Page 443 - Handbook of Thermal Analysis of Construction Materials

Section 2.0 - Calcium Aluminate Cements 419

active cement components. The parameters which affect strength, e.g.,
degrees of hydration (D ), transformation (D ), and carbonation (D ) can be
t
c
h
estimated from TG curves.
The transformation (conversion) and carbonation processes are
considered to be diffusion controlled first order reactions with rate con-
stants k = 0.48 and k = 0.007. One unit of conversion results in a 0.57
c carb
fold increase in porosity and a corresponding unit of carbonation, 0.18
times decrease in porosity. Therefore, the change in porosity (∆P) due to
these processes can be expressed as follows:

Eq. (5) ∆P = 0.57 (1 – D ) e -0.46t – 0.18 (1 – D ) e -0.007t
t
c
where t is the time in years and D and D are as previously defined.
c
t
The hydration of CA, CA , and C A after different days of
2 12 7
hydration was reported by Das, et al. [10] They used DSC methods in their
analysis. The CA calorimetry peaks were similar to those described
previously (Fig. 15a).
Endothermic peaks at 175° and 300°C were attributed to the
dehydration of CAH and AH gel, respectively. The peak observed at
10
3
275°C (1 day) may be due to the formation of a complex C AH phase.
2 8
C AH formed after 28 days as indicated by the presence of a peak at 230°C.
2
8
The peak at 320°C denotes the dehydration of C AH (significant at 7 days
3 6
of hydration). XRD results (dehydration of AH gel and gibbsite transfor-
3
mation occur at about the same temperature) reveal the formation of
crystalline AH and the C AH phases after 28 days hydration. Dehydration
6
3
3
of AH and C AH occur at very close temperatures, and the DSC peaks
6
3
3
-1
overlap (particularly with a slow scanning rate, e.g., 10°C min ). These two
dehydration peaks are well separated in DSC at high scanning rates (Fig.
16). The activation energy, E for the dehydration of AH and C AH have
a
3
6
3
been calculated using Kissinger’s relation. [11]
V m )
( E /
Eq. (6) 2 = exp − a RT m
T
m
where E = activation energy, V = scanning rate, T = peak transition
m
a
temperature, and R = gas constant. Estimates of E for the dehydration of
a
-1
-1
AH and C AH are 107.16 kJ mol and 35.58 kJ mol , respectively.
3
3
6