## 4 Conclusions  
1. The slump and slump flow of the cement mortar made by replacing sand with the TPSW decreased as the amount of TPSW added increased. Since the TPSW was  
Figure 15: SEM images of 5% TPSW cement mortar at different ages.  
less smooth than natural sand, the slump and slump flow were reduced. When the W/C increased, the slump and slump flow increased due to the increase in the water content. When the setting time was prolonged, the W/C ratio had a greater impact than the substitution amount.
2. The compressive strength decreased as the amount of TPSW substitution increased because the waste PS was less water-absorbent than natural sand during mixing, which resulted in more water molecules. The cementing ability was degraded and pores were formed. A W/C of 0.5 and a TPSW substitution amount of 5% could effectively achieve the economic benefits of waste recycling.
3. As the amount of TPSW added increased, the flexural strength decreased. When the W/C was 0.5 and at the age of 91 days, the strength was reduced by 9.4% when the substitution amount was 5%. When the substitution amount exceeded 10%, the strength was reduced by more than 10%, showing that a W/C of 0.5 and a TPSW substitution amount of 5% were the most effective at removing waste for waste recycling.
4. Since the ultrasonic pulse velocity depends on the internal density of the specimen, when the W/C was 0.4 and the TPSW substitution amount was 5% at the age of 56 days, the ultrasonic pulse velocity was greater than 4500 m/s, indicating that the specimen quality was good. As the substitution amount and W/C increased, pores were formed inside, so the ultrasonic pulse velocity decreased. The velocity increased with age.
5. As the W/C and the waste PS substitution amount increased, the number of pores in the specimen increased, and the number of internal pores increased; at later ages, the water absorption distinctly decreased. The water absorption had a relative relationship with the ultrasound and strength. The higher the ultrasonic pulse velocity was, the denser the specimen, the lower the water absorption, and the greater the strength.
6. The weight loss results for the cement mortar show that the W/C ratio had a greater impact than the substitution amount. The weight loss increased with the W/C. As the substitution amount increased and as the sulfate resistance of the TPSW became greater than that of the natural sand, the weight loss tended to decrease. The weight loss was greater at an early age. The hydration was relatively complete at late ages, so the weight loss of the specimen tended to decrease.
7. The resistivity decreased as the W/C increased. Since the TPSW had a higher resistivity than natural sand, the resistivity increased with the substitution amount and age.  
Conceptualization, H.-Y.W.; Methodology, C.-H.W. and S.-L.T.; Formal analysis, C.-C.H.; Investigation, C.-F.L.; Resources, H.-Y.W.; Data curation, C.-C.W.; Writing-review & editing, C.-C.H. and H.-Y.W. All authors have read and agreed to the published version of the manuscript.  
This research received no external funding.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
First author [PERSON] was employed by the Hoei Construction Co., Ltd., Taiwan. The research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.