## Result and Discussion
### Contact Angle  
The middle part of each \(50\times 50\times 50\) mm specimen at 28 curing days was cut and its surface was divided into nine parts. After dropping a 3 \(\mu\)L water drop on each part, the surface contact angle was measured and averaged. The WR-In exhibited the largest surface contact angle followed by ZWR, Sand-Co, FA-Co, OPC, and Silica-Co. In the surface contact angle measurement to examine the hydrophobicity of each specimen, average values were obtained based on the results of measuring the OPC specimen three times. In the case of OPC, the surface contact angle could hardly be measured because a water drop was dropped and absorbed at the same time. The standard deviation of OPC was \(\pm 10^{\circ}\) and the measurements were \(13^{\circ}\), \(24^{\circ}\), and \(21^{\circ}\), respectively. When Sand-Co, Silica-Co, and FA-Co were measured using the same method, deviations of less than \(\pm 15^{\circ}\) were observed. The WR-In and ZWR specimens exhibited no significant difference with deviations of \(\pm 7^{\circ}\), and slight differences did not significantly reduce hydrophobicity. Table 11 shows the results of measuring the surface contact angle using the sessile drop method at 28 days of age.  
Figure 7: Results of the compressive strength test.  
The contact angle was found to be 133\({}^{\circ}\) for the WR-In specimen, in which the water repellent was directly mixed, and 124\({}^{\circ}\) for the ZWR specimen. For the OPC and Silica-Co specimens, the contact angle could not be measured because it was too small, and their contact angles were determined based on the several measurements. Figure 8 shows the measured contact angles. The contact angles of the WR-In and ZWR specimens were larger than 90\({}^{\circ}\), which indicates hydrophobicity. For these specimens, it appears that water was not absorbed through pores and the repulsive force was generated by the surface tension, because the pores inside concrete was filled with the water-repellent's active ingredient and powder.  
Figure 9 shows the method image of measuring the contact angle. The WR-In exhibited hydrophobicity with a contact angle of 130\({}^{\circ}\), but its compressive strength was only 30% of that of OPC. The strength of ZWR was 83% of that of the reference sample and its contact angle was measured to be 124\({}^{\circ}\), confirming that ZWR exhibited excellent performance compared to the compressive strength reduction rate. In the case of FA and SF, it appears that the surface was not completely coated with the water repellent even after immersion in it and drying because the surface was smooth and spherical and had few internal pores and, thus, the fixing ability was insufficient. Inside the cement mortar, some of the water-repellent's active ingredient was attached to the interface of FA and SF, and some remained in the form of particles, thereby exhibiting a small surface contact angle. It appears that the water-repellent active ingredient that remained as particles lowered the compressive strength by interfering with bonding inside cement mortar and delaying hydration.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**Name**} & \multicolumn{4}{c}{Contact Angle (\({}^{\circ}\)) 28 Days} \\ \cline{2-6}  & \multicolumn{4}{c}{Measurement Average} & \multicolumn{2}{c}{Normalized Values} \\ \cline{2-6}  & 1 st & 2 nd & 3 rd & Total & OPC 28 Days (\%) \\ \hline OPC & - & 20 & - & 20 & 100 \\ \hline Sand–Co & 85 & 80 & 72 & 79 & 395 \\ \hline Silica–Co & 20 & - & - & 20 & 100 \\ \hline FA–Co & 74 & 60 & 69 & 68 & 340 \\ \hline WR–In & 136 & 130 & 133 & 133 & 665 \\ \hline ZWR & 122 & 124 & 128 & 124 & 620 \\ \hline \hline \end{tabular}
\end{table}
Table 11: Result of contact angle (esssile drop method).  
Figure 8: Result of contact angle test.