铬酸型钙矾石与硫酸型钙矾石的差异性研究

The differences between the characters of chromate ettringite and of ettringite

人工合成硫酸型钙矾石和铬酸型钙矾石晶体的X射线衍射谱显示,铬酸型钙矾石的晶格比硫酸型钙矾石的稍大一些。把铬酸型钙矾石放在SO_4^(2-)浓度为1240mg/L的硫酸钠溶液中长期浸泡并跟踪监测发现,随着浸泡时间的延长,铬酸型钙矾石中的CrO_4^(2-)被SO_4^(2-)替换的比例升高,6个月时其置换率达到了近26％。分析认为这是因硫酸型钙矾石的溶解度远小于铬酸型钙矾石而引起的不全等溶解的结果。采用2种加热方武来研究2种人工合成钙矾石的热稳定性的差异。一种是连续加热实验,即把合成的硫酸型钙矾石或铬酸型钙矾石及其合成液混合物由25℃至60℃逐级加热,并在每个温度下恒温1小时。第二种加热方法是分次加热,即把硫酸型钙矾石或铬酸型钙矾石与石灰水混合,并分为若干份,每份混合物在不同的温度级别下恒温受热1小时。液相化学监测结果表明:这2种加热试验所得的结果十分接近;硫酸型钙矾石液相中SO_4^(2-)的浓度在40℃～50℃时浓度最低,说明硫酸型钙矾石在该温度下最为稳定;而铬酸型钙矾石最为稳定的温度是45℃～60℃。硫酸型钙矾石周围液相中SO_4^(2-)的浓度在70℃～75℃左右开始增大,说明硫酸型钙矾石于此温度开始出现热分解,不过90℃时其分解量仅为13％左右。铬酸型钙矾石则在80℃以后开始出现热分解。

The lattice of chromate ettringite appears a little bigger than that of ettringite according to their X-ray diffraction patterns. When chromate ettringites are dipped into sodium sulfate solution which the concentration of sulfate is 1240mg/L, the amount of the substitution of sulfate for chromate in it increases with time. After 6 months contact, the substitutions come up to nearly 26 percent. The substitution is believed to result from the difference of their dissolubility which can give rise to incongruent dissolution between ettringite and chromate ettringite. The differences of the thermal stability between these two ettringites are discovered according to two kinds of experiments. In the first experiment which is used continues heating way, synthetic ettringites or chromate ettringites were heated from the grade of 25℃ to grade of 60℃ with their solutions. During heated, the mixture was heated at constant temperature for 1 hour at every grade. In the second kind of experiment in which mixture samples were heated separately, synthetic ettringites or chromate ettringite were heated at every temperature grade with their lime solutions separately. During these two experiments, chemical changes in the solutions were monitored all the time in order to determine the concentrations of sulfate or chromate. The monitoring data show that similar results were achieved from the two experiments. They all indicate that the concentration of sulfate in ettringite solution becomes the lowest at 40℃～50℃. That's to say, the most suitable temperature for ettringite formation is at the region of 40℃～50℃. However, the best temperature for chromate ettringite formation is at 45℃～60℃. The concentrations of sulfate in ettringite solution begin to increase at 70℃-75℃, indicating ettringites begin to decompose significantly at that temperature. The amount of the decomposition of ettringite is about 13 percent at 90℃ according to monitoring data. It appears that chromate ettringitea start to break down at 80℃ and the amount of the decomposition reaches 62 percent at 90℃. Overall, compared with ettringite, chromate ettringitea have so features: bigger lattices, smaller crystals, higher solubility, worse thermal stability. These characters were believed to result from the week binding of chromate to other components in the lattice. This essential distinction can also bring about other unstable natures such as high carbonation rate, more solubility in acid water, et al.