三聚磷酸钠对二水石膏晶体生长习性与晶体形貌的影响

三聚磷酸钠是建筑石膏高效缓凝剂.用扫描电镜和X射线光电子能谱研究了三聚磷酸钠对建筑石膏凝结硬化、液相离子浓度与过饱和度、二水石膏晶体生长习性与形貌的影响.从晶体生长的角度分析了三聚磷酸钠对二水石膏的作用机理.结果表明:三聚磷酸钠显著改变了二水石膏晶体的生长习性,使其由针状变为短柱状或板状,且晶体尺度增加;三聚磷酸钠通过化学作用选择吸附在二水石膏晶体(111)面,抑制其在c轴方向的生长;改变了晶面在不同轴向的相对生长速率,使二水石膏晶体生长习性和形貌发生了改变;三聚磷酸钠使石膏液相离子浓度和过饱和度降低,晶体成核几率和成核速率降低,导致二水石膏晶体粗化.

EFFECT OF SODIUM TRIPOLYPHOSPHATE ON CRYSTAL GROWTH HABIT AND CRYSTAL MORPHOLOGY OF GYPSUM DIHYDRATE

Sodium tripolyphosphate (STPP) is a highly efficient retardant, and is commonly used in building gypsum. The effects of STPP on the setting and hardening, ion concentration in liquid phase and supersaturation degree, crystal growth habit and crystal morphology of gypsum dehydrate were investigated by scanning electron microscope and X-ray photoelectron spectroscopy. The retarding mechanism of STPP was analyzed from the viewpoint of crystal growth. The results indicate that STPP changes the crystal growth habit of gypsum dihydrate, leading to large crystal size and transformation of the crystal shape from needle-like to short prismatic or plank-shaped. The STPP is selectively chemisorbed on the (111) face of gypsum crystal. It inhibits the growth in c axis directions and alters the relative growth rate of crystal faces in different directions, resulting in the transformation of the growth habit and morphology of dihydrate gypsum crystal. The addition of STPP in gypsum decreases the ion concentration and supersaturation degree in gypsum liquid phase. Then, the nucleation probability and the nucleation rate of gypsum dihydrate crystal are reduced, as a result, the gypsum dihydrate crystal is coarsened.