柠檬酸对建筑石膏水化的影响及其机理研究

系统研究了柠檬酸对建筑石膏凝结硬化过程及二水石膏晶体形貌的影响，以及不同pH值下的柠檬酸缓凝效果，并结合光电子能谱分析技术对其作用机理进行了分析．结果表明：柠檬酸使建筑石膏的水化放热变缓，液相过饱和度和早期水化率降低；柠檬酸与二水石膏表面的钙离子产生化学结合，其结合层覆盖在二水石膏晶胚表面，降低了晶核的表面活性，增加了成核势垒，使二水石膏晶体成核数目减少，速率减慢，诱导期延长；柠檬酸对二水石膏[111]面钙离子的选择性吸附，抑制了二水石膏在C轴上的生长，改变了二水石膏的晶体形貌，使其由针状转化为短柱状；由于二水石膏晶胚减少，晶体生长时间较充分，导致晶体尺寸明显粗化，且柠檬酸掺量越大，晶体粗化现象越严重．

Effect of Citric Acid on the Process of Hydration of Building Gypsum and Its Retarding Mechanism

Effect of citric acid on the setting and hardening of building gypsum and crystalline habit of dihydrate and relationship between pH value and retarding effect were analyzed. Retarding mechanism of citric acid was discussed from the standpoint of crystal growth by X-ray photo technology. The results obtained indicate that the addition of citric acid delays the hydration of building gypsum, correspondingly leading to heat evolution slowing down and reduction in degree of supersaturation in liquid phase and hydration degree at early stage. Due to the complexion between citric acid and Ca~(2+), the surface energy of embryo newly formed is greatly decreased, correspondingly the number of nuclei lessen, the growth rate of dihydrate is slow down and thereby the introduction period is prolonged, so the crystal have enough time and space to grow. At the same time, because of preferred adsorption of citric acid on [111] face, the growth of C axis is strongly inhibited, consequently leading to the transformation of dihydrate crystal from needle-like to short prismatic. Furthermore the higher the dosage of citric acid, the larger the dihydrate crystal is.