We have applied atomistic simulation techniques to model the continuous CdCO3-CaCO3 solid solution and to investigate the thermodynamic properties. All inequivalent substitutional configurations were considered for the single unit cell and the (2 × 1 × 1) supercell for the full range of Cd concentrations, as well as doping between 0 and 16.7 mol.% Cd for the (2 × 2 × 1) supercell. Our calculations show that segregation of Cd2+ and Ca2+ is energetically favourable when compared with any other cation distribution. No cation ordering is expected at high temperature and the system behaves as an ideal solid solution above 200 K, due to the configurational entropy. The lattice parameters change with the Cd concentration, where they are predicted to follow a linear relationship, due only to cation substitution but not cation ordering.