Spray-dried Na phosphates and a soln. of Na(3)PO(4) from an industrial plant were mixed together to prep. Na(5)P(3)O(10) of increased d. at 80 degrees C and varying mass ratio (optionally after addition of the reaction product). The product was calcined at 450 or 550 degrees C. The highest d. of the calcined product (0.915 g/mL) was achieved when the raw materials were mixed at the mass ratio 0.5 and some amts. of Na(5)P(3)O(10) were added to the reaction mixt.