Raw Material Characterization and Experimental Details for Investigation of The Ca(OH) 2 -

Fly ash was obtained from the Ha-dong coal-fired power station in South Korea. The oxide composition of fly ash was determined using an X-ray fluorescence (XRF) spectrometer (S8 Tiger;

Bruker, Germany); the results are presented in Table 6 with the loss on ignition (LOI) of fly ash, measured by TG (SDT Q600; TA instruments, USA). Based on the American Society for Testing and Materials (ASTM) C 618, the fly ash used in this study was classified as Class F fly ash because the sum of SiO2 + Al2O3 + Fe2O3 was 85.3% [118].

Table 6: Chemical composition of the raw fly ash used in this study.

Oxide Weight (%)

SiO₂ 55.5

Al₂O₃ 22.2

Fe₂O₃ 7.6

CaO 4.5

K₂O 1.8

MgO 1.7

Na₂O 1.7

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TiO₂ 1.3

SO₃ 1.3

P₂O₅ 1.3

SrO 0.4

BaO 0.2

MnO 0.1

ZrO₂ 0.1

Loss on ignition 1.0

Figure 24: XRD pattern of the raw fly ash.

XRD analysis of raw fly ash was conducted using an X-ray diffractometer (D8 Advance; Bruker AXS, Germany) with an incident beam of Cu-Ka radiation ( = 1.5418Å) over a 2 scanning range of 8°–60°. The X’pert High-Score program [119] was used along with the Inorganic Crystal Structure Database (ICSD) [120] to interpret the XRD data. It was deduced that the raw fly ash consisted of quartz, mullite, and magnetite, as shown in Figure 24. The XRD result contained a hump in the 15°–25° range due to the presence of amorphous phase of the raw fly ash [115].

10 20 30 40 50 60

Raw fly ash

Mullite (98-015-8098)

Quartz (98-003-9830)

Magnetite (98-007-7590)

2 Theta degree (Cu-Ka₁) Amorphous phase

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(a)

(b)

Figure 25: Particle size distributions in (a) the raw fly ash and (b) the sugar.

Figure 25 shows the particle size distributions in the raw fly ash and the sugar, as determined by laser diffraction particle size analysis (HELOS; Sympatec, Germany); the diffractometer was equipped with a RODOS dispersing unit. The fly ash contained ~10 vol.% of particles over 100 μm in size. The size of the sugar particles was mostly between 100 and 1,000 μm.

Table 7: Mixture proportions and descriptions of the paste samples.

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 10,000

Densitydistribution (log.) Cumulative distribution (Volume %)

Particle size (μm) Cumulative distribution

Density distribution (log.)

< Raw fly ash >

Median particle size = 467.52 μm Mean particle size = 457.86 μm

0 10 20 30 40 50 60 70 80 90 100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5

0 1 10 100 1,000 10,000

Particle size (μm)

Cumulative distribution (Volume %)

Densitydistribution (log.)

< Sugar >

Cumulative distribution Density distribution (log.)

Median particle size = 467.52 μm Mean particle size = 457.86 μm

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Group Label Binder

(wt%) Sugar

(wt%) w/b Temp. (°C) Re-

mixing Testing

FA CH NC V C X T M I p

No-sugar Su0

69 20 11 0

0.42 23 ● ●

Su0-23C 0.30 23 ●

Su0-90C 0.30 90 ● ● ● ●

Su0-23C-(sol) 2.00 23 ● ●

Su0-90C-(sol) 2.00 90 ● ●

Su0-R 0.42 23 ● ●

Su0-R-23C 0.30 23 ● ●

Su0-R-90C 0.30 90 ● ●

0.5%- sugar

Su0.5

69 20 11 0.5

0.42 23 ● ●

Su0.5-23C 0.30 23 ●

Su0.5-90C 0.30 90 ● ● ● ●

Su0.5-23C-(sol) 2.00 23 ● ●

Su0.5-90C-(sol) 2.00 90 ● ●

1.0%- sugar

Su1.0

69 20 11 1.0

0.42 23 ● ●

Su1.0-23C 0.30 23 ●

Su1.0-90C 0.30 90 ● ● ● ●

Su1.0-23C-(sol) 2.00 23 ● ●

Su1.0-90C-(sol) 2.00 90 ● ●

Note. FA: Fly ash, CH: Ca(OH)², NC: Na²CO³, R: re-mixing (re-stirring), V: Vicat needle test, C:

compressive strength test, X: XRD, T: TG, M: MIP, I: ICP-OES, p: pH measurement. ●: test conducted. In each sample, the weight sum of all binder components was 100 wt%. The specific gravities of the binder and sugar were following as: Na²CO3 : 2.53g/cm³, Ca(OH)2 : 2.24g/cm³, Fly ash : ~2.1 g/cm³, and Sugar : 1.59g/cm³.

Three groups of pastes were prepared with varying contents of sugar – (1) pastes without sugar (Samples with Su0 in their labels), (2) pastes with 0.5% sugar with respect to the binder weight (Samples with Su0.5 in their labels), and (3) pastes with 1.0% sugar with respect to the binder weight (Samples with Su1.0 in their labels). The detailed mixture proportions of the samples are shown in Table 7; the weight percent of each binder component (i.e., fly ash, Ca(OH)2, and Na2CO3) was maintained at a constant value (69:20:11) in all the samples. Extra pure (98%) Ca(OH)2, Na2CO3, and sugar (C6H12O6) were used in this experiment.

