PURPOSE: To produce high purity ettringite for building materials having self-flame- retarding effect by adding a specified amt. of lime raw material such as CaO and gypsum to a NaAlO2 soln. followed by reaction under restricted conditions.

CONSTITUTION: To a NaAlO2 soln. are added about 1.5 mols of lime raw material such as CaO, esp. CaC2 sludge and about 1.5 mols of gypsum to 1 mol of NaAlO2, and they are reacted at ordinary temp. W90°C for above 5 hr. The yielded ettringite is used in manufacture of building materials having self-extinguishing properties, esp. ceiling and a heat insulating board. The gypsum used is calcined gypsum, hemihydrate gypsum or gypsum, esp. gypsum formed in bulk as a byproduct of exhaust gas desulfurization.