Page 188 - Handbook of Thermal Analysis of Construction Materials

Section 4.0 - Superplasticizers 171

Figure 11. Effect of sulfonated naphthalene formaldehyde on viscosity of cement paste.

In concrete, slump can be increased substantially by the addition of
superplasticizers. For example, a slump of 260 mm from an initial slump
value of 50 mm is obtained by an addition of 0.6% SMF; the same value is
realized with an addition of 0.4% SNF. The time when the superplasticizer
is added to concrete affects the slump value. By adding the admixture with
the mixing water, the slump is increased, but even higher values result if the
admixture is added a few minutes after the concrete is mixed with water.
Higher than normal workability of concrete containing a superplas-
ticizer is maintained for about 30–60 minutes, and then there is a rapid
decrease in slump, termed slump loss. To control or extend the workability,
the superplasticizer should be added at the point of discharge of concrete.
Some admixtures are added to superplasticizers to control slump loss. The
acrylate-based superplasticizers are claimed to possess good slump reten-
tion qualities.
The rate of hydration of cement and cement compounds is influ-
enced by superplasticizers. The reported data on the effect of superplasticizers
on the hydration of C A and C A + gypsum systems are contradictory
3
3
because of the variations in the materials and conditions of hydration.
There is a general consensus that SNF and SMF retard the hydration of