The reductive effect of slowly cooled blast furnace slag and its mechanism were studied and the conclusion described below was obtained. Slowly cooled blast furnace slag and granulated blast furnace slag from the same source had nearly the same sulfur content but they differed markedly in the content of soluble sulfur. The slowly cooled blast furnace slag contained more dissolved sulfur than the granulated blast furnace slag and much of it was thiosulfate ion. Sulfur in the slowly cooled blast furnace slag was localized as calcium sulfide. The reason for the ample elution of thiosulfate ion from the slowly cooled blast furnace slag was deemed due to the oxidation of calcium sulfide, which had been exposed through crushing, to calcium thiosulfate. Both the granulated blast furnace slag and the slowly cooled blast furnace slag exhibited a negative value of redox potential; both showed reducing effects. The slowly cooled blast furnace slag exhibited a greater negative value in redox potential than the granulated blast furnace slag which revealed that the former had greater reducing power than the latter. Hexavalent chromium decreased more slowly when coarse particles of the slowly cooled blast furnace slag were used; this clearly showed that the speed of reduction could be regulated by controlling the particle size of the slowly cooled blast furnace slag. The slowly cooled blast furnace slag showed excellent reducing power in a low pH range and far surpassed that of the granulated blast furnace slag. On the other hand, the granulated blast furnace slag, which bound ample water, removed hexavalent chromium from its liquid phase as hydration proceeded.
