Carbon Sequestration Through the Production of Precipitated Calcium Carbonate From Waste Concrete

Abstract

As a result of human activities, rising concentrations of atmospheric carbon dioxide (CO2) are causing climate change. One method to sequester CO2 involves the production of synthetic limestone (CaCO3) from calcium silicate minerals and CO2 via mineral carbonation. Certain grades of CaCO3, including Precipitated Calcium Carbonate (PCC), are sold as a filler and pigment to several industries. Therefore, if marketable as PCC, the production of CaCO3 via mineral carbonation may be an economical method to help mitigate climate change. The global market for PCC offers the potential to utilize several million tonnes of CO2 per year. Waste cement is a suitable source material for PCC production via mineral carbonation and can be recovered from waste concrete as a byproduct of aggregate recycling practices. The objective of this research was to investigate the possibility of sequestering CO2 by producing PCC via the recovery and carbonation of waste cement calcium. An acid (HCl) was used to allow the complete leaching of calcium, and so the dissolved calcium could be separated from the residual material via filtration to enable PCC recovery. A purification step via pH adjustment preferentially precipitated co-leached impurities such as iron and silicon. CaCO3 precipitation was later induced by adding Na2CO3 that can be produced by the absorption of CO2 from flue gases or the atmosphere (air capture) using NaOH. HCl and NaOH can be recycled via bipolar membrane electrodialysis. Although this method may require over twice the energy of the conventional PCC manufacturing process, low carbon electricity can be used and negative process emissions are currently feasible in multiple Canadian provinces. Lower energy carbonation methods could be used where stationary sources of concentrated CO2 are located, however air capture is possible and the use of HCl and a purification step allows for the complete carbonation and utilization of waste cement calcium. This minimizes emissions from hauling residues and may ensure that significantly more CO2 is absorbed by the waste cement than would otherwise be passively absorbed by the material over several decades of atmospheric exposure.