Page 310 - Handbook of Thermal Analysis of Construction Materials

Section 6.0 - Thermal Analysis of Superplasticizers 287

The physico-chemical phenomena associated with the setting of
cement is complex. The time of setting is defined in ASTM in terms of the
penetration of a needle to a specific depth. Ramachandran [36] examined by
DSC several cement samples containing water reducers, retarders, and
superplasticizers that had just set. In all the mixes, an endothermal peak
typical of Ca(OH) was detected, suggesting that at the time of set at least
2
some C S must hydrate. It was also observed that the superplasticized
3
cement paste had hydrated to a lesser extent than the reference cement paste
at the time of setting. This indicates that physical forces are also involved
in the setting phenomenon.

6.0 THERMAL ANALYSIS OF
SUPERPLASTICIZERS

The analysis of superplasticizers is not easy as the manufactured
product may not be pure. Techniques such as IR and UV spectroscopy yield
useful, but not adequate, data so that analysis by other tools becomes
necessary. Thermal analysis methods have been applied with some success
to evaluate superplasticizers. The results could also be used for quality
control purposes.
Milestone [38] carried out DTA of superplasticizers and other ad-
mixtures in an atmosphere of oxygen and obtained several thermal inflec-
tions that could be used for differentiation of superplasticizers of similar or
dissimilar types. In one example, two superplasticizers, thought to be
identical, yielded different DTA patterns. In Fig. 20, two superplasticizers,
apparently identical, show different patterns.
DTG/TG has been used to differentiate various types of admixtures
including superplasticizers. [39] It was found that maximum weight loss
occurred at 400°C for the sodium SMF admixture. Sodium SNF showed
maximum weight loss at about 900°C. Six industrial superplasticizers (5
SNF and 1 SMF type) were subjected to DTG/TG investigations and
thermal stabilities were found to be different so that the data could be used
as finger prints for identification purposes. [40]
Evolved Gas Analysis (EGA) technique has also been applied to
investigate superplasticizers. [41] In this technique, the admixture is heated
at a constant rate and the evolved gas, such as water, is measured. In water-
containing compounds, water is adsorbed and measured coulometrically. In