CHAPTER 2. RAW MATERIALS AND EXPERIMENTAL TECHNIQUES
3.4. Reaction Products Formed In The Calcium Oxide-Activated Fly Ash Binder With The Calcium
Figure 16 presents the XRD patterns of the CaCl2-CaO-activated fly ash pastes with a w/b = 0.4 at 3 and 28 days after being treated by the solvent-exchange and drying procedure. In all samples, quartz (SiO2, ICDD PDF-2 no. 98-008-9277), mullite (3Al2O3∙2SiO2, ICDD PDF-2 no. 98-004-3298), and calcite (CaCO3, ICDD PDF-2 no. 98-004-0107) were found in common. Among these phases, quartz and mullite existed in the raw fly ash (see Figure 13), while calcite was newly formed, possibly due to carbonation. When CaCl2 was present, hydrocalumite (Ca2Al(OH)6Cl·2H2O) was newly synthesized, while Ca(OH)2 mostly disappeared.
Compressive strength(MPa)
w/b=0.4 w/b=0.5 w/b=0.6 w/b=0.7 w/b=0.8 w/b=0.9 28 days
CC0 CC5 CC10 CC15 Sample group
0 5 10 15 20 25 30 35 40 45 50
28
(a) (b)
Figure 16. XRD patterns of the samples, after treatment using a drying procedure, with a w/b = 0.4 at (a) 3 days, and (b) 28 days.; HC: hydrocalumite, M: mullite, Q: quartz, CH: calcium hydroxide, C: calcite.
It is worth noting that Ca(OH)2, which was generated from the hydration of CaO, was present in the CC0-0.4 until 28 days, while not being found at all in the samples containing CaCl2 at 3 and 28 days [see ▲CH with the dashed line in Figure 16]. The consumption of Ca(OH)2 was frequently reported to be the main cause of the strength improvement of the Ca(OH)2-activated fly ash binder with additional activators [71, 76]. When Na2CO3 was incorporated in the Ca(OH)2-fly ash paste, Ca(OH)2
was completely consumed in a chemical reaction within the first 3 days, resulting in a notably greater strength than that of the sample without Na2CO3 [71]. In this study, the incorporation of CaCl2 into the CaO-fly ash paste also led to the rapid consumption of Ca(OH)2. Although the atmospheric carbonation would cause the consumption of Ca(OH)2 to some extent, the whole depletion of Ca(OH)2 in the samples with CaCl2 was likely due to two following main reasons: (1) the formation of hydrocalumite
10 20 30 40 50 60
Quartz (98-008-9277)
Mullite (98-004-3298)
Hydrocalumite (98-005-1890)
Ca(OH)2(98-007-3468)
Calcite (98-004-0107)
CC0-0.4
3 days
C
▲CH
■HC
Q ▲CH
M
2 Theta degree (Cu-Ka1)
▲CH
CC5-0.4
CC10-0.4 CC15-0.4
Quartz (98-008-9277)
Mullite (98-004-3298)
Hydrocalumite (98-005-1890)
Ca(OH)2(98-007-3468)
Calcite (98-004-0107)
10 20 30 40 50 60
28 days
Q
M ▲CH
▲CH
C
2 Theta degree (Cu-Ka1)
▲CH
■HC
CC0-0.4
CC5-0.4 CC10-0.4 CC15-0.4
29
and (2) the C-S-H formation from the enhanced pozzolanic reaction between amorphous phase of fly ash and Ca(OH)2. First, hydrocalumite was only identified in the samples with CaCl2, while not in the sample without CaCl2 [see ■ HC in Figure 16]. Given that, in the samples with CaCl2, Ca(OH)2
completely disappeared, and the hydrocalumite formation might be closely related to the consumption of Ca(OH)2. Second, it is clear that the Ca(OH)2-consuming pozzolanic reaction was boosted in the samples containing CaCl2 given that these samples produced much higher strengths than did the CC0- 0.4 (see Figure 15). This second argument would be supported by Figure 17, which presents the overlapped XRD patterns of raw fly ash, CC0-0.4, and CC5-0.4 at 28 days, and its enlargement for better visibility. By comparing these patterns, two observations were obtained: (1) the amorphous hump at 17°–27° from fly ash decreased due to the CaCl2 incorporation, and (2) the C-S-H peak near 29°–33°
[49, 96] largely increased in the presence of CaCl2. Given that the amorphous phase of fly ash is dissolved by OH- in a high pH environment [88, 97], these should be related to the pozzolanic reaction of the amorphous phase of fly ash with Ca(OH)2 as the C-S-H peaks simultaneously increased with the reduction of the amorphous hump. Thus, the incorporation of CaCl2 in the CaO-activated fly ash system likely led to more formation of C-S-H. Note that the compressive strengths of the samples with CaCl2 were much higher than that of the sample without CaCl2 (see Figure 15). Hence, the XRD results support the compressive strength results.
