Page 307 - Handbook of Thermal Analysis of Construction Materials

284 Chapter 7 - Superplasticizing Admixtures

The effectiveness of a superplasticizer may also depend on its
degree of poly-condensation. Vovk, et al., prepared a sodium SNF type
superplasticizer of polycondensation degree (n) from 2 to 17 using liquid
chromatography. [17] Table 6 gives the heat developments in cements
containing a polymer with n varying between 2 and 17. The higher the
degree of polycondensation, the greater the extent to which it will combine
with the hydration products of Al-containing products. In Table 6, as n
increases from 2 to 10, the value of the main exothermic peak intensity (C S
3
hydration peak) decreases, and it occurs at later times. With further increase
in the value of n (17), the thermal peak is enhanced, and the peak appears
much earlier.

Table 6. Heat Evolution in Cement Containing SNF of Different Degrees
of Polycondensation

n dQ/dt Induction Period Peak Appearance
(hrs) (hrs)

- 1.6 3 12
2 1.0 6 20
5 0.9 10 35
10 0.7 35 65
17 1.2 26 40

In another investigation, Basile, et al., [37] added 0.4% SNF of
average molecular weights 260, 290, 480, and 640 to cement and followed
by DTG the amounts of calcium hydroxide formed at 8 hours and beyond.
The results are tabulated in Table 7. The minislump results showed that the
fluidizing effect increased as the molecular weight of SNF was increased.
The superplasticizers retarded the hydration from 8 hours to 1 day. At 7 and
28 days, the amounts of Ca(OH) formed increased in the presence of
2
superplasticizers. At higher molecular weights, more Ca(OH) seems to
2
have formed.