Analysis on Retardation Mechanism for Low Cement Castable Setting by Boric Acid Addition

摘要

A boric acid retarding mechanism for low cement castable containing alumina cement has studied. With the addition of boric acid to water solution percolated from alumina cement dispersion liquid, gel precipitate containing a large amount of aluminum hydroxide is formed along with the lowering of Al and Ca ion concentration and pH value. However, the temporarily lowered Al ion concentration and pH value gradually increase with the lapse of time. The dissolution behavior of the formed gel precipitate is similar to that of Al(OH)_3 gel. It was concluded that selective permeability of gel precipitate formed on alumina cement to water and ions as well as drastic change of gel precipitate dissolution caused by pH value change are major factors for retation mechanism of alumina cement hydration by boric acid. In low cement castable system, coagulation time is delayed in proportion to added amount of boric acid, and capillary water pH value at coagulation time exhibits almost the same value regardless of the amount of added boric acid. At coagulation, Al ion elution into capillary water is observed. These results indicate that retardation mechanism of sole alumina cement hydration by boric acid can be applied for retardation mechanism of low cement castable setting.