## 5 Discussion on the Microscopic Mechanisms of RAIW Destruction  
Compared to the natural aggregate infill wall materials (NAIW), recycled aggregate infill wall (RAIW) materials possess greater nonuniformity, more complex internal structures, and higher levels of randomness. However, the interfacial transition zone between the cement and recycled aggregate is weaker. Improving the composition, structure and properties of the interfacial transition zone is one of the methods used to enhance and improve the mechanical properties of RAIW. Changes in the water-cement ratio, amount of recycled aggregate, aggregate-cement ratio, and amount of additives have important effects on the transition layer's structure. (1) For the RAIW, because of the porosity and high water absorption characteristics of the recycled aggregates relative to those of natural aggregates, a large part of the moisture mixed into cement-based materials is absorbed by the porosity of the recycled aggregates and tiny particles, forming the internal free water. The remaining water penetrates into the internal part of the cement-based materials, acting as a binder and participating in hydration reactions. In the case of a low water-cement ratio, most of the moisture is absorbed by the pores of the recycled aggregate and tiny particles, and only a little water penetrates into the internal part of the cement-based materials to participate in the hydration reaction, resulting in an insufficient number of hydrated cement reactions. Most of the cement cannot form the C-S-H cementitious system; thus, the interfacial aggregate connection is weak--that is to say, the mechanical properties of the RAIW are weak. In the case of a high water-cement ratio, a lot of moisture that is not absorbed by the pores of recycled aggregate and the tiny particles penetrates into the internal part of the cement-based materials. These moisture levels exceed the amount of the water required for the hydration reaction. Although the hydration of the cement is conducted sufficiently, the remaining moisture causes the concentration of the hydration production (cementitious system) to decrease. In addition, has also been shown that the use of too much water increases the heat in the hydration process, resulting in increased porosity of the interfacial aggregate connection, thus decreasing the mechanical properties of the RAIW. The water-cement ratio curve can be seen in Figures 4 and 5. When the water-cement ratio was less than 0.8, the moisture that penetrated to the hydration cement-based materials did not reach the standard of bond water. Therefore, the mechanical properties were lower than when the water cement ratio was 0.8. When the water-cement ratio was more than 0.8, the use of excessive water caused the concentration of the hydration production to decrease, and a lot of heat of hydration was released; eventually, the mechanical properties of the RAIW degenerated. Figure 6: Intuitive analysis of the measured and theoretical strengths. (2) Since the recycled aggregate was made with the waste concrete and abandoned brick block, a large number of micro-cracks and initial damage between the interfacial of the old structure was present. Moreover, recycled mixtures contain lots of tiny particles; these tiny particles made the interfacial between the old and the new parts of the RAIW less thin, and a serious drying shrinkage effect appeared after the formation of the RAIW forming, leading to the strength of RAIW declining further. It can be summarized that the mechanical properties of RAIW decrease as the aggregate-cement ratio and the recycled aggregate replacement rate increase. The aggregate-cement ratio and aggregate replacement rate curve are shown in Figures 4 and 5, respectively. When aggregate-cement ratio and aggregate replacement rate increased, the proportion of the recycled aggregate contained in the RAIW also increased.