Changes in geotechnical properties of urban soils during carbonation

Urban soils that contain materials derived from demolition undergo carbonation as a consequence of a reaction between Ca derived from cement and concrete, and carbonate ultimately derived from the atmosphere. Estimated rates of CaCO₃ accumulation are equivalent to the removal of 150T CO₂ ha^-1 yr^-1. Although this process has benefits for carbon capture, the formation of CaCO₃ within the soil potentially affects the geotechnical properties. Trial pits at a number of locations have consistently yielded CaCO₃ contents of 20 wt% from 0-1m depth. Carbonate has formed in intergranular space within the soil, possibly with improvements in strength but also with changes in permeability that may affect through drainage. This paper investigates the limits to CaCO₃ formation imposed by soil porosity for a number of artificial soils: 'well sorted sand or gravel', 'sand and grave, mixed', 'silt' and 'clay'. Calculated values for CaCO₃ for the various soil types are used to determine limits to CaCO₃ formation within the first 1 m of sediments at the Science Central Site in Newcastle upon Tyne, UK, using information derived from five trial pits within made ground.