Experimental Study On Preparation And Physical And Mechanical Properties Of Red Mud Magnesium Phosphate Cement

Magnesium phosphate cement(MPC)is a new type of cementitious material formed by the reaction of dead-burned magnesium oxide(Mg O)and acid phosphate.According to the type of phosphate is divided into magnesium ammonium phosphate cement(MAPC)and magnesium potassium phosphate cement(MKPC).MPC has excellent mechanical properties,but its application is limited by high heat of hydration,high price and poor water stability.Mineral admixtures can not only reduce the cost of MPC,but also change the hydration process and hydration products of MPC,thus improving its mechanical properties and water resistance.Red mud is an industrial solid waste discharged from bauxite mines after refining Al₂O₃,which has a strong alkalinity resulting in a low utilization rate.In this project,by pre-treating the red mud to partially replaced the dead-burned Mg O and NH₄H₂PO₄or KH₂PO₄.By systematically studying the effect of variation of red mud admixture on its fluidity,mechanical properties,water resistance,setting time,hydration temperature and heat of hydration.And through XRD,SEM and thermodynamic analysis of the hydration reaction,we studied the composition,morphology and hydration hardening process of red mud MPC with the change of red mud dosing and the influence mechanism.The above experimental study and theoretical analysis led to the following conclusions.1)Red mud pretreatment process and mechanism research.By pretreating the red mud with excess NH₄H₂PO₄or KH₂PO₄,the red mud showed a clear Na H₂PO₄diffraction peak.The content of soluble potassium and sodium ions and activity index in red mud were increased to different degrees.2)Research on physical and mechanical properties of MPC of red mud.On the basis of the M/P and W/C determined in the MAPC reference group test,the amount of pretreatment red mud blending was adjusted.The fluidity of both MAPC and MKPC decreased with the increase of red mud admixture.MAPC mechanical properties showed a trend of first decreasing and then increasing.At the red mud dosing of 30%reached 64.3 MPa as 92.7%of the reference group.Water resistance increased and then decreased with the amount of red mud admixture.The mechanical properties of MKPC showed a trend of increasing and then decreasing with the increase of red mud dosing.The compressive strength reached 47.8 MPa at 10%admixture,which was 2.3 MPa higher than the reference group.Water resistance continued to increase with the increase of red mud dosing.Red mud MAPC had higher mechanical properties than MKPC and lower water resistance than MKPC.3)Study on the hydration process and mechanism of MPC of red mud.Red mud MAPC setting time decreased and then increased with the dosing of red mud.The peak hydration temperature at 10%red mud dosing was higher than the reference group.Peak temperature decreased gradually with increasing red mud dosing.The setting time of red mud MKPC increased with the increase of red mud admixture.The hydration temperature and exotherm decreased with the increase of red mud mixing.The theoretical value of thermodynamic analysis was close to the measured value when the red mud dosing was low.The increase of Na H₂PO₄content in the red mud blending system led to a large difference between the measured and theoretical values of its heat of hydration.XRD analysis showed that the main mineral composition of the red mud MPC at all ages were unhydrated Mg O and struvite,and there were no obvious diffraction peaks of red mud hydration products.The intensity of the characteristic peak of struvite increased slightly with the age of hydration.The Mg O diffraction peak gradually decreased with the increase of red mud doping.SEM analysis showed that hardened MPC with unhydrated Mg O as the matrix.The crystallinity of struvite increased with age,growing from microscopic short columnar crystals to well-crystallized columnar crystals.With the increase of red mud admixture,the crystallinity of struvite in MPC decreased significantly and the density increased.