Figure 17. Influence of CaCl2 on the amorphous hump at 17°–27° (2θ) and C-S-H peaks at 29°–
33° (2θ) in the XRD results, which were measured for the samples with a drying process, at 28 days.
Figure 18 shows the XRD results for the samples at 3 days, which were prepared without any drying procedure. Earlier studies [93, 98, 99] were used to identify calcium oxychloride in this study
10 20 30 40 50 60 13 20 30 34
40 50 60
(1) Decrease in glassy hump
(2) Formation of C-S-H hump Raw
fly ash
CC5-0.4 CC0-0.4
28 days
2 Theta degree (Cu-Ka1) 2 Theta degree (Cu-Ka1) A: Raw fly ash
B: CC0-0.4 C: CC5-0.4
A
B C
B C
A
30
because there was no reference pattern for calcium oxychloride in the ICDD PDF-2 database. Calcium oxychloride (denoted CAOXY) only appeared when the content of CaCl2 was used over 10 wt% (see broken lines in Figure 18).
In Figure 18, it is worth noting that, with an increasing CaCl2 content, while the reflections of hydrocalumite were not changed in the peak intensity, those of calcium oxychloride became stronger.
This implies that, when CaCl2 was used over 10 wt%, the CaCl2 might be mostly used for synthesizing calcium oxychloride rather than hydrocalumite. Interestingly, the CC10-0.4 and CC15-0.4 at 3 days showed more distinct calcite peaks than those of the CC5-0.4 at 3 days (see peaks at 29.5° with arrows in Figure 16 (a)]. The decomposition of calcium oxychloride was reported to yield calcite [99]; thus, some portion of early calcite formation in the CC10-0.4 and CC15-0.4 samples at 3 days was likely related to the presence of calcium oxychloride.
Figure 18. RD patterns of the samples at 3 days without a drying procedure; HC:
hydrocalumite, CAOXY: calcium oxychloride.
The TG curves with differential thermogravimetry (DTG) curves, a differential form of TG, at 28 days are presented in Figure 19. The TG results confirmed the formation of C-S-H, hydrocalumite, Ca(OH)2, and calcite. Unlike CC0-0.4, the samples with CaCl2 showed significant weight loss below 160°C, which is attributed to the dehydration of both C-S-H and hydrocalumite [70, 77, 100, 101].
Increasing CaCl2content CAOXY
10 20 30 40 50 60
2 Theta degree (Cu-Ka1)
CC0-0.4 without drying 3 days
2 Theta degree (Cu-Ka1) CC5-0.4
without drying
CC10-0.4 without drying
CC15-0.4 without drying
12
10 11 10 11 12 10 11 12 10 11 12
CC15-0.4
HC HC HC
CC10-0.4 CC5-0.4
CC0-0.4
CAOXY
CAOXY 10 wt%
0 wt% 5 wt% 15 wt%
31
Hydrocalumite is known to undergo three-step thermal decompositions in TG: (1) dehydration below 120°C, (2) dehydroxylation around 290°C, and (3) anion decomposition above 670°C [100-102].
