Page 386 - Handbook of Thermal Analysis of Construction Materials

364 Chapter 9 - Non-Portland Cement Binders and Concrete

CAC of structural quality can be made even if complete conversion
occurs provided the w/c ratio is sufficiently low. At low w/c ratios, the water
released during the conversion reacts with the anhydrous grains not utilized
in the initial hydration reaction, and fills the pores. A major factor preventing
loss in strength is the grain size of the converted minerals, viz., C AH and
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AH . At low w/c ratios, the low porosity is maintained by the packing of the
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small crystallites while at higher w/c ratios, the porosity is increased by the
larger crystals. [24]
It is also suggested that the cubic hydrate (C AH )does not have a
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binding capacity. For example, the strengths of the aluminate hydrates are
thought to be in the order: CAH > C AH > C AH .
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Work by Ramachandran and Feldman [25] has indicated that the
conversion of C A to C AH under certain conditions, in fact, enhances
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strength. Using high compaction methods, they obtained calcium
monoaluminate discs for which the effective water/aluminate ratio for
hydration was 0.15. They found that, compared with a microhardness value
of 195 MPa for the unhydrated compacted CA sample, those hydrated at 20
or 80°C for 2 days developed hardness of 1074 and 1574 MPa respec-
tively. [26] These results show that neither the high temperature nor the
formation of the cubic phase is detrimental to strength development. At
80°C, the reaction proceeds very rapidly and accelerates the conversion of
CA to C AH and AH phases. As the particles of CA in the compact lie very
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close to each other, direct bond formation between C AH products is
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enhanced. At 20°C, however, direct bond formation due to C AH may not
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be favored, as C AH and AH products are transported and recrystallized
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in the pores by the initial formation of hexagonal phases.
CAC may contain appreciable amounts of calcium aluminoferrite. [27]
The contribution of this compound becomes important if CAC is made at
low w/c ratios. Ramachandran and Beaudoin [28] investigated the physico-
mechanical characteristics of C AF prepared at w/s ratios between 0.08 and
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1.0 and hydrated for various periods at temperatures of 23 or 80°C or
autoclaved at 216°C. In pastes hydrated at 23°C, with w/s ratios of 0.3–1.0,
it was found that the lower the w/s ratio, the higher the microhardness
values: 373 and 59 MPa, respectively, at a w/s ratio of 0.3 and 0.5.
Significant increases in microhardness were observed in samples hydrated
at an effective w/s ratio of 0.08. Unhydrated, pressed C AF had a value of
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314 MPa; that hydrated at 23 and 80°C had values of 1128 and 1933 MPa
respectively. A few samples prehydrated at 23 or 80°C were autoclaved at
216°C. The unhydrated pressed C AF sample and the two prehydrated at 23
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and 80°C, having initial microhardness values of 314, 1128, and 1933 MPa,