Calcium phosphate cements were investigated as potential lost circulation control materials for geothermal wells. The calcium phosphate cements were produced by reacting calcium aluminate (high alumina) cement with sodium phosphate compounds. Pumpable formulations with thickening times of up to 2 h at temperatures between 25 and 90 degrees C were developed and characterized. The materials showed rapid set behavior, early strength development, low permeability and acceptable durability in hydrothermal environments. Strengths up to 4 MPa were achieved 4 h after mixing and permeabilities were of the order of 10(-9)-10(-7) cm/s at 24 h. Partial replacement of calcium aluminate cement with ground granulated blast furnace slag was found to reduce the amount of borax retarder required to maintain pumpability at elevated temperatures and pressures.