The production of Portland cement stands to be a major source of CO2 emissions and lack of durability in extreme climate zones. The present study describes a novel multi-waste geopolymer composite made from waste foundry sand (WFS), Mexican grass ash (MGA), and ground granulated blast furnace slag (GGBS) which is activated by a new mechano-carbonation process that uses CO2 as a curing agent. The unique activation process allows for the in-situ formation of carbonate and the densification of the microstructure at room temperature through a combination of low CO₂ curing and mechanical refining. Four activation methods were developed and compared, such as mechano-carbonation, mechanical activation, thermal treatment, and room temperature curing. Among all techniques, the composite created by mechano-carbonation showed the highest values of 43.5 MPa in compressive strength, 6.4 MPa in flexural strength, 2570 kg/m3 in density, and 3% in water absorption. The microstructural analyses (XRD, FTIR, and SEM) proves that the presence of finely dispersed CaCO₃ and C–(A)–S–H gels, leading to a dense and low-permeable matrix which could withstand freeze/thaw, chloride and sulfate attacks. Mechano-carbonation lead to carbon management at a large scale and creation of durable binders for the sustainable construction and restoration of existing structures, and fosters multi-waste recycling and adoption of low-energy processes.