Study On High Temperature Resistance Modification Of Magnesium Phosphate Cement And In-plane Shear Between Cfrp Sheets And Concrete

Magnesium phosphate cement（MPC）has several advantages of rapid setting time,high early strength,good bonding strength with other materials,l ow drying shrinkage and favorable abrasion resistance.Therefore,it could be extensively applied in construction repairs and reinforcement projects.The properties of MPC under high temperature were reported by seldom researchers;however,the modification methods of high temperature resistance and the mechanisms are not clearly.Meanwhile,the use of external carbon fiber sheets bonded by organic glue has been systemically researched and widely applied to reinforce the concrete structure all over the world.The carbon fiber has a favorable high-temperature resistance under anaerobic conditions,while the properties of epoxy adhesive,being used to bond the carbon fiber sheets and concrete,would sharply deteriorate when exposed to a high temperature.To enhance the high-temperature resistance and the application of external bonded carbon fiber sheets,the substitution of organic epoxy adhesive by inorganic adhesive with favorable high-temperature resistance is necessary.The modification effect of wollastonite on MPC and the mechanism was one of the key points in this paper.The results showed that the flexural strength and compressive strength of the modified MPC increased with the increasing content of wollastonite.The modified MPC had the excellent freeze-thaw resistance,dry-wet resistance,water resistance and sulfate resistance,but the mechanical properties would dramatically decrease in strong acid and alkali environment.The results of SEM-EDS analysis showed that the wollastonite did not participate in the chemical reaction,and the reaction products of MPC were not affected.Furthermore,the pore structure of MPC was filled by the wollastonite with micro fiber shape.Hence,the micro structure MPC and the compressive strength were improved.The results of the mechanical properties of samples exposed to high-temperature showed that the mass loss of MPC was less affected by the addition of wollastonite.The compressive strength of modified MPC did not decrease when exposed to the high temperature of 600 ℃.Although the wollastonite did not participate in the reaction of MPC and did not affect the reaction products and phase morphology,the wollastonite itself had the excellent chemical stability at the high temperature of 1000 ℃.It was this micro fibrous wollastonite restricts the dehydration deformation of the reaction products of MPC;hence the mechanical properties of samples were not decreased dramatically.The shear strength of the carbon fiber sheets bonded by modified MPC at different temperature and environment was the other key point in this paper.The results showed that the shear strength was enhanced over than 40% by the substitution of modified MPC.Simultaneously,the shear strength loss was dramatically decreased after exposed to the high temperature of 105⁵00 ℃.The MPC specimens didn’t successfully fail through the failure mode of the complete debonding for all of the treatments under high temperatures ranging from 105 ℃ to 500 ℃,perhaps mainly due to the reduction of the MPC’s bond strength at high temperatures.The shear strength after exposure to dry-wet cycling,water immersion and MgSO₄ solution attack of carbon fiber sheets bonded by modified MPC was better than that of epoxy adhesive.However,the sulfuric aci d,Na OH solution and Na2SO₄ solution attack resistance of MPC was worse than that of epoxy adhesive.As a word,the high-temperature resistance of MPC was enhanced by the addition of wollastonite.The bonding strength of modified MPC was better than that of epoxy adhesive,and it can be used as a new inorganic adhesive to strengthen concrete structures.Although the MPC specimens failed through interlaminar slip of fiber strips instead of complete debonding,the excellent bonding property under normal temperature and high-temperature demonstrated the feasibility of the substitution epoxy adhesive by modified MPC in the application of construction structure strengthening.In addition,the modified MPC as adhesive to bond the carbon fiber sheets and reinforce concrete structures has a better economy and environmental friendliness.