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A Vicat needle test was carried out to evaluate the retardation effect of sugar on the Ca(OH)2- Na²CO³-activated fly ash paste; the initial and final setting times [97, 115] of four selected samples (Su0, Su0.5, Su1.0, and Su0-R) were measured using an auto Vicat apparatus (E044N; Matest, Italy) at room temperature. Note that Su0-R was prepared by re-stirring Su0 immediately after its final set was completed (i.e., re-mixing of the fully stiffened Su0 sample). To prepare samples for the Vicat needle test, fly ash, Ca(OH)², and Na²CO³ with/without sugar were mixed with de-ionized water at a water-to- binder weight ratio of 0.42, as prescribed by the ASTM C187 standard [117], for 3 min. Later, these fresh pastes were molded into conical rings within 2 min. In the case of the paste sample without sugar (i.e., Su0), the Vicat needle test was carried out every 1 min, while for those samples with sugar (i.e., Su0.5 and Su1.0), it was performed after every 15 min because the sample without sugar showed a very rapid setting behavior. In this study, Su0 was almost set within 15 min after mixing was started; to evaluate the possible influence of re-mixing on the plasticity of the pastes, Su0-R was prepared by vigorously re-mixing the Su0 sample at 358 rpm using a mixer, after which the Vicat needle test was carried out on Su0-R with a penetration time interval of 15 min.

To prepare samples for strength testing, after dry-mixing raw materials, de-ionized water was added at a w/b ratio of 0.30 to each sample; subsequently, the pastes were mixed for 3 min in a mechanical mixer. The fresh pastes were cast in cubic brass molds (50 × 50 × 50 mm³). The casting process was completed within 5 min before any initial setting started. Two different curing temperatures were used, 23 °C (Samples with 23C in their labels) and 90 °C (Samples with 90C in their labels) . All the samples were cured at 99% relative humidity for 3, 6, 12, 18, and 24 h. In order to compare the influence of sugar addition on strength development of the binder at the different curing temperautres, the curing periods were set identical at both 23 and 90 ºC. It is worth noting that Su0-R-23C and Su0-R-90C represent Su0-R samples cured at 23 °C and 90 °C, respectively; these samples were prepared to examine the effect of remixing on sample strength.

Compressive strength testing was carried out on three specimens of each type of mixture. The fractured specimens were collected to prepare samples for XRD and TG analyses. The collected samples were finely ground and immersed in acetone at room temperature for 2 days to stop further reaction.

The ground samples were then kept in a vacuum desiccator at –60 cmHg for 3 days.

XRD was performed on the samples at 3, 6, 12, 18, and 24 h using the same equipment and software used for analyzing raw fly ash. In addition, immediately after the Vicat needle test at room temperature, the samples were analyzed by XRD to identify what reaction products might be responsible for the rapid setting of samples without sugar. From the Vicat needle test samples, four types of pastes were selected – (1) Su0 before initial set, (2) Su0 after initial set, (3) Su0.5 at 45 min, and (4) Su1.0 at 45 min. Although the Su0 sample set rapidly within a few minutes, the samples with sugar did not set at 45 min, which is

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the minimum required time for normal initial setting [121]. In order to stop further reaction, they were solvent-exchanged with acetone and vacuum-dried for 2 days in the same manner as described earlier.

TG was performed using a TG/DSC1 instrument (Mettler Toledo, USA) in a nitrogen atmosphere.

Powdered samples were tested in the temperature range of 23–1000 °C at a heating rate of 20 °C/min.

Pore size distributions and cumulative pore volumes of the hardened samples were estimated by MIP (Autopore IV; Micrometrics, USA). To prepare samples for MIP testing, the hardened pastes were saw-cut into 5 × 5 × 5 mm³ cubic specimens and immersed in isopropanol with a volume ratio of 240:1 for 14 days to stop further reaction. Lastly, the samples were vacuum-dried for 2 days to remove any residual water or isopropanol. Note that the XRD, TG, and MIP analyses were performed only on samples cured at 90 °C because the samples cured at 23 °C barely hardened even after 24 h.

For ICP-OES and pH measurement, diluted samples of w/b = 2.0 were prepared at 23 °C (Samples with (sol) in their labels). The powder mixtures were mixed with de-ionized water at w/b = 2.0 and stirred using a heating magnetic stirrer for 1 (only for pH measurement), 3, 6, 12, and 24 h, while the temperature was maintained at either 23 °C or 90 °C. During the agitation process, the samples were kept sealed to prevent evaporation of water and carbonation by the CO² present in ambient air, which may affect the pH of the solutions. Before testing, all the samples were vacuum-filtered to separate out the aqueous phase. ICP-OES was conducted using an ICP optical emission spectrometer (700-ES;

Varian, USA) with a 40 MHz free-running radio-frequency generator and axially viewed plasma. pH values were measured using a pH meter (HI 3320; Hanna Instruments, USA). In this study, these techniques were used to examine the (1) calcium complexation of Ca(OH)² by sugar and (2) dissolution of fly ash. Thus, calcium, silicon, aluminum, iron, and sodium were selected as the target elements for ICP-OES.