Study On The Properties Of Magnesium Phosphate Cement With Mineral Admixtures And Fibers

Magnesium phosphate cement（MPC）has the characteristics of quick setting and hardening,high early strength,great volume stability and excellent adhesion to old cement concrete,which has potential applications in the field of rapid repairment of concrete structures.However,MPC also suffers from shorter setting time and worse toughness,which seriously limit its application.In view of the main problems existing in MPC,this paper prepared MPC with mineral admixtures and fibers,aiming to retard the setting and enhance the fracture toughness of MPC.Firstly,metakaolin,limestone and coal gasification slag were selected as admixtures to prepare MPC,the workability of MPC pastes were evaluated.The effect of mineral admixtures on the properties of MPC were analyzed according to hydration temperature test,hydration products analysis,thermogravimetric analysis,microstructure observation and strength test.Secondly,MPC composites were prepared with different types of fibers,and the toughening effect of fibers on MPC were discussed.Finally,MPC was placed in water,Na Cl solution and Na₂SO₄solution,the corrosion resistance of MPC was evaluated by compared to the samples under air curing.The results reveale that metakaolin could prolong the setting time and decrease the flowability of MPC,it is conductive to the mechanical properties of MPC through the activity effect and filling effect.Limestone powder is not only detrimental to the workability of MPC,but also against the strength formation of MPC,So it is not suitable as a mixture.Coal gasification slag is beneficial to the workability of MPC,it has no adverse effect on the strength of MPC as little amount.Meanwhile,coal gasification slag can adjust the appearance of MPC.In terms of fiber,basalt fiber shows great bond behaviour with MPC matrix,which can improve the strength of MPC mortar obviously.The MPC mortar mixed with polyvinyl alcohol（PVA）fiber exhibits better deformation performance,and PVA fiber can promote the toughening effect of MPC apparently.Bagasse fiber can absorb the water,with many pores and low strength,which adversely affects the fluidity and the strength of MPC mortar.The bagasse fiber is not suitable for preparing MPC.In terms of the corrosion resistance,the reaction degree of Cl^-and SO₄^2-with MPC are extremely low.The water effect can promote the hydration degree of MPC and dissolve part of struvite.The effect of solution on strength is ranked in decreasing order:water>Na Cl solution>Na₂SO₄ solution.In addition,water corrosion is detrimental to the bonding behaviour between fibers and matrixes,reducing the strengthening effect of fiber and resulting in lower mechanical properties of MPC mortar.The strength retention rate of MPC mortar in the corrosion environment is higher than 91.3%at 45 days,indicating that MPC mortar has great corrosion resistance.