Table 1. ASTM C 494 admixtures.
Initial set time,
Category
Function
deviation from control
A
Water-reducing
1 hr earlier to 1.5 hr later
B
Retarding
13.5 hr later
C
Accelerating
13.5 hr earlier
D
Water-reducing and retarding
13.5 hr later
E
Water-reducing and accelerating
13.5 hr earlier
F
High-range water-reducing
1 hr earlier to 1.5 hr later
G
High-range water-reducing and retarding
13.5 hr later
None of the manufacturer's admixtures conforming to this standard contained added chlorides. Setting
times are based on tests conducted at 23C.
C 109 [ASTM 1989a]). The mortar intentionally was
of hydration. Their advantage in the cold is that the
made without an air-entraining admixture to avoid
earlier the cement consumes water, the earlier the con-
strength variations caused by variable air contents.
crete becomes immune to frost damage. Retarders slow
The mortar was mixed in a Hobart mixer according
down cement hydration, making them useful for off-
to ASTM C 305 (ASTM 1989b). All but the high-range
setting early stiffening caused by high temperatures or
water reducers were incorporated into the mixing
high accelerator doses.
water before being placed in the mixing bowl (high-
Mortar vs. concrete
range water reducers were added during the final minute
of mixing). The mixer was run at low speed while the
Instead of concrete, mortar was used to evaluate the
cement was added to the water for 30 seconds. Mixing
performance of each admixture. Mortar simplified mix-
was stopped, the sides of the bowl were scraped down,
ing operations, reduced material handling procedures,
and permitted smaller test specimens. To the admix-
and mixing ran for another 45 seconds while sand was
added. The mixer was then stopped for a minute and a
ture, mortar acts like concrete. It has the same cement
half before running at medium speed for the final
and water to react with, similar void spaces to fill, and
minute.
the familiar aggregate-to-paste transition zones to deal
with. The only difference between mortar and concrete
Test procedure
is that mortar does not contain coarse aggregate where
Five commercial admixtures plus calcium chloride
concrete does.
were tested for their effects on fresh and hardened prop-
The mortar consisted of one part Type I portland
erties of mortar. All tests were compared to control
cement (Table 2) to 2.8 parts ASTM C 33 (ASTM
mortar containing no admixtures and all mixing took
1997a) sand on a weight basis. The cement-to-sand ra-
place at room temperature.
tio used in this study is not a ratio typical to concrete
On the fresh mortar, each admixture was evaluated
but it is a ratio commonly used in mortar studies (ASTM
for its effect on flow (workability), setting time (early-
age hydration rate), and freezing point depression (frost
Table 2. Chemical composition of Dragon
resistance). The commercial admixtures were tested at
Products Co. Type I cement.
their lowest and highest doses recommended by the
manufacturer, while the calcium chloride was tested at
Weight
its maximum allowable dose (2% by weight of cement).
Compound
percent
Flow tests were conducted according to ASTM C 109
(ASTM 1989a), setting times according to ASTM C
Silicon dioxide
20.6
Aluminum dioxide
4.9
403 (ASTM 1981), and freezing points were obtained
Ferric oxide
2.4
from a thermocouple embedded into a dummy cylin-
Calcium oxide
63
der of fresh mortar placed in a coldroom.
Magnesium oxide
2.8
Once each admixture was characterized for its ef-
Sulphur trioxide
3.3
fect on fresh mortar, it was combined in various ways
LOI
1.71
Equivalent sodium oxide
1.32
to improve the ability of mortar to gain strength at tem-
2
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