Page 240 - Handbook of Thermal Analysis of Construction Materials

222 Chapter 6 - Retarding and Water Reducing Admixtures

2.0 LIGNOSULFONATES

The lignosulfonate-based admixtures have been used more widely
than other water reducers. They are capable of reducing water requirements
and retarding the setting times of concrete. They influence the dispersion
and the hydration rate of the individual cement compounds, and, thus, the
cement itself. Techniques such as XRD, DTA, DSC, TG, DTG, and
conduction calorimetry have been used extensively to follow the hydration
of cement and cement compounds containing different types and amounts
of lignosulfonates (LS).

2.1 Tricalcium Aluminate

Tricalcium aluminate hydrates to form, initially, hexagonal phases
identifiable by endothermal effects in DTA at 150–200°C and 200–280°C.
These are converted to a cubic phase of formula C AH which exhibits
3 6
endothermal effects at 300–350°C and 450–500°C. The addition of ligno-
sulfonate influences the rate of formation of these phases and their inter-
conversions to the cubic phase. Depending on the amount of lignosulfonate,
the hexagonal phase may be stabilized even up to fourteen days or more with
lignosulfonate, but in that hydrated without the admixture, the cubic form
may appear at six hours or earlier (Fig. 1). [1]
The mechanism by which the hexagonal phase of calcium alumi-
nate is stabilized by calcium lignosulfonate (CLS) can be examined by
treating this phase with lignosulfonate and subjecting it to DTA studies.
Adsorption-desorption studies have indicated that the hexagonal phase
adsorbs lignosulfonate irreversibly. There is evidence that lignosulfonate
enters the interlayer positions of the hexagonal phase. Figure 2 shows the
thermograms of the hexagonal hydrate containing CLS and that of CLS
[2]
itself. An intense exothermic peak at about 360°C is common to CLS and
the hexagonal phase containing CLS, but is reduced to a small hump in
vacuum DTA. The important difference is that an exothermic peak appear-
ing at 730°C in the sample treated with CLS is absent in the CLS sample.
This exothermic peak signifies oxidation of the strongly bound CLS in the
interlayer positions. The weight loss curves obtained through TG investi-
gation also show evidence of the formation of a complex between the
hexagonal phase and CLS.