Page 226 - Handbook of Thermal Analysis of Construction Materials

208 Chapter 5 - Accelerating Admixtures

Conduction calorimetric curves of cement hydrated in the presence
of different amounts of TEA have also been obtained. [18] In Fig. 16,
conduction calorimetric curves of cement hydrated in the presence of 0, 0.1,
0.25, 0.35, and 0.5% TEA are given. Immediately on contact with water all
the samples evolved heat (not shown) that is caused by the heat of wetting,
the hydration of free lime, and the reaction of C A to form ettringite. The
3
magnitude of this peak increased with the amount of TEA added. This
indicates that TEA accelerates the formation of ettringite. The second peak,
occurring after 9–10 hours in cement without TEA is caused by the
hydration of C S. A small hump at about 22 hours denotes the formation of
3
low sulfoaluminate from the reaction between ettringite and excess C A. In
3
the curves of samples containing TEA dual peaks appear between 10 and
15 hours. One of them represents the ettringite-monosulfate reaction that is
accelerated by TEA. The other peak represents the hydration of C S,
3
triggered by the aluminate-gypsum reaction. This peak occurs at a later
period compared to that registered for C S (with no TEA). DTA curves
3
indicated that at 20 hours lower amounts of lime were formed in samples
with TEA. The setting times were in the range of 4.3–4.8 hours in samples
containing less than 0.05% TEA, but a rapid setting occurs (2–6 minutes)
at 0.1–0.5% TEA levels. This may be associated with the accelerated
formation of ettringite.
Calcium formate of formula Ca(HCOO) is a by-product in the
2
manufacture of polyhydric alcohol, pentaerthritol. It is a powder and has a
low solubility of about 15% in water at room temperature. It is a popular
non-chloride chemical that is advocated in practice. Many non-chloride
accelerating admixture formulations contain formates. Calcium formate is
an accelerator for the hydration of C S; at equal concentration, however,
3
CaCl is more effective in accelerating the hydration of C S (Fig. 17). [19]
2 3
The hydration of C S is also accelerated by calcium formate. The
2
increase in compressive strength of the formate-treated dicalcium
silicate samples in relation to the reference sample is evidence of the
acceleratory effect.
In the hydration of portland cement, although initially larger
amounts of heat are developed in the presence of Ca formate, at later ages
the heat may be slightly lower or equal to that of the reference material.
Calcium formate accelerates the hydration of all types of cement. Figure 18
gives the relative amounts of heat produced by adding 0.18 molar calcium
chloride, calcium nitrite, and calcium formate to Type V cement. Calcium
chloride is the best accelerator followed by calcium formate. [12]