Enhanced weathering for CO2 removal using carbonate-rich crushed returned concrete; a pilot study from SE Ireland

Abstract

A 1.21-ha pilot study in SE Ireland investigated crushed returned concrete (CRC), applied at a rate of 7.5 tonnes/ha as a soil amendment for carbon dioxide removal (CDR) by enhanced weathering (EW) over a 10-month period. Most, but not all amended sites showed enhanced concentrations of calcium and bicarbonate in shallow soil waters compared with adjacent controls. Soil pH increased rapidly by 0.3 to >1 pH units (5.75 ± 0.25 (n = 8) in controls to 6.5 ± 0.7 (n = 24) at amended sites), with greatest increases in soils that had lowest initial pH (<6.0). Aside from one site where concentrations continued to increase, calcium (1.76–0.39 mM Ca) and bicarbonate (4.06–0.64 mM) values in soil waters declined gradually over the course of the study, consistent with rapid initial dissolution and progressive exhaustion of the fine-grained fraction, as indicated by a shrinking-core model. Most sites exhibited high bicarbonate to cation ratios (1.2–2.2) consistent with CRC dissolution by soil carbonic acid, although two sites adjacent to the field-boundary river that floods periodically showed evidence for strong acid (nitric) dissolution. Enhanced bicarbonate levels in soil waters at amended sites compared with controls, coupled with a downward-flush hydrological model driven by annual effective rainfall to calculate bicarbonate export and a simple stoichiometric correction for nitric acid weathering, indicate gross CDR rates in the range zero to 0.52 tonnes CO2/ha (69 kg CO2/tonne), limited by bicarbonate export rather than weathering rates. Further optimisation of the CDR efficiency of this material should be possible using strategies to avoid or pre-neutralise non-carbonic soil acids and by taking account of sub-aerial carbonation at the post-crushing, pre-amendment stage that arises when the material is crushed in preparation for EW applications. A simplified life-cycle analysis indicates that a discount of about 11% should be applied to account for CO2 emissions from crushing, transport and spreading operations.