# Effect of Adding Waste Polyethylene and GGBFS on the Engineering Properties of Cement Mortar
## 3 Results and Discussions
### Ultrasonic Velocity  
The ultrasonic velocity measurement method will not cause any damage to the cement products, and the simple operation is a method that the engineering field is willing to  
Figure 8: Tensile strength of cement mortars with the addition of different ratios of waste PE and GGBFS replacement.  
adopt. The measured wave velocity value is closely related to the compactness of the cement products. Generally speaking, the pulse wave velocity is proportional to the square root of the elastic modulus of concrete and inversely proportional to the square root of the concrete density. The elastic modulus of concrete is proportional to the square root of the compressive strength according to the ACI 318 recommendation, and the pulse wave velocity is proportional to the fourth root of the compressive strength. That means that when the compressive strength of concrete increases with the increase in the material's age, the pulse wave velocity will also increase slightly. However, at a later curing period of the material, the pulse wave velocity in the concrete does not change sensitively with the strength.  
After 3 days, the W/B was 0.5, and the ultrasonic velocities varied from 3862 m/s to 3589 m/s with different proportions of waste PE, as shown in Table 6 and Figure 9. After 28 days, the wave velocities of the control group and other groups with different proportions of GGBFS (0%, 10%, and 20%) replacing cement were 4192 m/s, 4300 m/s, and 4386 m/s (increased by 2.6% to 4.5%). This result indicated that the replaced volume of GGBFS increased from 0% to 20%, and the ultrasonic velocity tended to rise slightly due to the pozzolanic reaction of GGBFS. After 91 days, the ultrasonic velocities of the control group and other groups with 1% to 4% added waste PE were 4616 m/s, 4554 m/s, and 4310 m/s, while the ultrasonic velocity decreased by 1.3% and 6.6% compared with the control group. This result indicated that the ultrasonic velocity decreased due to the increment in the waste PE.  
\begin{table}
\begin{tabular}{c c c c c c c} \hline
**W/B** & **GGBFS (\%)** & **PE (\%)** & **3 Days** & **7 Days** & **28 Days** & **56 Days** & **91 Days** \\ \hline \multirow{8}{*}{0} & & 0 & 3862 & 3975 & 4192 & 4391 & 4616 \\ \cline{2-7}  & & 1 & 3830 & 3973 & 4137 & 4302 & 4554 \\ \cline{2-7}  & & 2 & 3800 & 3911 & 4083 & 4256 & 4462 \\ \  
After 91 days, the wave velocities of the groups without added waste PE but with different proportions of GGBFS (0%, 10%, and 20%) to replace cement were 4616 m/s, 4801 m/s, and 4903 m/s, respectively. From 28 to 91 days, the wave velocity of the control group increased by 10.1%. When the replaced volumes of GGBFS were 10% and 20%, the wave velocities increased by 11.7% and 11.9%, respectively. This result indicated that the groups with GGBFS added produced pozzolanic reactions in the late period, which filled the internal holes of the sample and increased its compactness. After 56 days, when the replaced volume of GGBFS was 20%, and 1% to 2% of waste PE was added, the ultrasonic velocity was higher than 4500 m/s, which indicated that the concrete was of good quality.