Page 398 - Handbook of Thermal Analysis of Construction Materials

376 Chapter 9 - Non-Portland Cement Binders and Concrete

Particulates in the form of different calcium aluminate hydrates,
e.g., CAH , C AH , and C AH , or pre-hydrated CAC/CH (H-CAC/CH)
2
6
3
8
10
have varying effects on ettringite formation and expansion of expansive
cement pastes.
Mixtures containing different calcium aluminate hydrates can be
prepared by hydrating calcium aluminate cement (CAC) at different tem-
peratures (Table 2). Either CAH , C AH , or C AH is the major compo-
2
10
6
3
8
nent for each different mixture. The major component of the H-CAC/CH
mixture also is C AH . Addition of CH to CAC appeared to promote C AH 6
3
3
6
formation. The use of calcium aluminate mixtures prepared from CAC
hydration rather than pure synthetic calcium aluminates is based on practical
considerations.
Table 2. Preparation Conditions of Calcium Aluminate Hydrate Mixtures
Name of Curing Main Mineral Moisture
Mixtures Temperature (°C) Component State

M-CAH 0 CAH Paste
10 10
M-C AH 23 C AH Paste
2 8 2 8
M-C AH * 38 C AH 6 Dry Powder
3
6
3
H-CAC/CH 23 C AH 6 Paste
3
*Cured at 38°C for 2 months followed by 24 hours drying at 85°C.

The effect of different calcium aluminate hydrates on free expansion
is shown in Fig. 7. It is apparent that the expansion of expansive cement
pastes containing the CAH or C AH mixture started at very early ages
10 2 8
(about 1 hour), and developed mainly in the first 3 hours. Their 3-hour
expansion and ultimate expansion values are 0.58 and 0.65% respectively.
The expansion of specimens containing additives based on anhydrous CAC
and H-CAC/CH mixture developed after 3 hours. The former had the largest
ultimate expansion (around 0.8%) at about 10 days; the latter a much lower
expansion than the cement pastes containing expansive additives based on
CAH or C AH mixtures. Its ultimate expansion was about 0.45%. The
2
10
8
cement paste containing the expansive additive based on the C AH
3 6
mixture showed very delayed and lower expansion compared to the other
samples.