Abstract

The production of ordinary portland cement (OPC) is a CO₂ intensive process. Specifically, OPC clinkering reactions not only require substantial energy in the form of heat, but they also result in the release of CO₂; i.e., from both the decarbonation of limestone and the combustion of fuel to provide heat. To create alternatives to this CO₂ intensive process, this paper demonstrates a new route for clinkering-free cementation by the carbonation of fly ash; i.e., a by-product of coal combustion. It is shown that in moist environments and at sub-boiling temperatures, Ca-rich fly ashes react readily with gas-phase CO₂ to produce robustly cemented solids. After seven days of exposure to vapor-phase CO₂ at 75 °C, such formulations achieve a compressive strength of around 35 MPa and take-up 9% CO₂ (i.e., by mass of fly ash solids). On the other hand, Ca-poor fly ashes due to their reduced alkalinity (i.e., low abundance of mobile Ca- or Mg-species), show limited potential for CO₂ uptake and strength gain—although this deficiency can be somewhat addressed by the provision of supplemental/extrinsic Ca agents. The roles of CO₂ concentration and processing temperature are discussed, and linked to the progress of reactions and the development of microstructure. The outcomes create new pathways for achieving clinkering-free cementation while enabling the beneficial utilization (“upcycling”) of emitted CO₂ and fly ash; i.e., two abundant, but underutilized industrial by-products.