**5. Conclusions**

The present study investigated the effects of different SiC sludge contents and alkaline activator solutions on the geopolymer reaction and microstructures of metakaolin-based geopolymers subjected to different curing times. The geopolymer with the optimal SiC sludge replacement level and NS/SS ratio contained a high heat evolution value (990.6 W/g), excellent mechanical properties (6.42 MPa), and high deconvolution percentages of Q4(3Al, 2Al, and 1Al). The conclusions can be summarized as follows. The high NaOH concentration prompted the dissolution of the initial solid in the alkaline activator, which caused the first exothermal peaks to be more intense. The 1.6NS10SCS (SiO2/Al2O3 molar ratio of 1.51) had favorable mechanical strength due to an increasing SiO2/Al2O3 molar ratio and a synergistic effect between the SiC sludge and the metakaolin. The geopolymers with high replacement levels of SiC sludge not only contained dissolved silica and alumina from metakaolin but also underwent polycondensation on a geopolymer reaction of SiC sludge-based geopolymers. A greater amount of SiC sludge (over 20%) may block the synergistic effect between the SiC sludge and metakaolin. The results of this study have shown that a geopolymer with an optimal SiC-sludge

replacement level and NS/SS ratio expresses a synergistic effect that allows an improvement of both the geopolymer reaction and the strength of metakaolin-based geopolymers. This renders SiC sludge as promising additives for the production of metakaolin-based geopolymers. For future work, we expect to analyze other important properties of SiC sludge-based geopolymers by thermal conductivity, and fire performance, which was a promising alternative as fire resistance materials in the construction industry.

**Author Contributions:** Writing—original draft preparation, K.-W.L.; writing—review and editing, K.-L.L.; T.-W.C.; W.-T.L.; Y.-W.L. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by Ministry of Science and Technology of the Republic of China, Taiwan (Contract No. MOST-109-2221-E-197-012-MY3).

**Acknowledgments:** The authors thank the Ministry of Science and Technology of the Republic of China, Taiwan, for supporting this research financially.

**Conflicts of Interest:** The authors declare no conflict of interest.
