CA缓凝剂对再生石膏性能的影响及其机理

系统研究了柠檬酸缓凝剂对再生石膏性能的影响，以及其性能变化规律，并结合化学反应相关理论、红外光谱和扫描电镜对其作用机理进行了分析。结果表明：加入柠檬酸缓凝剂后，原生石膏的凝结时间延长，强度大幅度的降低；而相比未掺加缓凝剂的再生石膏(R-P)而言，掺加缓凝剂的再生石膏(R-CP)标稠需水量降低，凝结时间延长，1 d强度有所降低，但干强度却有小幅度的升高。对于未掺加缓凝剂的原生石膏(POP)而言，其对应的再生石膏需水量增加，凝结时间延长，强度降幅达40-50%；而对于掺加缓凝剂的原生石膏(POCP)而言，R-CP有较小幅度地需水量增加，凝结时间和干强度降低，1 d强度却有所升高。分析表明，原生石膏中加入柠檬酸缓凝剂后，柠檬酸与Ca2+结合形成CaCit-，阻止石膏晶体在C轴上的生长，延迟水化，形成粗棒状的晶体，从而降低石膏的强度；而其对应的再生半水石膏依旧保持原有的短棒状，降低其用水量，且R-CP中的CaCit-可以再次与石膏中Ca2+结合，起到效果较差的缓凝作用，延迟再生石膏的水化。

Influence of CA Retarder on Properties of Recycled Gypsum and Its Mechanism

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.