Oyster shells are one of the most abundant seafood wastes in Vietnam which have not been deposed efficiently, causing many negative effects on the ecosystem and the quality of human life. In this study, the recycling of waste oyster shells was investigated to provide a calcium carbonate-rich material for improving and stabilizing the pH value of acidified water. Characterization of the recycled shells using various techniques proved that the thermal treatments at temperatures from 120 to 220 °C led to no significant changes in the chemical composition and calcite-based layer structure of oyster shells. Testing the performance of the recycled oyster shells in the adjustment of the aqueous pH value was conducted in a solution containing NH4Cl (0.5 g/L) as a simulation of the acidified aquaculture phase. As expected, using 1 g of the 220 °C -calcinated oyster shell for 2 L of the simulated solution, the solution pH rapidly increased from 6.3 to 7.7 in the first 2 h, reached the extreme value (~ 7.8) after 24 h and slowly decreased to the stabilized pH range of ~ 7.2 after 5-6 d. It was proved out that the oyster shell amount significantly affected the solution pH value. Importantly, the elemental analysis showed that the content of toxic transition metals including Cd, Pb, As, and Cr in the oyster shell was below the detection limit of each other, implying great potential for the application in the real aquaculture environment in Vietnam.