Civil Engineering Reference
In-Depth Information
2.6
Tricalcium Silicate-Tricalcium Aluminate-
Lignosulfonate-Water System
Adsorption of lignosulfonate on C
3
A during hydration is much
higher than that on the hydrating C
3
S. In the system C
3
A-C
3
S-H
2
O, C
3
A
may act as a sink for lignosulfonate.
[14]
Ramachandran
[16]
studied the
effect of CLS on the hydration of C
3
S containing C
3
A, hexagonal
aluminate hydrate, and cubic aluminate hydrate. The degree of hydration
of C
3
S was monitored by determining the DTA peak area due to Ca(OH)
2
decomposition.
The effect of the addition of CLS on the hydration of C
3
S in the
system C
3
S-C
3
A-H
2
O can be demonstrated with reference to Fig. 7.
[15][16]
The rate of hydration is estimated by the amount of Ca(OH)
2
formation.
Curve 1 refers to the hydration of C
3
S with water. By the addition of 0.8%
calcium lignosulfonate (curve 2), the hydration is completely inhibited
even up to thirty days, as evident from the absence of Ca(OH)
2
. When 5%
C
3
A is added to the mixture of C
3
S and lignosulfonate, curve 3 results. The
hydration of C
3
S is retarded owing to the CLS, but not completely
inhibited as in curve 2 because most of the admixture is consumed by the
hydrating C
3
A. Curve 4, in which C
3
S and C
3
A are hydrated for five minutes
prior to the addition of CLS, shows a much higher retardation of the C
3
S
hydration compared to curve 3. This shows that the pre-hydrated
mixtures adsorb less lignosulfonate and allow greater adsorption to take
place on C
3
S. At larger additions of the retarder (curve 5), even though C
3
A
adsorbs initially a large amount of the admixture, larger concentrations
are still present to effectively retard C
3
S hydration for a long time
(compare curves 5 and 3).
The combined effect of Na-lignosulfonate (0.3%) and Na
2
CO
3
in
the hydration of C
3
S (80%) + C
3
A (20%) is approximately similar to that
in the presence of lignosulfonate except for complete blocking of C
3
S and
C
3
A hydration for at least thirty minutes (Fig. 8).
[14][17]
At higher dosages
of both lignosulfonate and carbonate (0.9%), an induction period of one day
occurs.
[17]
The influence of other organic water reducers, such as glucose
and sodium gluconate at additions of 0.1-0.3% on the C
3
S (80%) + C
3
A
(20%) system appears to be similar to that of lignosulfonate except for a
better retarding action of Na-gluconate.
[14]
