(Received 7 September 2000; accepted 30 September 2002)
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ASTM C-359, a standard test method for early stiffening of Portland cement (mortar method), is used to assess false set and flash set behaviors of cement in mortar. This test method defines cement false set as “the early development of stiffness . . . . . .without evolution of much heat, which stiffness can be dispelled and plasticity regained by further mixing without addition of water,” while flash set is described as “the early development of stiffness . . . . . .usually with the evolution of considerable heat, which stiffness cannot be dispelled nor can the plasticity be regained by further mixing without addition of water.” Therefore, if penetration is regained after remixing, then the early stiffening is considered as false set. If remixing cannot restore the penetration, the early stiffening is considered as flash set. Although ASTM C-359 does not specify the causes of false set and flash set, it is generally understood that false set is mostly due to re-hydration of dehydrated gypsum in the cement, while flash set is believed to be the result of gypsum deficiency in the cement. The deficiency of gypsum allows rapid hydration of aluminate phase. Our test results indicated that in some cases ASTM C-359 alone could not be directly used to explain the origin of false set or flash set. It is found that early stiffening behavior is related to the amount of gypsum and degree of gypsum dehydration, to the clinker mineralogy and morphology, and to the physical property such as cement fineness.
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