₂ sequestration with RCA using bio-based materials. The rate-limiting steps of CaCO₃ precipitation will be investigated to facilitate the carbon capture process in RCA. RCA will be treated with CA solution and carbonation will be applied to achieve CaCO₃ precipitation. Various environmental conditions are interesting to study, such as different levels of CO₂ concentrations (natural or accelerated carbonation), carbonation duration, temperature, and relative humidity (dry or wet conditions).

The physical-chemical properties of modified RCA will be measured, including thermogravimetric analysis (TGA) for analyzing the CH and CaCO₃ content in RCA, X-ray diffraction (XRD) for studying the CaCO₃ crystal structure, scanning electron microscopy (SEM) will be conducted to investigate the microstructure and CaCO₃ morphology. Water absorption and mechanical tests will be conducted to evaluate the modification performance of the RCA.

Student work:

The student(s) is(are) expected to carry out RCA carbonation experiments in the lab, which involve working with RCA and carbonation chamber. The student(s) will be able to work with characterization techniques such as TGA, water absorption, and mechanical tests. XRD for the CaCO₃ crystal structure study, and SEM for the microstructure study of RCA, will be carried out by the supervisor (Xiulin) with the help of the student(s). The student(s) will help to analyze the data obtained from the tests and try to discuss the results.

At the end of the project, the student(s) will be equipped with the knowledge of various material science related characterization technologies and the knowledge of the RCA carbonation.

Technical requirements:

The student(s) need(s) have basic knowledge about concrete