# Experimental Study on Preparation and Characteristics of Concrete Modified by Construction Waste
## 2 Materials and Methods  
### Raw Materials  
Cement: P-O52.5 grade ordinary cement produced by Zhangjiakou Xuanhua Jinyu Cement was used, and all its indicators meet the specification requirements. The basic performance indicators are shown in Table 1. Cement clinker particle size distribution curve is shown in Figure 1.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline SSA (m2/kg) & Loss & \multicolumn{2}{c}{Setting Time (min)} & \multicolumn{2}{c}{Bending Strength (MPa)} & \multicolumn{2}{c}{Compressive Strength (MPa)} \\ \cline{3-7}  & & Initial Setting & Final Set & 3 d & 28 d & 3 d & 28 d \\ \hline
335 & 1.55\% & 90 & 240 & 4.5 & 8.1 & 21.6 & 57.5 \\ \hline \end{tabular}
\end{table}
Table 1: Physical performance indicators of cement.  
Recycled aggregate surface modifier: KH-560 methacryloxy functional group silane; its general formula is RSiX3, a light yellow transparent liquid with a refractive index of (nD25) 1.4260-1.4280 and a specific gravity of (dD25) 1.065-1.072. The regenerated aggregate was put into the modifier solution for impregnation and modification. After impregnation, the regenerated aggregate was taken out and placed in an environment of 20 \(\pm\) 5 \({}^{\circ}\)C, drained for 1 d, and then dried at 105 \(\pm\) 5 \({}^{\circ}\)C to constant weight. Then, the related performance of the regenerated aggregate was tested.  
Polycarboxylic acid superplasticizer: water reduction rate of 30%, solid content of 30%. Silica fume: The silica fume produced by Beijing Lihong Chuangye International Co., Ltd. (Beijing, China) has an average particle size of 0.045 \(\upmu\)m, and its properties are shown in Table 2.  
The waste concrete is obtained from the project site, the steel bar is removed, the crusher is broken and screened, and a suitable particle size of recycled aggregate is selected. The production process of waste concrete recycled aggregate is shown in Figure 2.  
The porosity of concrete is divided into total porosity and effective porosity. Total porosity is the percentage of the volume of all voids in concrete to its total volume. It is  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Name** & **SiO2 (\%)** & **SSA (m2/kg)** & **Loss (\%)** & **Cl\({}^{-}\) (\%)** & **Moisture (\%)** & **Water Proportion** \\  & & & & & **(\%)** \\ \hline silica fume & 86 & 21,000 & 3.5 & 0.2 & 1.85 & 110 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Physical parameters of silica fume.  
Figure 1: Cement clinker particle size distribution curve.  
Figure 2: Waste concrete recycled aggregate preparation process.  
the main factor affecting the strength of concrete. The relationship between the porosity of concrete and the particle size of crushed stones is shown in Figure 3a. If the cement dosage is kept constant, as the size of coarse aggregate increases, its specific surface area becomes smaller, and there is less water demand, which allows a decrease in the water cement ratio. This is beneficial for improving concrete strength and reducing costs. However, the probability of internal defects occurring in large particle aggregates is relatively high, while small particle aggregates are relatively dense, and due to their generally low water cement ratio, the reduction in water cement ratio caused by the above factors is not significant. However, it can increase the bonding area with cement and improve the strength of concrete. At the same time, if the coarse aggregate particle size is too large, it will reduce the interfacial bonding performance between mortar and coarse aggregate, and seriously affect the impermeability and crack resistance of concrete. The relationship between compressive strength and particle size of concrete is shown in Figure 3b. Therefore, the particle size of recycled waste concrete aggregate selected in this test is 15-20 mm.