A method for controlling the formation of a hydroxyapatite bone filler from dry calcium phosphate precursors in an aqueous solution uses coated sodium phosphate powder. The sodium phosphate powder is coated with a water soluble cellulose. Until the cellulose dissolves in the aqueous solution setting of the calcium phosphate cements proceeds slowly but when the exposed sodium phosphate particles start to solubilize in the aqueous solution the setting rate increases.