Page 397 - Handbook of Thermal Analysis of Construction Materials

374 Chapter 9 - Non-Portland Cement Binders and Concrete

Table 1. Compressive Strength Values of the Three OPC/CAC Pastes

Hydration Time (h) 92.5/7.5 (MPa) 80/20 (MPa) 20/80 (MPa)
1 — 0.7 —
2 — 0.8 —
4 — 1.2 3.5
8 0.7 1.7 25.5
16 4.5 2.8 36.1

21 5.8 2.7 36.8
24 8.2 2.7 43.8
48 16.2 2.6 40.7
72 23.5 10.7 46.7

The early strength of the binary paste systems is attributed to the
above reactions. [44] The OPC normally provides the sulfate and the CAC is
the primary source of aluminates. The amount of ettringite formed depends
on the available amount of sulfate and aluminates. In the 92.5/7.5 paste, the
small amount of ettringite formed does not contribute significantly to strength
of the paste (Table 1). It is apparent that the aluminates are the limiting
reactants. An estimate of the CAC/CS ratio is 0.78 (assuming a 4% gypsum
content in OPC and 40% aluminate content in CAC in the calculation) which
is much smaller than the value of 1.43 suggested as required to develop early
strength. [44]
In the 80/20 paste, the CAC/CS ratio is, however, about 2.50. Rapid
formation of a large amount of needle-like ettringite crystals occurs. This
contributes to the quick-set. The early strength is usually less than a few
MPa (Table 1). It is much lower than that contributed by C S, C S, and CA
3 2
hydration. It was also suggested by Bensted [45] that the fast formation of
CAH /C AH due to the CA/C A hydration may contribute to the early
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3
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2
strength of the binary paste system. The work referred to in Table 1 does not
seem to support Bensted’s suggestion. Significant early strength develop-
ment in the pure CAC paste or even in the 20/80 paste system does not occur
in spite of rapid formation of CAH /C AH .
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