One chemistry, many pathways: Comparing carbon-negative cement, enhanced weathering, and ocean alkalinity enhancement

We compare carbon dioxide removal pathways involving carbon-negative cement and carbonate concrete (CNCm and CNCb), enhanced rock weathering (ERW), and ocean alkalinity enhancement (OAE). All ultimately rely on alkalinity from enhanced weathering of alkaline silicate rocks, even when calcium and magnesium are extracted from seawater. We assess them using five practical criteria: carbonation efficiency, measurement, reporting, and verification (MRV), location flexibility, process optimization, and co-benefits. Bicarbonate-dominant carbon sequestration, including OAE, offers high carbonation efficiency, reasonable MRV, coastal or riverine deployment at moderate to alkaline pH, and lower energy use per unit alkalinity. However, co-production of carbon-negative cement or carbonate concrete may be economically most attractive by offsetting carbon storage costs, with further benefits from metal recovery and waste stabilization. We highlight the need for integrative design and systematic optimization.