Ion Diffusion In Recycled Aggregate Concrete Under Drying-Wetting Cycles And Compound Salt Corrosion

Under the guidance of national policies such as the"carbon peaking and carbon neutrality"strategy and"the 14th Five-Year Plan"for the development of a circular economy,recycled aggregate concrete has been applied to new concrete structures.However,recycled aggregate concrete（RAC）has a multi-interface transition zone structure,which makes its durability significantly lower than that of natural aggregate concrete.The climate in the northwest region of China is dry,with evaporation far exceeding precipitation,and the soil and groundwater in the region contain a large amount of corrosive ions.During the service life of RAC structures,corrosive ions are transmitted through the RAC interface transition zone and pores,Causing a decrease in the durability of RAC,resulting in steel corrosion and failure of component bearing capacity,which seriously threatens the safety and durability of in-service RAC structures,and hinders the application of RAC in engineering.Therefore,studying the diffusion of compound salt corrosion ions in RAC,revealing the laws and mechanisms of ion diffusion,is of great reference value for promoting the scientific and reasonable application of RAC in steel reinforced concrete structures in the northwest region.Based on the climate and environmental characteristics of the northwest region of China and the durability characteristics of concrete structures,this paper used a7.5%Mg SO₄+7.5%Na₂SO₄+5%Na Cl compound salt solution as the corrosive medium to simulate the durability corrosion process of RAC structures.The compound salt corrosive test was conducted under drying-wetting cycles.Solid-liquid extraction method and electrochemical method were used to test the content of Cl^-,Ca²⁺,Na⁺and p H value in the corroded RAC.Using diffusion depth and corrosive time as axes,the changes in ion content in the corroded RAC were analyzed,with a focus on the influence of mineral admixture types and replacement rates on the ion content in the corroded and an ion diffusion model considering the mineral admixture types and replacement rates was established.X-ray diffraction was used to analyze the mineral composition of the corroded RAC,and the variation characteristics of mineral composition of corrosive products were studied to reveal the ion diffusion mechanism of compound salt corrosive in RAC.The following conclusions were drawn in this paper:（1）The Cl^-content of RAC in corrosion increases rapidly with the increase of dry and wet cycles.In the same number of dry-wet cycles,the Cl^-content decreases from high to low in the following order:fly ash-slag RAC,fly ash-kaolin RAC,fly ash-silica fume RAC.Moreover,the Cl^-content increases with the increase in the substitution rate of slag,silica fume,and kaolin.The RAC with 20%fly ash and 10%slag has the lowest Cl^-content,while the RAC with 10%fly ash and 20%silica fume has the highest Cl^-content.（2）The p H value and Ca²⁺content of corroded RAC decrease with the increase in the number of dry-wet cycles but increase with the increase in the corrosive depth.The p H value and Ca²⁺content of fly ash-slag RAC decrease slowly,while those of fly ash-silica fume RAC decrease quickly.Moreover,the p H value and Ca²⁺content decrease with the increase in the substitution rate of slag and silica fume.After 180dry-wet cycles,the stable depth of p H value for the RAC with 20%fly ash and 10%slag is minimum,and that for the RAC with 20%fly ash and 10%silica fume is maximum（3）The diffusion coefficient of Cl^-is significantly related to the substitution rate of mineral admixtures.The Cl^-diffusion coefficient of fly ash-slag RAC is lower than that of fly ash-silica fume RAC.The decrease in the substitution rate of fly ash and the increase in the substitution rate of slag,silica fume,and kaolin lead to an increase in the Cl^-diffusion coefficient.An ion diffusion model considering the type and substitution rate of mineral admixtures in RAC is established by introducing a time attenuation coefficient.（4）As the number of wet-dry cycles increases,the diffraction peak intensity of the corrosive products such as gypsum,anhydrite,brucite,and magnesium chloride in RAC gradually increases,while the diffraction peak intensity of portlandite decreases rapidly or even disappears.With the increase of corrosive depth,the diffraction peak intensity of gypsum,anhydrite,brucite,and magnesium chloride decreases until it disappears.At a distance of 4mm from the surface after 180 wet-dry cycles,the RAC with 20%fly ash and 10%slag has the lowest diffraction peak intensity of gypsum,anhydrite,brucite,and magnesium chloride,while the RAC with 20%fly ash and10%silica fume has the highest.At a distance of 10mm from the surface after 180wet-dry cycles,the diffraction peaks of gypsum,anhydrite,brucite,and magnesium chloride disappear in the RAC with 20%fly ash and 10%slag and the RAC with 20%fly ash and 10%metakaolin,while the diffraction peak of portlandite increases.Gypsum diffraction peak is still present in the RAC with 20%fly ash and 10%silica fume.