Sodium tripolyphosphate (STMP, Na₃P₃O₉) is a phosphate-based functional additive in building materials, not a primary cementitious material. Its main function relies on chelating Ca²⁺ ions after dissolving in water, adsorbing onto particle surfaces, and slowly hydrolyzing into linear polyphosphates. STMP is an essential additive in the production of gypsum boards, significantly enhancing the final product's fire resistance and mechanical properties.