Influence of phosphorus impurities on the structure and performance of cellulose-ether-modified gypsum

• Soluble phosphorus impurities decrease the supersaturation of the gypsum solution. • The generation of calcium phosphate retards the hydration reaction of gypsum. • Soluble phosphorus leads to the disintegration of the HPMC colloidal association. • The disintegration of HPMC decreases water retention of the HPMC-modified gypsum. • Soluble phosphorus makes a looser structure and deceases the mechanical strength. This study examined the influence mechanism of phosphorus impurities in phosphogypsum on the structure and properties of gypsum. The influence of four phosphorus impurities, namely Ca 3 (PO 4 ) 2 , CaHPO 4 , H 3 PO 4 , and Ca(H 2 PO 4 ) 2 on the hydration process, water retention, and mechanical properties of hydroxypropyl methyl cellulose ether (HPMC)-modified gypsum was examined through inductive coupled plasma emission spectrometer, scanning electron microscopy, and nanoparticle size testing. The soluble phosphorus impurities exert a significant retarding effect on HPMC-modified gypsum, which decreases the heat of hydration in the early stage. Moreover, soluble phosphorus impurities significantly decrease the water retention and compressive and flexural strengths of modified gypsum. Although slightly soluble CaHPO 4 exerts a similar adverse influence on the structure and properties of modified gypsum, the effect is weaker than that of soluble phosphorus impurities. Insoluble Ca 3 (PO 4 ) 2 does not affect the properties of modified gypsum. Soluble phosphorus in the liquid phase decreases the supersaturation of the gypsum solution and metathesises with the Ca 2+ in the solution to produce insoluble calcium phosphate. This product covers the surface of the crystal nucleus, delays the hydration reaction of gypsum, and decreases the amount of heat released in the early hydration process. Moreover, a coarse overlap of the crystal growth and loose structure of the hardened body is observed, and the flexural and compressive strengths decrease. Increase in the amount of soluble phosphorus in the solution decreases the polarity of the hydroxyl group in the HPMC molecule and strength of the hydrogen bond within the association complex. This phenomenon leads to the disintegration of the large colloidal association and the less swelling, owing to which the HPMC molecule cannot effectively block the water transmission channel in the mixture, resulting in decreased water retention of the HPMC-modified gypsum.