Study On Bond Properties Of Magnesium Phosphate Cement Based Materials

Cement concrete is the largest and most widely used building materials in the world.A large number of existing concrete structures indicates a great potential demand of repairation and reinforcement works.Magnesium phosphate cement(MPC)with quick development of early strength has been regarded as a excellent material in the field of repairation.The present research of MPC mainly concentrated on the hydration process,microstructure,physical and mechanical properties,involved a few study on the bonding properties,but lacked the study on the volume stability of MPC.Namely,previous studies primarily focused on the super early strength of MPC,but paid a little attention to long-term durability.Bond performance of MPC was studied from three aspects including free state,constraint state and bond durability.The improvement of bonding performance and bonding mechanism of MPC were discussed.The results contributes to the application of magnesium phosphate cement based materials in engineering repair.Firstly,the bond behavior of MPC under free condition was studied.Based on the result of MPC's workability and the compressive strength,the bonding property of MPC was characterized by flexural bond strength and shear bond strength.The fracture characteristics of the interface region was analyzed.The results show that MPC material has excellent bonding properties,more than 3.0 Mpa of 2h flexural bond strength and 5.0 Mpa of one-day flexural bond strength,indicating its advantages in repairing.The ratio of composition in which the W/B(water binder ratio)and M/P(MgO and phosphate)are the main factor affects the bond performance of MPC.The phosphate content of MPC material has an optimum range.The bonding performance of MPC is good when the W/B is 0.12~0.14,M/P is 2~3.The flexural bond strength of MPC has a good correlation with the compressive shear bond strength.Considering the requirements of repairing materials in practical engineering,the bonding property of MPC materials under the restrained condition was studied.The early deformation and interfacial adhesion of MPC were studied and compared to ordinary portland cement-based materials(referred to as OPC materials)and sulphoaluminate cement-based materials(SAC materials).Meanwhile,the development of fracture and permeability at the interface between MPC and matrix were observed,and then combined with the bond strength to characterize the bonding properties of MPC.The result shows that early volume stability which was mainly affected by the composition ratio of MPC has significant influence on the adhesion of MPC under the restraint condition.Most MPC materials in the initial(after the final condensate 0~4h)and early(1d~10d)period will have a certain expansion.The expansion rate could be up to 3.5×10-4,which is conducive to the interface bonding property under the restraint condition.The volume stability of MPC materials was significantly better than that of OPC/SAC materials.And,the drying shrinkage ratio of MPC materials could be only 1/4~1/5 of OPC/SAC materials at same ages.Under the restraint condition,the MPC material still has excellent bond performance,specificaly,the flexural bond strength of 2h and 1d can reach 3.8MPa and 5.4MPa.By the observation of the interface and free water seepage test,the bonding interface of MPC material has no crack;the water permeability time is greater than 72 h.But the OPC/SAC material appears 23.6 m visible cracks after 4 hours later;the water penetration time is less than 17 min.The interfacial bonding quality of MPC material is better than that of OPC/SAC.The durability of interface bonding between MPC and concrete was also studied in different environments.The effects of fresh water,simulated sea water as well as 3% MgSO4,10% NaCl and 10% MgSO4 solutions on the interfacial bond strength of MPC materials were investigated.The results show that the bond strength and the durability of MPC with air curing are better than that with water curing.With immersed in fresh water,the bonding strength of MPC materials decreases at early ages,but could gradually recovered to 94% of the samples with air curing.MPC material has better corrosion resistance in fresh water.MPC samples immersed in 10% MgSO4 solution has higher bond strength than the specimens cured in air.MPC can be used as an emergency repair material,even used in structural repairing with durability requirements,especially in the oceans and high-saline environments.Finally,the morphology characteristics of the interface between MPC and matrix at macro and micro scale are analyzed.The formation mechanism of bond strength between MPC and old concrete is revealed.Also,the performance improvement of MPC is investigated by the methods of compound phosphate and mineral admixture.The results show that the bonding strength of the interface between MPC and the matrix is mainly based on the physical mechanical bite.MPC exhibits better performance when the compound phosphate(potassium dihydrogen phosphate: ammonium dihydrogen phosphate 0.7:0.3)is used.Compared with MPC prepared with single phosphate,the average bond strength of MPC prepared with compound phosphate at each age is about 40% higher.The addition of a certain amount of fly ash and silica fume also benefits the bond strength improvement of MPC at each age.The bond strength can be increased by 20%.Also,there is an improvement for the compressive strength development and water resistance.Based on the research on the bond performance of MPC and the influencing factors,it is indicated that MPC can be used as the material for repairing and strengthening concrete structure.MPC is an ideal material for rapid repair and long-term performance maintenance of the existing concrete structures.