Development of modified techniques for accelerated carbonation of recycled concrete fines

Abstract

The generation of waste concrete, the shortage of natural aggregates, and the high CO2 emission are three parallel issues encountered by the construction industry. Carbonation on the waste concrete has been reckoned as a comprehensive solution to address the three issues at a time, because it improves the qualities of crushed waste concrete, facilitates their re-utilization as secondary aggregates, and absorbs CO2. However, with the emerging need to improve carbonation efficiency and effectiveness, manual control over carbonation via different approaches was investigated in this thesis. Meanwhile, the applicability of carbonation to a wider range of waste concrete was examined. Recycled concrete fines (RCFs), the fine fractions produced by crushing waste concrete that have a higher amount of residual cement paste and higher specific surface area, thus higher porosities, inferior properties and higher CO2 reactivities, are selected as the main objects of interest. Firstly, the effectiveness of a magnesium-based modification on conventional pressurized dry carbonation, aiming at enhancing the compactness and roughening of the surface textures of RCFs, was tested. Secondly, an aqueous carbonation approach, aiming at taking advantage of the dissolution-precipitation behaviour to create a shell-core structure for the benefit of better bonding capacity, was used. Meanwhile, the influence of aqueous carbonation on RCFs, with a particular focus on the particle size effect, was investigated. Aiming at further improving the carbonation efficiency, the aqueous carbonation approach was modified through the use of alkali solutions and elevated temperatures, and its effect on RCFs was evaluated. Additionally, the applicability of carbonation treatment on the RCFs derived from blended concrete with different supplementary cementitious materials (SCMs) was also examined. The results suggested that the magnesium-modified carbonation was characterized by the capacity to create a more densified microstructure and roughened 3D surface texture due to the formation of Mg-calcite clusters. Meanwhile, the aqueous carbonation was found to develop a shell-core structure of RCFs that featured an outmost calcite coating, a silica-rich layer, a partially decalcified layer and an uncarbonated matrix. Besides, the aqueous carbonation, due to the rapid connection of the pore network in the presence of the bulk water environment, had a strong ability to promote the diffusion of aqueous carbonate species and thus increase the carbonation efficiency. Additionally, the effect of aqueous carbonation on RCFs was found to be particle-size dependent, as the large particles (>0.6 mm) could have densified microstructure while the finer particles were more likely to be decomposed to have a higher reactivity. As a comparison, the enhanced aqueous carbonation approach by integrating the use of sodium hydroxide solution and elevated temperature has an extraordinary capacity to enhance the early carbonation rate that the dry carbonation was less likely to break through. More importantly, the carbonation behaviours of RCFs made from blended concrete were found to be closely linked to the type of SCMs and their dosages. Overall, the research presented in this thesis was expected to enhance our understanding of the control over carbonation technique and lay the foundation for the industrial production of RCFs with higher qualities.