Civil Engineering Reference
In-Depth Information
In studies above, the time of termination of the induction period is
taken to evaluate the relative setting times of cements containing retarders.
The setting times of cement mortars are determined according to ASTM
C403 using the Proctor needle. It is an arbitrary method and the time at
which the setting occurs may involve both physical and chemical
phenomena. Ramachandran
[33]
obtained the DSC curves of set mortars
containing different types of retarders with or without a superplasticizer
(Fig. 19). All of them exhibited endothermal peaks at about 100° and
450-500°C indicating the formation of Ca(OH)
2
at the time of set. Thus, it
is evident that at the time of set some hydration of C
3
S takes place. The
degree of hydration of C
3
S for the samples is different, depending on the
admixture used. It, therefore, appears that the morphological and other
factors also play a part in the setting phenomenon. It appears that the setting
time occurs at some point after the induction period has terminated, but
before the time at which the exothermic peak appears in the conduction
calorimetry.
8.0
SLUMP LOSS
Addition of a superplasticizer to concrete results in significantly
increased slump values. This increase is transient, however, and is gener-
ally not maintained beyond a period of 30-60 minutes. One method of
controlling this loss is to add a retarder in the superplasticized concrete.
Conduction calorimetric and different thermal methods may be adopted to
examine the relative retardation action of admixtures. Of the several
retarders studied, calcium gluconate was found to retard the hydration to a
maximum extent. In Fig. 20,
[33]
the conduction calorimetric curves show
that by the addition of 0.1-0.2% gluconate there is retardation of hydration
of cements. However, in combination with 0.1-0.2% sulfonated melamine
formaldehyde, there is increased retardation. It was found that by the
addition of 0.05-0.1% sodium gluconate, the slump loss can be decreased.
For example, with only gluconate added to concrete, the slump value is
about 40 mm at 2 hours, but, in combination with SMF, the slump is as high
a 140 mm
(Fig. 21).
[33]
