calcium oxide vs limestone

calcium oxide

noun
  • A white powderous substance, CaO, normally made by heating calcium carbonate. 

limestone

noun
  • An abundant rock of marine and fresh-water sediments; primarily composed of calcite (CaCO₃); it occurs in a variety of forms, both crystalline and amorphous. 

  • Pertaining to or made of limestone. 

How often have the words calcium oxide and limestone occurred in a corpus of books? (source: Google Ngram Viewer )