Necessity to promote a recycle of waste plaster board is mounting, since the waste plaster board has been legally prohibited to dispose into inert landfill site. However, it is concerned that the recycled waste plaster board (recycled gypsum) produces hydrogen sulfide gas, so that it is one of the interference with the recycling. There is a possibility to control the hydrogen sulfide gas production from the recycled gypsum by artificially changing into alkaline condition or adding iron oxide. In this study, we discussed the hydrogen sulfide gas production from a ground improvement with the recycled gypsum, quick lime, and/or iron oxide powder. In addition, unconfined compression strength of the ground improvement is evaluated in order to obtain an incentive of the recycling. Adding only the recycled gypsum cannot improve strength of a soft ground having much amount of fine particles. Quick lime was required for strength improvement. If pH of the ground improvement is greater than 9, it was observed that the hydrogen sulfide gas production potential is negligible. Increasing pH by adding the quick lime is able to more effectively control the production of the hydrogen sulfide gas than adding the iron oxide powder. The production of the hydrogen sulfide gas is able to be controlled by adding the quick lime more than half of the amount of gypsum addition.
