Study On Ion Transport Characteristics Of Recycled Concrete Based On Percolation Theory

In recent years,the construction industry has experienced rapid growth.However,this has resulted in a corresponding increase in construction waste.This accumulation of waste not only seriously pollutes the natural environment but also represents a significant loss of resources.One potential solution is the production of recycled aggregate from construction waste,which can not only mitigate the environmental impact of construction waste but also significantly conserve natural resources.However,due to the existence of old mortar on the aggregate surface,there is a complex and changeable multiple ITZ in recycled concrete,which provides a convenient channel for ion transmission.It is of great significance to explore the change process of pore structure in recycled concrete under the condition of ion erosion and its influence on ion transport mechanism.The multiple ITZ structure of recycled concrete shows significant percolation characteristics.The introduction of percolation theory is helpful to study the transport characteristics of erosive ions in the complex internal structure of recycled concrete and quantitatively characterize the internal structure of recycled concrete.In this study,metakaolin and fly ash were used as modified mineral admixtures.To investigate the transport of sulfate ions in recycled concrete under wet and dry circulation conditions,analyze the pore structure characteristics after erosion and explore the percolation characteristics,various testing methods were employed,such as backscattered electron imaging(BSE),EDS surface scanning,mercury injection test(MIP),and X-ray computed tomography(X-CT).The following research work was carried out in this study:(1)This study investigates the effects of metakaolin and fly ash on the compressive strength and sulfate ion concentration distribution of recycled concrete under a dry-wet cycle.The findings reveal that the compressive strength initially increases,followed by a decrease that accelerates with increasing dry-wet cycle age.As the metakaolin content increases,the strength of recycled concrete gradually improves,with the most significant enhancement observed at a content of 5%.In contrast,the strength of recycled concrete first rises,then decreases with increasing fly ash content,with the highest compressive strength observed at a content of 16%.Furthermore,the mineral admixtures can enhance the permeability resistance of sulfate ions to the recycled concrete,and the concentration of sulfate ions decreases as the mineral admixture content increases.(2)Backscattered electron imaging(BSE)technology and image analysis software were used to analyze the two-dimensional pore structure of recycled concrete after slicing and polishing.The influence of sulfate ions on the pore structure of recycled concrete was quantitatively characterized using percolation theory.The results indicate that the wet seat rate and wet seat combination number of recycled concrete with different mixtures initially decrease,then increase with the increase of the number of wet and dry cycles,indicating that the difficulty of connecting between seats increases initially and then decreases.Additionally,the number of clusters gradually increases with sulfate attack,while the maximum cluster first decreases and then increases.Moreover,with the increase of admixture content,the percolation probability of recycled concrete gradually decreases,and the fractal dimension of the two-dimensional system decreases.Finally,the percolation probability initially decreases,then increases with the increase of dry and wet cycles.(3)The X-CT test method is used to scan the recycled concrete,and 3D image processing software is used to reconstruct the internal pore structure,so as to realize the true representation of the internal structure and quantitatively characterize the influence of sulfate ion erosion on the change of pore structure.At the same time,in order to show more obvious percolation characteristics of porous media,the pore network model is established by using pervious concrete,the pore and throat characteristics are quantitatively analyzed.The results reveal that the surface porosity of recycled concrete exhibited significant fluctuations with height,and its distribution is uneven along the height.Sulfate ion attack led to gradual pore connection and enlargement,which is reflect in the internal cluster structure showing the infiltration path of ions and the spatial anisotropy of recycled concrete,emphasizing that ion invasion is a dynamic process.The percolation probability increases with the number of wet and dry cycles,indicating an increase in the connectivity of recycled concrete.Additionally,compared to recycled concrete,pervious concrete have a more uniform pore structure with higher connectivity,as reflect in its average coordination number of pores reaching 3.0.