This is the chemical reaction pathway for the conversion of calcium (Ca) to carbonic acid (H2CO3):

Ca (calcium) reacts with oxygen to form CaO (calcium oxide):
Ca + O2 -> CaO

CaO reacts with carbon dioxide to form CaCO3 (calcium carbonate):
CaO + CO2 -> CaCO3

CaCO3 reacts with water to form carbonic acid (H2CO3):
CaCO3 + H2O -> H2CO3

Therefore, the overall reaction pathway from calcium to carbonic acid is:
Ca -> CaO -> CaCO3 -> H2CO3