Study On The Durability Damage Pattern Of Recycled Concrete Of Iron Tailings Under The Effect Of Compound Erosion

The durability of hydraulic concrete has long been a problem faced by hydraulic buildings,especially those serving in the western region,where the harsh environment in which they are located has led to hydraulic buildings being affected by the service life of hydraulic concrete.With the impact of carbon peaking,carbon neutral and green sustainable development policies,how to solve the waste and tailings pollution after the buildings are demolished is also a new problem we are facing.Therefore different proportions of recycled coarse aggregates can be combined with iron tailings sand and mixed into concrete,which not only realises the green sustainable development policy,but also has the effect of reducing project costs.This paper therefore aims to realise the application of recycled iron tailings concrete in the practical engineering of hydraulic buildings in the North West and to investigate the main causes of deterioration to hydraulic concrete.In this paper,freeze-thaw cycle tests,freeze-thaw cycle,sulphate attack tests,freeze-thaw-dry-wet-salt attack tests and nine groups of recycled iron tailings concrete with different proportions(0%,30% and 50% replacement of recycled coarse aggregate and 0%,30% and 50% replacement of iron tailings sand),each of which was mixed with the same amount of air-entraining agent to improve the resistance of the concrete to freeze-thaw cycles,were designed in turn.The study was carried out using accelerated indoor tests.The macroscopic properties of the recycled iron tailings concrete are summarised by observing the changes in the deterioration index of the recycled iron tailings concrete under different environmental conditions and with different ratios.The Weibull distribution was used to characterise the damage to the concrete in terms of its relative dynamic modulus of elasticity,and the damage superposition principle was used to construct a multifactor coupled life prediction model for the recycled iron ore concrete.The detailed conclusions obtained are as follows.(1)Under freeze-thaw cycles,it was found that the mass loss and relative dynamic modulus of concrete of different proportions decreased with the increase of the number of cycles,and when the set number of freeze-thaw cycles was reached,the rate of mass loss was the lowest for RAC0-IOT0 and RAC3-IOT3 concrete,only 0.8% and 0.89%,and the rate of change of relative dynamic modulus was also the lowest for RAC0-IOT0 and RAC3-IOT3 concrete.IOT0 and RAC3-IOT3 concretes.In the freeze-thaw and sulphate attack environments,as in the freeze-thaw-dry-wet-salt attack environment,the mass and relative dynamic modulus of elasticity increased to varying degrees in the initial stages,probably due to the presence of sulphate,which caused a chemical reaction between the concrete and the sulphate ions,resulting in the filling of the internal pores and an increase in mass and relative dynamic modulus of elasticity.After 200 cycles,it was found that the mass loss and the relative dynamic modulus of elasticity were lower in the concrete mixed with iron ore tailings than in the concrete without iron ore tailings.(2)Observing the concrete in the three environmental conditions,it was found that the internal pores of the concrete were mainly small and medium-sized pores in the early stage of the experiment,but after the start of the test,the internal pores of the concrete continued to develop and the number of large pores and cracks increased,while compared to the freeze-thaw cycle test,to which sulphate attack was added,the development of pores was faster than in the single-factor,while compared to the concrete in the two-factor condition,the three-The pore development rate of the concrete under the three-factor(freeze-thaw-dry-wet-salt attack)test was faster compared to the concrete under the two-factor condition.Analysis of the microscopic morphology revealed that a large number of rod-shaped products appeared inside the two-factor and three-factor concrete,as well as white flocculent microscopic products,which were analysed as crystalline products of sulphate,and that the multiple interfacial structures characteristic of the recycled coarse aggregates inside them were the main cause of the low durability of the recycled concrete.(3)Using a two-parameter Weibull probability distribution model,a three-factor model for predicting the life of recycled iron tailings concrete under the action of freeze-thaw cyclic damage model,the results of the study: the expected service life of concrete in a pure freeze-thaw environment is the longest of the three environmental conditions;the expected service life of B RAC5-IOT5 concrete under freeze-thaw-dry-wet-salt intrusion conditions is19.18 years The longest expected service life is 24.56 years for B RAC0-IOT5,which has the shortest expected service life.