CHAPTER 2. RAW MATERIALS AND EXPERIMENTAL TECHNIQUES
3.2. Raw Material Characterization and Experimental Details for Investigation of Calcium Oxide-
The raw fly ash was collected from the Ha-dong power plant in South Korea. The chemical composition of the fly ash is given in Table 3, which was determined by an X-ray fluorescence spectrometer (S8 Tiger; Bruker, Germany). This fly ash was Class F type, as it contained 87.5% of SiO2 + Al2O3 + Fe2O3 according to the ASTM C 618 [83].
Table 3: Chemical compositions of raw fly ash
Formula Oxide weight %
SiO2 56.8%
Al2O3 23.3%
Fe2O3 7.4%
CaO 5.0%
K2O 1.7%
MgO 1.5%
TiO2 1.4%
Na2O 0.9%
SO3 0.9%
P2O5 0.6%
BaO 0.1%
SrO 0.1%
ZrO2 0.1%
22
MnO 0.1%
Powder X-ray diffraction (XRD) (D8 Advance; Bruker AXS, Germany) was conducted on the fly ash with an incident beam of Cu-Ka radiation ( = 1.5418 Å) for a 2θ scanning range of 8–60°. The X’pert High-Score Plus program [84] and an International Center for Diffraction Data (ICDD) PDF-2 database [85] were used for the XRD analysis. Corundum (NIST RMS 676a, crystalline alumina 99.02 %
± 1.11 %) was added as an internal standard [86]. The results showed that the raw fly ash contained crystalline quartz, mullite, and magnetite, as shown in Figure 13. The weight percentages of amorphous and crystalline phases of fly ash are shown in Table 4; the raw fly ash had a relatively low weight content of the amorphous phase compared to general fly ashes in South Korea [87]. The amorphous content of Class F fly ash can vary largely from 60% to 90% as it depends on source of coal or burning procedure in South Korea [25, 88].
Figure 13: The XRD patterns of raw fly ash.
Table 4: The contents of the crystalline and amorphous phases in fly ash.
Phase Content (%)
Quartz 19.0
Mullite 21.2
Magnetite 1.6
Amorphous 58.2
Total sum 100.0
10 20 30 40 50 60
Raw fly ash
Mullite (98-009-9328)
Quartz (98-004-2498)
Corundum (98-002-4851)
Magnetite (98-007-7590)
2 Theta degree (Cu-Ka1)
23
The particle size distribution of the fly ash was estimated using a laser diffraction particle size analyzer (HELOS, Sympatec, Germany) with a RODOS dispersing unit (see Figure 14). The fly ash consisted of more than 10% of large particles over 100 μm and median particle size of the fly ash was 23.27 μm.
Figure 14: Particle size distribution of the raw fly ash
The mixture proportions are shown in Table 5. The fly ash was replaced with CaCl2 at 0, 5, 10, and 15 wt% in the total weight of binder to find the optimal CaCl2 content for strength improvement, but the CaO content was fixed at 20 wt%. CaCl2 was not an additive chemical in this study, but rather a replacement unlike the role of CaCl2 for the portland cement because we developed a new cementless binder having fly ash, CaO, and CaCl2 as main material components. In addition, previous studies have reported that the mixture of CaCl2, Ca(OH)2, and H2O may generate calcium oxychloride phase (3Ca(OH)2∙CaCl2∙12H2O) and the formation of calcium oxychloride is largely affected by the weight ratios among CaCl2, H2O, and Ca(OH)2 [89, 90]. Thus, in order to investigate the formation of calcium oxychloride with varying CaCl2 content, we fixed the weight ratio of H2O and CaO in the samples with an identical w/b. The water-to-binder weight ratio (w/b) of samples varied from 0.4 to 0.9. The fresh CaCl2-CaO-fly ash paste retained a suitable consistency without segregation in the entire range of the w/b in this study.
0 10 20 30 40 50 60 70 80 90 100
0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1
0 1 10 100 1,000
Cumulative distribution (Volume %)
Densitydistribution (log.)
Particle size (μm)
Cumulative distribution Density distribution (log.)
