## 5 Conclusions  
WCP was heated by 200 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 600 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, and 800 \({}^{\circ}\)C and added in mortar, replacing 30% cement. Some properties of WCP and the mechanical strength of Mortar were determined. The following conclusions were obtained.  
1. The particle size of waste concrete powder changed after heat treatments of 200 \({}^{\circ}\)C, 400 \({}^{\circ}\)C, 600 \({}^{\circ}\)C, 700 \({}^{\circ}\)C, and 800 \({}^{\circ}\)C; the treatment of 700 \({}^{\circ}\)C decreased the particle size of WCP; and its D90 size was reduced by 55% with comparison to the original WCP.
2. More mass loss and free calcium oxide generation in WCP occurred after the heat treatment in 800 \({}^{\circ}\)C; furthermore, some active materials in WCP treated at 800 \({}^{\circ}\)C were determined by XRD detection.
3. The activity index (AI), which was measured by the ratio of mechanical strengths between the mortar with 30% replacement of cement by WCP to those of the original mortar, demonstrated that the WCP treated at 700 \({}^{\circ}\)C could be used in concrete as an admixture.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **Day** &
\begin{tabular}{c} **Reference** \\ **Mortar** \\ \end{tabular} & \multicolumn{5}{c}{**Replacement 30\%**} \\ \cline{3-10}  & & **0\({}^{\circ}\)C** & **200 \({}^{\circ}\)C** & **400 \({}^{\circ}\)C** & **600 \({}^{\circ}\)C** & **700 \({}^{\circ}\)C** & **800 \({}^{\circ}\)C** \\ \hline Flexible strength & 3 & 5.44 & 3.58 & 3.33 & 3.66 & 3.81 & 4.91 & 3.96 \\ (MPa) & 7 & 7.27 & 4.43 & 5.72 & 6.52 & 5.15 & 6.42 & 5.36 \\  & 28 & 8.79 & 4.63 & 6.18 & 7.06 & 5.89 & 6.83 & 6.23 \\ \hline Compressive strength & 3 & 29.41 & 17.68 & 14.34 & 14.97 & 16.68 & 23.71 & 17.53 \\ strength & 7 & 46.82 & 25.16 & 26.20 & 29.92 & 23.16 & 33.43 & 28.44 \\ (MPa) & 28 & 55.34 & 41.91 & 38.98 & 42.64 & 39.82 & 47.71 & 36.98 \\ \hline \hline \end{tabular}
\end{table}
Table 4: Mechanical strengths of mortars with the heated WCP for replacing 30% cement.  
\begin{table}
\begin{tabular}{c c c c c c c c c} \hline \hline  & **Day** &
\begin{tabular}{c} **Reference** \\ **Mortar** \\ \end{tabular} & \multicolumn{5}{c}{**Replacement 30\%**} \\ \cline{3-10}  & & **0\({}^{\circ}\)C** & **200 \({}^{\circ}\)C** & **400 \({}^{\circ}\)C** & **600 \({}^{\circ}\)C** & **700 \({}^{\circ}\)C** & **800 \({}^{\circ}\)C** \\ \hline \multirow{3}{*}{AI of flexible strength (\%)} & 3 & 100 & 65.8 & 61.2 & 67.3 & 70.0 & 90.0 & 72.8 \\  & 7 & 100 & 60.9 & 78.7 & 89.7 & 70.8 & 88.3 & 73.7 \\  & 28 & 100 & 52.7 & 70.3 & 80.3 & 67.0 & 77.7 & 70.9 \\ \hline \multirow{3}{*}{AI of compressive strength (\%)} & 3 & 100 & 60.1 & 48.8 & 50.9 & 56.7 & 80.6 & 59.6 \\  & 7 & 100 & 53.7 & 56.0 & 63.9 & 49.5 & 71.4 & 60.7 \\ \cline{1-1}  & 28 & 100 & 75.7 & 70.4 & 73.5 & 72.0 & 86.2 & 66.8 \\ \hline \hline \end{tabular}
\end{table}
Table 5: AI values for heated WCP according to the mechanical strengths.  
**Author Contributions:** Conceptualization and writing--original draft preparation, Y.S.; data curation, C.O., S.L. and J.Z.; writing--review and editing, Q.T. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research was funded by the Shandong province key R&D program (Public Welfare) 2019, Grant number 2019 GSF109108, the scientific research foundation for the returned overseas Chinese scholars, State Education Ministry, grant number: (2013)1792 and the fund project of collaborative Innovation Center of green building of Shandong Province.  
**Acknowledgments:** The authors would like to thank [PERSON] for his contribution of SEM images detection.  
**Conflicts of Interest:** The authors declare no conflict of interest.