The present work investigates biomass wastes and their ashes for re-use in combination with mineralised CO2 in cement-bound construction products. A range of biomass residues (e.g., wood-derived, nut shells, fibres, and fruit peels) sourced in India, Africa and the UK were ashed and exposed to CO2 gas. These CO2-reactive ashes could mineralise CO2 gas and be used to cement 'raw' biomass in solid carbonated monolithic composites. The CO2 sequestered in ashes (125-414 g CO2/kg) and that emitted after incineration (400-500 g CO2/kg) was within the same range (w/w). The CO2-reactive ashes embodied significant amounts of CO2 (147-424 g equivalent CO2/kg ash). Selected ashes were combined with raw biomass and Portland Cement, CEM 1 and exposed to CO2. The use of CEM 1 in the carbonated products was offset by the CO2 mineralised (i.e. samples were 'carbon negative', even when 10% w/w CEM 1 was used); furthermore, biomass ashes were a suitable substitute for CEM 1 up to 50% w/w. The approach is conceptually simple, scalable, and can be applicable to a wide range of biomass ashes in a closed 'emission-capture' process 'loop'. An extrapolation of potential for CO2 offset in Europe provides an estimate of CO2 sequestration potential to 2030.