Median particle size = 23.27 μm
24 Table 5. Mixture proportions of paste samples
Group Label Binder (wt%)
Fly ash CaO CaCl2 Total w/b
CC0
CC0-0.4
80 20 0 100
0.4
CC0-0.5 0.5
CC0-0.6 0.6
CC0-0.7 0.7
CC0-0.8 0.8
CC0-0.9 0.9
CC5
CC5-0.4
75 20 5 100
0.4
CC5-0.5 0.5
CC5-0.6 0.6
CC5-0.7 0.7
CC5-0.8 0.8
CC5-0.9 0.9
CC10
CC10-0.4
70 20 10 100
0.4
CC10-0.5 0.5
CC10-0.6 0.6
CC10-0.7 0.7
CC10-0.8 0.8
CC10-0.9 0.9
CC15
CC15-0.4
65 20 15 100
0.4
CC15-0.5 0.5
CC15-0.6 0.6
CC15-0.7 0.7
CC15-0.8 0.8
CC15-0.9 0.9
The fly ash and CaO powders were fully dry-mixed with CaCl2 pellets and then mixed with de- ionized water for 5 min [91]. Note that we used newly purchased CaO and CaCl2 (extra pure, 98%) immediately after unpacking, and the relative humidity in the laboratory was quite low as we use a dehumidifier in the laboratory; thus, potential contact of the chemicals with moisture was likely very low. Three identical samples for each mixture proportion were cast in brass cubic (50 × 50 × 50 mm) molds to measure compressive strength. The samples were cured under 60 °C with 99% relative humidity for 3 and 28 days.
Compressive strength testing was performed on the triplicate samples of each mixture. After testing, fractured specimens were collected to prepare powder samples for the XRD and TG analyses. The finely ground powder samples were solvent-exchanged using acetone for 14 days to stop further hydration.
All samples were completely dried out in an ~60 cmHg vacuum desiccator for 3 days to remove any residual water and solvent [41].
Previous studies have noted that the calcium oxychloride phase (3Ca(OH)2∙CaCl2∙12H2O) is easily decomposed in a drying condition [92, 93]. Monosi et al. [93] reported that the calcium oxychloride
25
phase disappeared in the ground sample, washed with methyl alcohol. Thus, to identify whether calcium oxychloride formed in this study, additional powder samples were prepared without any drying procedure (i.e., neither the solvent-exchange nor the vacuum-drying method).
The XRD was carried out for the powder samples using the same instrument and conditions used to characterize the fly ash, and the measured patterns were analyzed using the same software and database. In particular, for the samples to detect the calcium oxychloride phase, the other model of XRD (D/MAX 2500V/PC; Rigaku, Japan) equipment, which is directly accessible to the author, was conducted upon grounding the samples because this process should be immediately performed before the instant decomposition of calcium oxychloride.
The TG was conducted on powdered, hardened pastes at 28 days using the TG instrument (TG/DSC1; Mettler Toledo, USA) with a heating rate of 20 °C/min in a nitrogen atmosphere from 25 °C to 1,000 °C.
Pore volumes and size distributions in the hardened samples at 3 and 28 days were estimated with a mercury intrusion porosimeter (Autopore IV, Micrometrics, USA). The samples for the MIP testing were made in 5-mm cubic pieces. To halt further hydration, these cubic specimens were immersed in isopropyl alcohol for 14 days and then stored in the vacuum desiccator for 2 days [41]. Note that all the XRD, TG, and MIP analyses were carried out only for the samples at a w/b = 0.4 as representatives because they produced the greatest strengths.
The ICP-OES was performed using a spectrometer (700-ES; Varian, USA) with a 40 MHz free- running radio-frequency generator and axially viewed plasma to investigate the characteristics in the initial dissolution of fly ash. This analytical technique may quantify dissolved elements in liquid solutions [94, 95]. In this study, the target elements were selected as silicon, aluminum, iron, and calcium because the raw fly ash was mainly composed of aluminosilicate phases and iron oxides (see Table 3). The concentration of calcium in the paste solution may indicate the initial dissolution degree of CaO and CaCl2. The sample preparation for ICP-OES analysis was conducted as follows: each mixture powder was mixed with de-ionized water at a w/b = 2 to obtain a sufficient quantity of aqueous phase [94, 95]; the aqueous mixture was agitated using a heating magnetic stirrer for 30 min while keeping the heating temperature at 60 °C, which is the same curing temperature used in this study; after agitating, the sample was centrifuged with 4,500 rpm for 5 min; the separate aqueous phase was used for the ICP-OES measurement.
3.3. Compressive Strength Of Calcium Oxide-Activated Fly Ash Binder With The Calcium