Abstract: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.