Although, below 160°C, the weight losses of the hydrocalumite and C-S-H overlapped with each other, the weight loss in 237–360°C was only attributed to the dehydroxylation of hydrocalumite. The samples containing CaCl2 distinctively exhibited the hydrocalumite DTG peaks around 290°C, while the CC0- 0.4 did not show any hydrocalumite peak, consistent with the XRD results. The large DTG peak around 450°C, which was the consequence of the decomposition of Ca(OH)2, was only found in CC0-0.4; this indicates that the presence of CaCl2 might accelerate the consumption of Ca(OH)2. The other DTG peaks above 600 °C were related to the decomposition of calcite and hydrocalumite [70, 101]. One might conjecture that filling of pores by the calcite formation could lead some strength improvement in this study; however, it should be noted that the TG and compressive strength results indicated that although the sample without CaCl2 produced more calcite than the other samples, it gained lower strengths. Thus, the filling of pores with calcite was not likely the cause for the strength gain in this study.
Figure 19.TG and DTG results for the samples with a w/b = 0.4 at 28 days.
0.00 0.02 0.04 0.06 0.08 0.10 0.12 0.14
70 75 80 85 90 95 100
0 100 200 300 400 500 600 700 800 900 1000
Deriv. Weight (%/°C)
Weight (%)
28 days Hydrocalumite
C-S-H & Hydrocalumite
Ca(OH)2
CC10-0.4 CC15-0.4 CC0-0.4 CC5-0.4 Calcite & Hydrocalumite
Temperature (°C) DTG
curves
TG curves
32
The weight losses, related to dehydration (i.e., at 60–160°C) and dehydroxylation (i.e., at 237–
360°C) of hydrocalumite, in TG at 28 days are presented in Figure 20. The weight losses for the dehydroxylation of hydrocalumite were quite similar among the samples containing CaCl2, even though the weight loss slightly increased with increasing CaCl2 content (see Figure 20 (a)). Therefore, the quantities of hydrocalumite are likely quite similar between the samples with CaCl2. This is consistent with the XRD results shown in Figure 18, which showed that the peak intensities for hydrocalumite were similar among the samples despite the higher CaCl2 content.
Figure 20 (b) shows the weight losses in 60–160 °C, which are due to the decomposition of both C-S-H and hydrocalumite. The weight loss in 60–160 °C clearly increased as the CaCl2 content increased. Thus, given that hydrocalumite showed only a slight difference in its formed quantity between the samples with CaCl2, Figure 20 (b) demonstrates that the weight loss of C-S-H followed the order: CC15-0.4 (largest) > CC10-0.4 > CC5-0.4 > CC0-0.4 (smallest), indicating that the samples with more CaCl2 produced more C-S-H phase. However, considering that C-S-H is the primary reaction product determining the strength, this TG result might appear inconsistent with the strength testing results because the greatest strength was obtained at a 5 wt% CaCl2 content. Thus, this inconsistency suggests the presence of an additional strong factor for strength determination in the CaO-CaCl2-fly ash system, which is discussed in Section 3.6.
(a) (b)
Figure 20. Weight losses in TG at 28 days: (a) at 237–360 °C (dehydroxylation of hydrocalumite) and (b) at 60–160 °C (dehydration of hydrocalumite and C-S-H).
1.0%
2.9% 3.0% 3.2%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
6.0%
7.0%
8.0%
9.0%
10.0%
Weight loss in TG
Weight losses related to hydrocalumite
at 237-360°C
2.2%
5.3%
6.1%
7.4%
0.0%
1.0%
2.0%
3.0%
4.0%
5.0%
6.0%
7.0%
8.0%
9.0%
10.0%
Weight losses related to C-S-H and hydrocalumite
at 60-160°C
Weight loss in TG
33
3.5. Dissolution Of Fly Ash In Aqueous Phase Of The Calcium Oxide-Activated Fly Ash Binder