Carbonation Kinetics of Waste Concrete

Mineral carbonation or CO₂ mineralization offers the possibility to reduce anthropogenic CO₂ emissions permanently by fixing the CO₂ into solid carbonates. The use of waste concrete as a calcium source to form carbonates has been investigated in literature, e.g. [1]. The overall mineralization process involves the leaching of calcium ions from the waste concrete, followed by the precipitation of the calcium ions as calcium carbonate. The kinetics of this process, however, is not well understood. In the current work, the dissolution kinetics of waste concrete was experimentally measured. A physical model was then developed that describes the experimental profiles. Additionally, experiments were performed that demonstrate the feasibility to mineralize waste concrete particles under flue-gas conditions.

Synthetic waste concrete was prepared from ordinary Portland cement, sand, and water. The concrete block was dried and cured for 28 days. It was then crushed into a sub-600 micron size fraction, and the properties of the particles were analyzed by various techniques, e.g. XRF (for chemical composition), XRD, TGA, pycnometry.

The dissolution of waste concrete in aqueous solutions was performed over a range of temperatures (25-60°C), CO₂ partial pressures (0.025 to 0.91 bar) and slurry densities (0.1 to 2 wt. %). The initial rate of dissolution of waste concrete was found to increase with temperature and CO₂ partial pressure. Under low slurry density conditions (0.1 - 0.25 wt. %), 80% of the calcium was leached from the waste concrete in 150 min. At higher slurry densities (2 wt.%), the extent of calcium leached into the solution dropped to about 25%. This decrease in the extent of dissolution with slurry density is due to equilibrium limitations to the dissolution of waste concrete.

A kinetic model for the dissolution of waste concrete was developed that can describe the experimentally measured profiles. The waste concrete was assumed to be composed of a mixture of multiple chemical species, namely calcium silicate hydrate, calcium hydroxide, calcium carbonate, and quartz. Surface complexation models were used to describe the dissolution rates of the individual chemical species. A population balance equation was used to describe the evolution of particle sizes (and reactive surface areas) during the dissolution process. Kinetic parameters were estimated by fitting the experimentally measured profiles, and the model was able to describe these profiles to a very good extent.

Finally, a set of single-step (batch) mineralization experiments were performed with flue gas (20 mol.% CO₂) at a pressure of 10 bar. At 25°C, 50% of the calcium in the waste concrete was mineralized within 150 min. The presentation will include additional results from the currently on-going experimental campaign.

1. Iizuka, A., Katsuyama, Y., Fujii, M., Yamasaki, A., & Yanagisawa, Y. (2004). AIChE Annual Meeting, Conference Proceedings, 79–86.