Hydration Properties And Preparation Of Functional Mortar Of Building Gypsum Containing Phosphorus And Fluorine Impurities

Phosphogypsum（PG）is an industrial solid waste produced in the wet production of acid.The comprehensive utilization of PG has attracted extensive attention in China and building materials industry.In this paper,the effects of H₃PO₄ and NaF on the properties and the modification effect of calcium hydroxide on phosphorus building gypsum（PBG）were explored.In order to increase the added value of PBG,two types of functional mortar were prepared,and their mix ratios were optimized,which promotes the comprehensive utilization of PG.The main results are as follows:（1）H₃PO₄ promoted the hydration ofβ-CaSO₄·1/2H₂O through increasing the supersaturation degree of CaSO₄·2H₂O.The adsorption and deprotonation of H₃PO₄could accelerate the setting ofβ-CaSO₄·1/2H₂O plaster and deteriorate the pore structure and strength.With the increase of the contents of H₃PO₄,the acceleration rate first increased and then decreased,and the compressive strength decreased.The content of H₃PO₄ should be controlled below 0.1%to prevent excessive loss of dry strength.（2）NaF promoted the hydration ofβ-CaSO₄·1/2H₂O through the nucleation of CaF₂and the catalysis of Na⁺,and the accelerations of CaF₂ and Na⁺could be superimposed on each other.NaF can shorten the setting time ofβ-CaSO₄·1/2H₂O,but it will cause a significant deterioration on pore structure and strength when the content of NaF was high.The NaF content should be controlled within 1.0%to prevent excessive strength loss.（3）At the condition of low content of Ca（OH）₂,it would react with ions to forms precipitation encapsulating crystals,which inhibited the hydration of PBG;At the condition of high content of Ca（OH）₂,it could provide nucleation sites and ensure the amount of Ca²⁺in pore solution,which contributed to the formation of CaSO₄·2H₂O.the low dosage of Ca（OH）₂ has a great negative effect on setting time and strength;the high dosage of Ca（OH）₂ could optimize the setting and strength.In addition,the addition of calcium hydroxide can significantly improve the volume stability of PBG.（4）For PBG-based self-leveling mortar（PSLM）,sulfoaluminate cement will cause a large loss of fluidity for 30 minutes.Replacing the sulfoaluminate cement with blast furnace slag（BFS）can reduce the loss of fluidity,and the BFS with smaller particle size is more effective.By mixing an sulfoaluminate cement and ultrafine BFS with an appropriate proportion,it can not only ensure that the contribution of sulfoaluminate cement to the 1d strength is not greatly weakened,but also effectively compensate for the loss of fluidity,so that PSLM with good performance can be prepared.（5）BFS and Ca（OH）₂ could increase the water resistance of PBG-based mortar.Ettringite prevented water erosion to CaSO₄·2H₂O by encapsulating CaSO₄·2H₂O crystals,and C（A）SH could also encapsulate CaSO₄·2H₂O crystals.The encapsulation of ettringite was the main factor that improved the water resistance of CaSO₄·2H₂O.Under the optimal mix ratio of PBG,BFS and Ca（OH）₂,ettringite was abundant and has a regular morphology,and the PBG-based mortar has the strongest water resistance.