Frequently Asked Questions
Cement & Concrete
Technology Home > FAQs > Early Stiffening,
False and Flash Set
Q: What causes early stiffening and what is the
difference between false set and flash set?
A: Early stiffening is a premature
loss of workability or plasticity of cement paste, mortar, or concrete.
This includes both false and flash set.
False set (plaster set) is evidenced by a significant
loss of plasticity without the evolution of much heat shortly after
mixing. From a placing and handling standpoint, false-set tendencies
in cement will cause no difficulty if the concrete is simply mixed
for a longer period of time or remixed without additional water
before being transported or placed. False set occurs when too much
gypsum dehydrates in the cement mill forming too much plaster (some
plaster in the cement is desirable). This leads to stiffening due
to the rapid reformation of secondary gypsum with interlocking needle-like
crystals. Additional mixing without added water breaks up these
crystals to restore workability. Ettringite precipitation can also
contribute to false set.
Flash set (quick set) is evidenced by a rapid
and early loss of workability in paste, mortar, or concrete. It
is usually accompanied by the evolution of considerable heat resulting
primarily from the rapid reaction of aluminates. If the proper amount
or form of calcium sulfate is not available to control the calcium
aluminate hydration, rapid stiffening takes place. Flash set cannot
be dispelled, nor can the plasticity be regained by further mixing
without the addition of water.
Proper stiffening results from the careful balance of the sulfate
and aluminate compounds, as well as the temperature and fineness
of the materials (which control the chemical reaction rates).
A balance among the ions in plastic concrete is necessary to prevent
early stiffening. The tendency for early stiffening may therefore
be attributed not only to individual cementitious materials, but
also to interactions between the various cementitious materials
and chemical admixtures. For example, some Class C fly ashes contain
significant amounts of aluminate phases and may disturb the balance
because the cement sulfates may not be sufficient to account for
them, leading to early stiffening. Some chemical admixtures, particularly
Type A water reducers, may also disturb the ion balance, with the
same result.
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| Conduction Calorimeter |
Cements are tested for early stiffening using ASTM C451 (paste method)
(AASHTO T 186), and ASTM C359 (mortar method) (AASHTO T 185), which
use the penetration techniques of the Vicat apparatus. However, these
tests do not address all of the mixing, placing, temperature, and
field conditions that can cause early stiffening. To detect early
stiffening caused by interactions between concrete ingredients the
use of conduction calorimetry has been suggested (FHWA
2006).
Click here for an overview of
available tests to avoid early stiffening in the field.
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