Page 249 - Handbook of Thermal Analysis of Construction Materials

230 Chapter 6 - Retarding and Water Reducing Admixtures

2.6 Tricalcium Silicate-Tricalcium Aluminate-
Lignosulfonate-Water System

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