1. A CEMENT COMPOSITION CONSISTING ESSENTIALLY OF: A CEMENT COMPONENT CONSISTING ESSENTIALLY OF FROM 25 TO 75 PARTS BY WEIGHT OF A GYPSUM CEMENT CONTAINING MORE THAN 66 WEIGHT PERCENT OF CALCINED GYPSUM AND FROM 75 TO 25 PARTS BY WEIGHT OF A HYDRULIC NATURAL CEMENT, THE TOTAL PARTS BY WEIGHT OF SAID GYPSUM CEMENT AND SAID HYDRAULIC NATURAL CEMENT IN SAID CEMENT COMPONENT ALWAYS EQUALING 100; FROM 10 TO 100 PARTS BY WEIGHT PER 100 PARTS BY WEIGHT OF SAID CEMENT COMPONENT OF DIATOMACEOUS EARTH; FROM 0.1 TO 10 PARTS BY WEIGHT PER 100 PARTS BY WEIGHT OF SAID CEMENT COMPONENT OF AN ADDITIVE SELECTED FROM THE GROUP CONSISTING OF ACID CARBOXYALKYL HYDROXYETHYL CELLULOSE MIXED ETHERS IN WHICH THE ALKYL GROUP CONTAINS FROM 1 TO 2 CARBON ATOMS, AND SALTS OF SAID MIXED ETHERS; AND FROM 1 TO 7 PARTS BY WEIGHT PER 100 PARTS BY WEIGHT OF SAID CEMENT COMPONENT OF AN ALKALI METAL SILICATE HAVING A SILCON DIOXIDE TO ALKALI METAL OXIDE MOL RATIO OF FROM 1 TO 2.5; SAID CEMENT COMPOSITION, WHEN SLURRIED WITH SUFFICIENT WATER TO PRODUCE A PUMPABLE SLURRY, HAVING THE PROPERTY OF BEING INSENSITIVE TO THE ACCELERATING ACTION OF SODIUM CHLORIDE CONTAMINANT ON THICKENING TIME.