**4. Conclusions**

In this study, cement and fly ash were used as cementitious materials and waste gypsum was used as the fine aggregates, which were suitable for CLSM. The compressive strengths at 28 and 56 days were 4.71 and 6.08 MPa, respectively. The air-curing method should be applied to avoid abnormalities such as volume expansion after curing the specimen in water. With the hydrated blending reaction, the waste gypsum in the CLSM specimen continued to react with cement and fly ash. The reaction produced more ettringite, C-S-H colloids and other hydrates with age, increasing the compressive strength. In waste gypsum, some components react with CO2 in the air, such as CaO and Ca(OH)2, but they have become stable, allowing for the specimen to maintain a reasonable volume. In addition, the volume and weight of the specimen did not change significantly after being immersed in seawater and tap water, and the appearance of the specimen was intact. Additionally, it was determined that waste gypsum can entirely replace fine aggregates in construction and can significantly increase the amount of reuse, thus achieving the effect of the green circular economy.

**Author Contributions:** Conceptualization, W.-T.L., A.C. and K.K.; methodology, K.K.; validation, W.-T.L., D.M. and M.Ł.; investigation, A.C.; resources, K.-L.L.; data curation, W.-T.L. and D.M.; writing—original draft preparation, W.-T.L. and K.K.; writing—review and editing, W.-T.L. and M.Ł.; visualization, A.C. and K.-L.L.; supervision, W.-T.L. and K.-L.L.; project administration, W.-T.L. and D.M. All authors have read and agreed to the published version of the manuscript.

**Funding:** The National Science and Technology Council (NSTC) in Taiwan supported this research with grant NSTC 111-2923-E-197-003-MY3 and the Polish National Centre for Research and Development in Poland, under the M-ERA.NET 3 program, grant number M-ERA.NET3/2021/144/3D-FOAM/2022.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** Publication cost of this paper was covered with founds of the Polish National Agency for Academic Exchange (NAWA): "MATBUD'2023—Developing international scientific cooperation in the field of building materials engineering" BPI/WTP/2021/1/00002, MATBUD'2023.

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