Study On The Durability Of Metal Aggregate Mortar-Concrete Substrate Interface Under Sulfate And Freeze-thaw Effects

The hydraulic concrete structures located in the northwest region of China have rapidly deteriorated under the combined effects of sulfate erosion,freeze-thaw cycles,and dry wet cycles.Therefore,the hot issue of repairing and strengthening damaged hydraulic concrete structures in this region has received widespread attention.Metal aggregate mortar,as a relatively new type of repair material,has excellent mechanical properties and durability.When using metal aggregate mortar to repair damaged concrete,the interface between metal aggregate mortar and base concrete is a weak area,and the durability of the interface greatly affects the repair quality.Therefore,it is necessary to conduct experiments and analysis on the interface between metal aggregate mortar and base concrete,Thus providing theoretical support for the repair construction of hydraulic concrete structures in the northwest region of China.This article first studied the durability of two types of metal aggregate mortars(metal aggregate cementitious mortar and metal aggregate polyurethane mortar);Secondly,durability tests were conducted on the interface between metal aggregate mortar and base concrete,and the changes in durability indicators after experiencing single sulfate dry wet cycle,single salt freezethaw cycle,and alternating sulfate dry wet cycle salt freeze-thaw cycle were studied;Finally,scanning electron microscopy and X-ray diffraction were used to analyze and study the microstructure and chemical products at the interface of the composite specimen after undergoing sulfate dry wet salt freezing cycles.The main conclusions of this article are as follows:(1)Both metal aggregate cement-based mortar and metal aggregate polyurethane mortar have high impermeability and low water absorption.Metal aggregate polyurethane mortar has higher impermeability than metal aggregate cement-based mortar,but lower water absorption than metal aggregate cement-based mortar.Compared to metal aggregate cement-based mortar,metal aggregate polyurethane mortar specimens undergo salt freeze-thaw cycles and dry wet cycles in two different sulfate solutions(5%Na2SO4 and 5%MgSO4),resulting in smaller mass loss rates and compressive strength changes,and better corrosion resistance.(2)The variation pattern of interfacial shear strength of metal aggregate mortar base concrete composite specimens can be divided into two stages:1)strength improvement stage:the pores inside the base concrete and at the interface between concrete and mortar are filled with corrosion products and crystal salts,which have a certain strengthening effect on the composite specimens;2)Strength reduction stage:During the dry wet cycle,the expansion force of erosion products and the pressure of sulfate crystallization cause a certain degree of damage to the pore structure.During the salt freeze-thaw cycle,in addition to the expansion stress caused by erosion products and salt crystallization,there is also the frost heave force generated by freeze-thaw that acts on the pore structure,reducing the shear strength of the composite specimen interface.(3)As the number of sulfate dry wet cycles and salt freeze-thaw cycles increases,the number of salt crystals at the shear failure interface gradually increases,and the scope of sulfate erosion gradually expands.Compared to MgSO4,Na2SO4 has a more severe deterioration in the durability of the metal aggregate mortar base concrete interface.Na2SO4 has both physical salt crystallization and chemical corrosion damage effects on the composite specimens,while MgSO4 mainly causes chemical corrosion damage to the composite specimens,with its physical crystallization damage effect lower than Na2SO4.In addition,MgSO4 and the hydration products of the base concrete will form a magnesium hydroxide dense layer on the surface of the specimens,This dense layer has certain erosion resistance.(4)The initial shear failure mode of metal aggregate cementitious mortar base concrete is the mixed failure mode of repair material and base concrete,and as the erosion time increases,its failure mode changes to the cohesive failure mode of base concrete;The shear failure mode of metal aggregate polyurethane mortar base concrete before and after erosion is the cohesive failure mode of concrete.With the increase of the number of cycles of single dry wet cycle,single salt freezing,and alternating dry wet cycle salt freezing,the number of concrete bonded to metal aggregate mortar after shear failure gradually increases,and the sulfate crystallization at the shear section gradually increases.(5)The effect of the alternating action of sulfate dry wet cycle and freeze-thaw cycle on the interface durability of metal aggregate mortar base concrete composite specimens is not a simple superposition effect of a single sulfate dry wet cycle and a single salt freezing effect,but rather the mutual promotion of the two factors,accelerating deterioration.(6)The microstructure and phase changes of the interface between metal aggregate mortar and base concrete after undergoing dry wet cycle salt freezing cycle were analyzed using SEM electron microscopy and X-ray diffraction.The results indicate that the interface width between metal aggregate mortar and base concrete increases after undergoing alternating action.The internal structure of the base concrete is loose after the alternate cycle of Na2SO4.Ca(OH)2 and CSH are consumed,and a large amount of ettringite and Na2SO4·10H2O are produced.After the alternate cycle of MgSO4,Mg(OH)2 and MSH are produced;After the metal aggregate cementbased mortar undergoes the alternate cycle of Na2SO4,the microcracks increase,and ettringite is also generated.Some metal iron is rusted and converted into Fe2O3.After the alternate cycle of MgSO4,Mg(OH)2 is generated in addition to metal iron rust;After undergoing alternating cycles of Na2SO4 and MgSO4,the metal aggregate polyurethane mortar showed no significant changes except for some iron aggregates that were corroded.This article studies the interface durability performance of metal aggregate mortar and base concrete through a combination of experimental research and theoretical analysis.The research results have important significance for the repair and reinforcement of hydraulic concrete in the high cold saline soil area of northwest China,and have certain reference value for practical repair engineering construction.