Page 230 - Handbook of Thermal Analysis of Construction Materials

212 Chapter 5 - Accelerating Admixtures

A comparison of the DTA curves of C S containing Ca nitrate,
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thiosulfate, and chloride shows that in terms of Ca(OH) formed at 7 hours
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all the compounds act as accelerators; calcium chloride, however, is the best
accelerator (Fig. 20). [21]

Figure 20. DTA curves of the hydration products of C S in the presence of calcium salts.
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In cold weather concreting, the rate of hydration of cement is very
slow. Attempts have been made to develop so-called antifreezing admix-
tures in which accelerators and a freezing point depressant are used.
Thiocyanates are potential accelerators that may be used in these formula-
tions. A systematic work has been carried out to determine the effect of
various thiocyanates such as Na, K, NH , Ca, and Li thiocyanates at dosages
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of 1.5 and 3%, on the hydration of cement cured at 20, 0, and -5°C. Both
conduction calorimetry and TG techniques were used. [22] All the thiocyan-
ates increased the rate of hydration of cement at all temperatures. The most
efficient early acceleration, as well as strength development, occurs with
3% Ca thiocyanate. A linear relationship exists between the amount of lime
formed and strength within a range of values (Fig. 21). The lines for the
paste cured at -5°C can be broadly divided into three groups. Pastes
containing 1.5% Na, Li, and K thiocyanate exhibit better strengths than the
reference material at lime contents greater than 3%. Best strengths are