In processing phosphate rock, the evolved fluorine is recovered largely as fluosilicic acid salts. A potentially high tonnage application of such byproducts is in the manufacture of portland cement. Data are presented on the effect of silicofluorides on the formation of calcium silicates, aluminates and aluminoferrite, and on the polymorphism of dicalcium and tricalcium silicates. Addition of 1 to 2 wtper thousand silicofluoride to the raw mixes of these cement constituents promotes their formation at lower temperatures than are ordinarily required and reduces uncombined lime. The presence of fluoride inhibits the formation of tricalcium aluminate and modifies the resulting dodecacalcium heptaluminate. Results are based on ambient temperature X-ray diffractometry, differential thermal and thermogravimetric analyses and quenching. Possible applications of the research are in the manufacture of new types of cements, reduction in clinkering temperatures and use of an environmental pollutant without damage to rotary-kiln refractories.