Influence of citric acid on the performances and change laws of recycled gypsum were studied. Its mechanism of citric acid was analyzed by the related theory of chemical reaction, FTIR and SEM. The results obtained showed that the addition of citric acid delayed the hydration, decreased the strength of paris of plaster remarkablely. The water demand, setting time and 1 d strength of recycled plaster with CA (R-CP) were decreased, prolonged and reduced, respectively, the dry strength, however, was increased in comparison to the recycled plaster without CA (R-P). Compared with paris of plaster without CA (POP), the water requirement, setting time and strength of R-P were increased, prolonged and substantially reduced, respectively, and the decreasing rate of strength was approximately 40-50%. The slow increase in water demand, slight decrease in setting time and rise in 1 d strength of R-CP were seen, and the dry strength, however, was improved in comparison with paris of plaster with CA (POCP). The analysis indicated that due to the complexion between citric acid and Ca2+ into CaCit-, the growth of C axis was strongly inhibited and the hydration was delayed, correspondingly leading to the transformation of dihydrate crystal from needle-like to short prismatic, which deceased the strength of POCP. The short prismatic and CaCit- are still existed in R-CP, which can decrease the water requirement and again complex with Ca2+, thus the setting time of R-CP was prolonged and hydration was delayed.
