## 2 Materials and Methods
### Mix Proportion and Specim Preparation  
Table 3 summarizes the mixed proportion of PUCM modified with NS. A total of 13 groups of PU-cement mortar containing different PU binders and NS content were prepared in this study. The PU binder was introduced to the cement mortar mixtures by cement weight at 0%, 10%, 15%, and 25%, while NS was introduced into the mortar mixture also by the weight of cement at 0 to 3% (interval of 1%) as summarized in Table 3. The constant water-to-cement (w/c) ratio of 0.25 was used to prepare all PU-modified cement mortar mixtures.  
The mortar containing PU binder modified with NS were produced based on the following steps: firstly, the dry materials, which includes sand, cement, and NS were placed in an SJ-15 cement mortar mixer and mixed thoroughly for 1 min with a slow speed. Then,  
\begin{table}
\begin{tabular}{c c c c c c c c c c c} \hline \hline \multirow{2}{*}{**Material**} & \multicolumn{8}{c}{**Oxides**} \\ \cline{2-11}  & **SiO2** & **Al2O3** & **Fe2O3** & **CaO** & **MgO** & **K2O** & **Na2O** & **SO3** & **TiO2** & **LOI** \\ \hline Cement & 23.270 & 4.410 & 2.450 & 62.850 & 1.420 & 0.480 & 0.210 & 2.570 & 0.080 & 1.820 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Chemical compositions of cement.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**PU Binder**} & \multirow{2}{*}{**Viscosity (CPS)**} & \multirow{2}{*}{**Appearance**} & \multicolumn{2}{c}{**Curing Age (h)**} & \multirow{2}{*}{**Tension Property (MPa)**} \\ \cline{3-4}  & & & & **Initial** & & **Final** \\ \hline Polyol & 35,000 & Grey white sticky & - & - & - \\ PAPI & 250 & Brown transparent & - & - & - \\ PU binder & - & - & 3.5 & 72 & 5.5 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Indexes of PU binder.  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline \multirow{2}{*}{**SU N**} & **Mixture ID** & **Cement** & **PU Binder** & **NS** & **Sand** & **Water** & **Superplasticizer** \\ \hline
1 & PUCM0-0 & 702 & 0.00 & 0.00 & 1404 & 175.5 & 14.04 \\
2 & PUCM10-0 & 702 & 70.2 & 0.00 & 1404 & 175.5 & 14.04 \\
3 & PUCM10-1 & 702 & 70.2 & 7.02 & 1404 & 175.5 & 14.04 \\
4 & PUCM10-2 & 702 & 70.2 & 14.04 & 1404 & 175.5 & 14.04 \\
5 & PUCM10-3 & 702 & 70.2 & 21.06 & 1404 & 175.5 & 14.04 \\
6 & PUCM15-0 & 702 & 105.3 & 0.00 & 1404 & 175.5 & 14.04 \\
7 & PUCM15-1 & 702 & 105.3 & 7.02 & 1404 & 175.5 & 14.04 \\
8 & PUCM15-2 & 702 & 105.3 & 14.04 & 1404 & 175.5 & 14.04 \\
9 & PUCM15-3 & 702 & 105.3 & 21.06 & 1404 & 175.5 & 14.04 \\
10 & PUCM25-0 & 702 & 175.5 & 0.00 & 1404 & 175.5 & 14.04 \\
11 & PUCM25-1 & 702 & 175.5 & 7.02 & 1404 & 175.5 & 14.04 \\
12 & PUCM25-2 & 702 & 175.5 & 14.04 & 1404 & 175.5 & 14.04 \\
13 & PUCM25-3 & 702 & 175.5 & 21.06 & 1404 & 175.5 & 14.04 \\ \hline \hline \end{tabular}
\end{table}
Table 3: Mix proportion of the mortar material (1 kg/m\({}^{3}\)).  
water containing superplasticizer was introduced to the dry mix and continued mixing for another 2 min. Subsequently, freshly prepared PU binders with various contents were introduced to the mortar mixes and we continued mixing until a homogeneous cement mortar mixture was achieved. Therefore, three specimens from each group were cast using prismatic specimens of a 40 \(\times\) 40 \(\times\) 160 mm\({}^{3}\) mold to evaluate mechanical properties. After 24 h, the beam specimens were de-molded and kept in the standard curing room (temperature = 20 \(\pm\) 2 \({}^{\circ}\)C and relative humidity \(\geq\) 99%) for 28 days before the testing. To achieve more reliable results, three samples from each PUCM sample were tested, and their average was measured as the test result. Figure 2 illustrates the systematic process of PUCM mixture and testing method.