This study explores the potential reuse of filter–pressed concrete sludge residue as a substitute for JIS–standard sand in cement mortar. Initially, mortar specimens were prepared using ordinary Portland cement with 10%, 20%, and 30% (by mass) of residue added. Compared to the mortar without additives, a slight increase in compressive strength was observed. Next, a Type B equivalent cement was prepared by mixing ordinary Portland cement with ground granulated blast furnace slag at a ratio of 60:40. Mortars were then produced with 10%, 20%, and 30% (by mass) of residue added, and their compressive strength was evaluated. The results indicated that even with up to 30% (by mass) residue addition, compressive strength slightly improved, confirming the material's performance retention. Furthermore, sulfate resistance testing was conducted by immersing Type B cement mortar specimens in 5% (by mass) sulfuric acid solutions. In the 5% (by mass) concentration test, high sulfate resistance was confirmed. Based on these findings, the prepared mortar was demonstrated to meet the standards set by the Japan Sewage Works Agency.

　This paper reports on the synthesis of calcium carbonate by the carbonation of gypsum dihydrate. The carbonation was carried out in sodium carbonate solution using gypsum dihydrate reagent for the fundamental study. The effects of reaction time, sodium carbonate solution concentration and reaction temperature on the carbonation were discussed. As for the reaction time, the carbonation gradually progressed and completed for 60 min at room temperature and 0.1 mol・dm^－3. In this time, calcite and vaterite were synthesized. It was confirmed that vaterite was generated on the surface of gypsum dihydrate. Vaterite separated from the surface of gypsum dihydrate due to crystal growth. On the other hand, a small amount of aragonite was formed as the sodium carbonate solution concentration increased. Regarding the reaction temperature, aragonite was formed at 40℃ and 0.1 mol・dm^－3. However, gypsum dihydrate remained at 50℃. The morphologies of calcite and vaterite were typical rhombohedral and spherical crystals. The aragonite shape was chestnut–like aggregation. The optimal condition of aragonite synthesis in this study was sodium carbonate concentration of 0.4 mol・dm^－3 and temperature of 40℃, the amount of aragonite in the sample was 58%.