Page 231 - Handbook of Thermal Analysis of Construction Materials

214 Chapter 5 - Accelerating Admixtures

exothermal heat effects that cannot be detected in the reference sample. In
Table 2, the setting times and strengths of the reference concrete (-7°C) are
compared with that containing the above admixture. The admixture devel-
ops better strengths than the reference and can be used in cold weather
concreting.

Table 2. Setting Time and Strength Development in a Newly Formulated
Admixture

Admixture Setting Times, Hrs Strength, MPa
Initial Final 1 d 3 d 28 d 1 yr

Air Entrained (Reference) 10.67 14.58 1.6 11.7 26.2 36.9
Air Entrained (Accelerator) 6.15 11.50 2.8 9.0 30.8 43.1
Non-Air Entrained + 5.50 11.50 3.6 8.7 34.2 46.3
Accelerator

Calcium nitrite has been used as a corrosion inhibitor in concrete.
It accelerates the hydration of cement. In Fig. 18, its acceleration effect on
Type V cement is obvious from the larger amount of heat developed
compared to that by the reference. [12] It appears to be a better accelerator
than calcium formate.
In cements, incorporation of calcium carbonate is permitted in
some countries. In Canada, the maximum limit is set at 5%. Calcium
carbonate is not an inert filler. It is known to react with calcium aluminate.
In a study of the hydration of tricalcium silicate in the presence of finely
divided calcium carbonate, Ramachandran [25] observed that the carbonate
acted as an accelerator. In Fig. 22, the conduction calorimetric curves
clearly show the accelerating influence of calcium carbonate. There was
evidence of formation of a complex of the carbonate with the hydrated
silicate. [25]
Alkalis such as NaOH, and Na salts of carbonate, aluminate, and
silicate are known to accelerate the hydration of cement and cause early
stiffening. In the hydration of tricalcium silicate with NaOH, there is an
acceleratory effect, as is evident from the conduction calorimetric curves