You would make a phosphate salt. Depending on its solubility, it may precipitate. Depending on the log K, your cola might no longer be acidic at all, and as oth

You would make a phosphate salt. Depending on its solubility, it may precipitate. Depending on the log K, your cola might no longer be acidic at all, and as others have suggested, not taste as nice, making the addition of both the phosphoric acid and calcium carbonate pointless. 2 H3PO4 + 3 CaCO3 -> Ca3(PO4)2 + 3 H2O + 3 CO2 The calcium phosphate phase listed above may not be the stable phase in cola solution at NTP, but is the simplest example.