Experimental Study On Properties Of Hybrid Fiber Reinforced Magnesium Phosphate Cement-based Composite

Magnesium Phosphate Cement（MPC）has many advantages,such as fast hardening,early strength,strong adhesion and good compatibility,and is suitable for rapid construction,emergency repair and reinforcement of buildings.However,MPC has shortcomings in crack resistance and deformation,and is prone to brittle failure when applied to pavement,bridge deck and other high impact or bending load conditions.Adding fiber into MPC can improve its toughness and give full play to its superior performance in the field of repair and reinforcement,but the improvement of MPC performance by a single fiber is limited.In order to break through the limitation of single-doped fiber reinforced magnesium phosphate cement-based materials,four kinds of fibers,end-hook type steel fiber（HSF）,micro-steel fiber（MSF）,polyvinyl alcohol fiber（PVAF）and basalt fiber（BF）,are selected for fiber blending in this paper to study the influence of different fiber blending methods on the mechanical properties of MPC foundation;Then select the test group with better content to further study its toughness characteristics and durability;Finally,the progressive crack resistance effect and reinforcement mechanism of the fiber are analyzed by combining the microscopic tests.The main contents and conclusions of the study are as follows:（1）The effect of fiber admixture and blending method on MPC strength and blending effect.The reinforcing effect of fibers on compressive strength is from superior to inferior:MSF>BF>PVAF;the reinforcing effect on flexural and splitting tensile strength is in the following order:MSF>BF>PVAF;the reinforcing effect of fiber blending on MPC strength is in the following order:HSF+MSF>HSF+BF>HSF+PVAF>PVAF+BF.The majority of blending groups have a negative blending effect on compressive strength.Most of the compressive strength in the fiber mixing group is negative hybrid effect,and the bending strength and splitting strength are basically positive hybrid effect except for the combination of PVAF+BF.The growth rate of MPC strength in the early stage is significantly higher than that in the later stage,which has obvious characteristics of early strength.（2）The influence of fiber content and mixing mode on the toughness characteristics of MPC With the increase of HSF content,the toughness characteristics of the single fiber blending group are continuously enhanced,and 1.5%is the optimal blending amount.The toughness of the fiber hybrid group increased with the increase in total fiber content,and the fracture toughness from the largest to the smallest was:HSF+MSF>HSF+BF>HSF+PVAF>PVAF+BF;the order of impact toughness from superior to inferior is:HSF+PVAF>HSF+BF>HSF+MSF>PVAF+BF.The two-parameter Weibull distribution can reasonably describe N₁ and N₂ of steel fiber hybrid reinforced cement-based materials.（3）The influence of fiber content and mixing mode on the durability of MPC.For the single blending group,the wear resistance first increases and then decreases with the increase of HSF content,and reaches the maximum value when the content is 0.5%;The impermeability pressure increases with the increase of HSF fiber content,and reaches the maximum value when the content is 1.5%.For the dual-doped group,the wear resistance from the best to the worst is:HSF+BF>HSF+MSF>HSF+PVAF>PVAF+BF;The impermeability pressure value from large to small is:HSF+BF>PVAF+BF>HSF+PVAF>HSF+MSF.The mass loss of the test piece is small after 20-80 freeze-thaw cycles and increases rapidly after more than 80 cycles;after 120 freeze-thaw cycles,the loss of compressive strength first increases and then decreases with the increase of fiber content in the case of HSF alone,and the minimum loss of compressive strength is 17.7%when the content of HSF is 1.5%;For the fiber mixing group,the fiber combination mode of resistance to strength loss from small to large is:HSF+PVAF>HSF+BF>HSF+MSF>PVAF+BF.（4）The MPC material itself has inherent defects and is prone to stress concentration and brittle damage.The fibers dispersed within the matrix will show different forms of damage due to differences in elastic modulus,mainly pull-out damage and fracture damage,and fiber blending can produce synergistic reinforcement.Fibre blending into MPC does not produce new compounds,mainly Mg O,Si O₂ and Mg（NH₄）PO₄?6H₂O.However,the fibers have a certain influence on the amount of hydration products generated in MPC.The hydration product diffraction peak is slightly reduced when HSF is blended alone,and the amount of hydration products generated is reduced for both HSF+MSF and HSF+PVAF blends,and increased for both HSF+BF and PVAF+BF blends.（5）HFRMPC material damage and fiber exerting crack arresting effects can be divided into three stages:the micro crack arresting stage before crack expansion starts,the multi-scale crack arresting stage where sub-micro cracks develop steadily,and the fiber-reinforced toughening stage where macro cracks destabilize and expand.Smaller sizes of MSF,BF and PAVF have an advantage in inhibiting microcrack generation and development,while larger sizes of HSF play a major role in controlling macroscopic crack expansion,with fibers acting as a step-by-step crackstopper at different structural levels and in different stress stages.The theoretical energy dissipation effect of the fibers corresponds to the actual energy dissipation in mechanical tests,and the theory is able to accurately predict the energy dissipation effect of the hybrid fibers.