低碱水泥凝结异常与机理分析

Abnormal Setting of Cement with Low Alkali and Its Mechanism

针对碱当量为0.10%的低碱硅酸盐水泥熟料出现石膏不能缓凝甚至促凝的异常现象,通过补充碱金属硫酸盐并与碱当量为0.64%的熟料进行对比,测定水泥凝结时间、流动度和黏度的变化,同时采用水化微量热仪和X射线衍射仪测量水化热和分析水化产物,以反映碱金属硫酸盐在低碱水泥早期凝结过程中的作用。结果表明:对于低碱熟料,外掺石膏使水泥SO_3含量达到0.50%,水泥初凝时间显著缩短,继续增加石膏掺量初凝时间虽有一定延长,但仍短于未掺石膏的熟料。碱金属硫酸盐有利于延长水泥凝结时间,但与水泥碱硫比即Na_2O_e/SO_3摩尔比有关,低于0.30时石膏缓凝作用不明显,高于0.60时继续增加碱含量会引起凝结异常,因此Na_2O_e/SO_3摩尔比在0.30~0.60范围内有利于凝结时间延长。适量碱金属硫酸盐能促进石膏与3CaO·Al_2O_3(C_3A)反应,有利于石膏发挥缓凝作用。

An abnormal phenomenon that the setting time of a low alkali clinker with 0.10% alkali equivalent cannot be prolonged or it is even shortened by gypsum was investigated. The performances such as setting time, fluidity and viscosity of cement were measured by adding alkali sulfates and comparing to a clinker with 0.64% alkali equivalent. The hydration process and product of cement were analyzed by isothermal calorimetry and X-ray diffraction to further clarify the effect of alkali sulfates in the early setting process of low alkali cement. The results show that the initial setting time shortens remarkably when SO3 increases to 0.50% in gypsum, and prolongs at a greater addition of gypsum, but it is still shorter than that of clinker without gypsum. The presence of alkali sulfates favors the increase of the setting time of cement, but is related to the alkali to sulfate molar ratio, i.e., Na2Oe/SO3 molar ratio. Gypsum alone cannot retard the setting time when Na2Oe/SO3 molar ratio is 〈 0.30. The further increase of alkali leads to abnormal setting when Na2Oe/SO3 molar ratio is 〉 0.60. Therefore, the Na2Oe/SO3 molar ratio in the range of 0.30-0.60 can favor the increase of the setting time. An appropriate amount of alkali sulfates can promote the reaction of 3CaO·Al2O3（C3A） with gypsum and favor gypsum to have the retarding effect.