The dominating effect of carbonate ions on regulating the properties of apatite drives further inquiry about its influence on amorphous calcium phosphate (ACP). To date, the maximum carbonate inclusion, and the influence on the densification of amorphous powders have not been reported. Carbonate-substituted ACP was synthesized and then crystallized by heating in a furnace (in air) or a pressure vessel (in water or steam). Characterization by X-ray diffraction showed a 21 wt.% carbonate (Ca/P = 2.6) solid solubility limit in ACP. Steam processing crystallized a monophasic apatite at the highest carbonate content: up to Ca/P of 2.4 (18 wt.% of carbonate). FTIR spectra showed carbonate absorption characteristic of the non-apatitic environment for the amorphous phase, but after crystallization showed carbonate, like in a phosphate site. Thermal analysis revealed that higher carbonate content amorphous powders are less thermally stable and crystallize at lower temperatures. Cold sintering of higher carbonate content amorphous powders produced a lower density and lower hardness. A significant decrease in hardness occurs after a Ca/P of 1.74 (8 wt.% carbonate). This is the first report on such wide carbonate content amorphous calcium phosphate (ACP) powders showing the solid solubility of carbonate, and the influence on the processability.