Preparation And Properties Of Magnesium Phosphate Cement With Boron Mud As Magnesium Material

Boron mud is the remaining weakly alkaline muddy waste residue in the production of borax using boron and magnesium minerals.Its accumulation not only wastes a large amount of land resources,but also causes soil alkalinity and water and environmental pollution due to its alkalinity.With the increasing demand for borax in today’s society,more and more boron mud is produced,and its comprehensive utilization has become one of the problems to be solved urgently in the industry.In view of the high content of magnesium in boron mud,magnesium phosphate cement was prepared by boron mud as magnesium material.Firstly,the changes of phase composition,reactivity,particle size and microstructure of boron mud before and after calcination were studied.The results show that the phase composition of boron mud is mainly magnesite（Mg CO₃）and magnesium olivine（Mg₂Si O₄）,the content of which is about 28.6%and 16.9%,respectively.After calcination,the magnesite in boron mud decompositions to form magnesite（Mg O）,the content of which is about 13.6%.The particle size of boron mud increases with the increase of calcination temperature.In addition,Boron mud-Magnesium phosphate cement（B-MPC）were prepared by using boron mud after different calcination systems,ammonium dihydrogen phosphate and borax.The results show that with the increase of calcination temperature and the prolongation of holding time,the setting time of B-MPC is prolonged,the fluidity is better,and the compressive strength is reduced.Based on the comprehensive analysis of the influence of calcination system on the basic properties of B-MPC,it is concluded that the best calcination system of boron mud is the calcination temperature of 850℃and holding time of 2 h.Secondly,the influence of the mass ratio of boron mud to ammonium dihydrogen phosphate（BM/P）,water-binder ratio（W/C）and borax content（B/BM）on the basic properties of B-MPC was studied.The results show that with the increase of BM/P,the setting time and fluidity of B-MPC decrease,and the compressive strength increases first and then decreases.With the increase of W/C,the setting time and fluidity of B-MPC are prolonged,and the compressive strength decreases.With the increase of B/BM,the setting time of B-MPC was prolonged and the fluidity was slightly increased,and the compressive strength decreased significantly in the early stage,but the change of compressive strength was small in the later stage.Based on the comprehensive analysis of the influence law of the batching ratio on the basic performance of B-MPC,the optimal batching ratio is BM/P=3,W/C=0.18,B/BM=5%.The fluidity of B-MPC prepared by the mixture ratio is 160 mm,the setting time is 16 min,the compressive strength can reach20 MPa for 3 h and 60 MPa for 28 d,and the compressive strength is increased in the later stage.Finally,the properties of magnesium phosphate cement（MPC）with partial substitution of dead burned Mg O by calcined boron mud were investigated.The effect of boron mud substitution ratio on the basic properties of MPC paste and mortar was studied.The results show that with the increase of the substitution proportion of boron mud,the setting time of the paste is slightly prolonged,the fluidity is decreased,and the compressive strength is firstly increased and then decreased.When the substitution proportion of boron mud was 40%,the compressive strength was the highest,and the compressive strength at 3 h and 28 d was 53.9 MPa and 93.1 MPa,respectively,which increased by 37.5%and 12.4%compared with the blank group.With the increase of the substitution proportion of boron mud,the setting time of mortar is prolonged,the fluidity is reduced,the flexural bond strength increases,and the compressive strength and flexural strength first increase and then decrease.