# Experimental Study on Preparation and Characteristics of Concrete Modified by Construction Waste
## 4 Analysis of Modification Mechanism  
The microstructure of the interface transition zone between recycled aggregate and cement slurry is shown in Figure 11. The macroscopic properties of materials are closely related to their microstructure, which can reflect their macroscopic properties. The interface transition zone has a significant impact on the performance of cement-based materials. Therefore, the microstructure of different interface transition zones is analyzed [45; 46; 47; 48; 49]. When conducting concrete compressive strength experiments, four representative samples that comply with electron microscopy scanning were selected at the fracture surface. After plasma sputtering and gold spraying treatment, the microstructure differences in each sample were scanned and analyzed under a scanning electron microscope to further analyze their mechanism of action on recycled aggregate concrete. The SEM images of the transition zone between recycled aggregates and cement-based slurries show the distribution of materials. The aggregates and cement-based slurries are bonded to each other, surrounded by sheet-like calcium hydroxide (CH) and needle-shaped ettrin9 ite (AFt). The bonding strength at the interface connection is low, and when the sample is subjected to load, it is easy to break in areas with poor bonding. After modification with modifiers, the water  
Figure 10: Concrete strength at various factor levels. consumption at the interface of recycled aggregate will be further reduced, resulting in more complete hydration reaction between cementitious slurry and the bonding surface of recycled aggregate, greatly enhancing the overall strength of concrete materials. Due to the adhesion of waste mortar on the surface, the surface of recycled aggregate is loose and porous, and a large number of micro cracks are distributed on the surface and inside, resulting in high water absorption and a low strength of recycled aggregate. The workability and strength of the prepared recycled concrete deteriorate. Based on the characteristics of recycled aggregates, surface modification methods can be considered from two aspects. One is surface chemical modification. Using modifiers to form a coating layer on the surface of the aggregate, covering the surface including pores and microcracks, making the inorganic aggregate surface organic while making it more compact, exhibiting good hydrophobicity and reducing its water absorption rate. The second is surface physical encapsulation modification, which does not change the surface hydrophilicity of inorganic aggregates. The modifier undergoes a hydration reaction with the waste mortar adhered to the surface of recycled aggregates, and the generated hydration products physically fill the pores and microcracks on the surface of recycled aggregates, making the surface more compact and reducing their water absorption rate. On the other hand, defects such as loose and porous surface waste mortar layer and micro cracks are cemented, strengthening the weak interface transition zone between waste mortar and aggregate, resulting in an increase in the strength of recycled aggregate and a decrease in crushing index. Silame coupling agent is a special type of silicon-based compound, with a structural formula usually represented as Y (CH2) nSiX3. The molecular structure contains two functional groups: X and Y groups. As a hydrolyzable functional group, X mainly affects the hydrolysis rate and efficiency of silane, with commonly used groups being methoxy and ethoxy, improving affinity between inorganic substances through chemical reactions or adsorption on their surfaces. The Y group is an active functional group, such as methyl, vinyl, and epoxy groups. Due to its affinity reaction with polymer molecules, it usually has a significant impact on the properties of composite materials when interacting with them.