Synthetic aggregates are fabricated from greater than approximately 70 wt % waste starting materials. Starting materials may be selected from granulated ground blast furnace slag, waste concrete fines, or sewage sludge ash, and mixtures thereof. The starting materials are bound together by a hydraulic cementitious binder either added to the starting materials or formed in situ. The waste starting materials, binder, and water are formed into pellets and subjected to a hydraulic reaction and carbonation in an atmosphere of greater than approximately 50% carbon dioxide at temperatures less than approximately 100° C. The resulting synthetic aggregate has a crush strength after a period of hardening equal to or greater than approximately 0.5 MPa.