an improved process for producing portland and portland type cement clinker wherein argillaceous and calcareous type materials are ground and blended to form a generally homogeneous mixture and subsequently burned in a kiln at a temperature in the range of about 1200 ° c . to about 1500 ° c . to form cement clinker , the improvement comprising incorporating into the mixture a fluorine containing mineral acid , selected from the group comprising fluotitanic acid , fluoboric acid , fluophosphoric acid , fluosilicic acid , and hydrofluoric acid , as a fluxing and mineralizing agent , in an amount of up to 3 . 0 % by weight based upon the weight of the dry solids in the mixture .