Raw Material Characterization and Experimental Details for Investigation of the Use Of Coal

Bottom ash was collected at the Ha-dong thermal power plant in South Korea. Commercial GGBFS (Chunghae, Korea) was used. Analytical grade CaO and CaCl2 (extra pure, 98%) were used as activators.

The oxide and elemental compositions of the raw materials, GGBFS and bottom ash, were determined using an X-ray fluorescence (XRF) spectrometer (S8 Tiger; Bruker, Billerica, MA, USA) and their losses on ignition (LOI) were measured with a thermal analyzer (SDT Q600; TA Instruments, New Castle, DE, USA). Table 8 shows the elemental and oxide compositions of the GGBFS and bottom ash.

66

Table 8. Elemental and oxide compositions of ground granulated blast furnace slag (GGBFS) and bottom ash by X-ray fluorescence (XRF).

GGBFS Bottom Ash

Element (atomic%) Oxide (wt%) Element (atomic%) Oxide (wt%)

Ca 59.5 CaO 45.1 Si 50.2 SiO2 60.4

Si 23.1 SiO2 33.6 Al 15.4 Al2O3 18.4

Al 9.7 Al2O3 13.3 Fe 13.3 Fe2O3 7.4

Mg 2.5 MgO 3.2 Ca 11.0 CaO 6.8

S 1.4 SO3 2.1 K 2.0 MgO 1.6

Ti 1.0 TiO2 0.8 Ti 1.7 Na2O 1.4

Mn 0.8 MnO 0.5 Na 1.5 TiO2 1.2

Fe 0.7 K2O 0.5 Mg 1.5 K2O 1.1

K 0.7 Fe2O3 0.4 Sr 0.8 MoO3 0.4

Na 0.3 Na2O 0.3 Mo 0.8 P2O5 0.3

Sr 0.1 SrO 0.1 Ba 0.7 BaO 0.3

Ba 0.1 BaO 0.1 Nb 0.5 SrO 0.3

Zr 0.09 ZrO2 0.1 P 0.3 Tb4O7 0.1

V 0.03 V2O5 0.0 Mn 0.2 MnO 0.08

Y 0.02 P2O5 0.0 Cl 0.06 SO3 0.06

P 0.01 Y2O3 0.0 S 0.03 Cl 0.03

Nb 0.00 Cr2O3 0.0 Cu 0.03 V2O5 0.03

LOI (wt%) 0.41% LOI (wt%) 0.68%

Powder X-ray diffraction (XRD) patterns for the GGBFS and bottom ash were collected using a high-power X-ray diffractometer (D/MAX 2500V/PC; Rigaku, Tokyo, Japan) with a Cu-Kα radiation (λ = 1.5418 Å) for a 2θ scanning range of 8°−60° in 2θ degrees. The obtained XRD patterns were analyzed with the X’pert High Score program [119] using the International Center for Diffraction Data (ICDD) PDF-2 database [143] and the Inorganic Crystal Structure Database (ICSD) [120].

Figure 36 shows the measured XRD patterns and identified phases of the raw GGBFS and bottom ash. In the mineralogical composition, the GGBFS contained only akermanite (Ca2MgSi2O7, ICSD PDF-2 no. 01-079-2425) while the bottom ash included several crystalline minerals such as quartz (SiO2, ICSD PDF-2 no. 01-087-2096), calcium aluminum silicate (Al1.77Ca0.88O8Si2.23, ICSD PDF-2 no. 00- 052-1344), mullite (3Al2O32SiO2, ICSD PDF-2 no. 01-079-1455), diopside (Ca1Mg1O6Si2, ICDD PDF- 2 no. 98-015-9054), and magnetite (Fe3O4, ICDD PDF-2 no. 98-015-8743). Amorphous humps were observed in both materials and are marked as shaded areas in Figure 36. The GGBFS mostly consisted of amorphous phase while the bottom ash contained a much smaller portion.

67

(a) (b)

Figure 36. XRD patterns and identified phases of the (a) raw GGBFS and (b) raw bottom ash.

Mg: magnetite.

The particle size distributions of the GGBFS and bottom ash were examined with a laser scattering particle size analyzer (HELOS; Sympatec, Clausthal-Zellerfeld, Germany) (Figure 37). It is worth noting that the mean particle size of the bottom ash (276.31 μm) was approximately 10 times larger than that of the GGBFS (27.00 μm).

Akermanite (Ca2MgSi2O7) (01-079-2425)

10 20 30 40 50 60

2 Theta degree (Cu-Ka1)

GGBFS Measured patterns

Reference patterns of identified phases Amorphous hump

Bottom ash

Quartz (SiO2) (01-087-2096) Calcium Aluminum Silicate (Al1.77Ca0.88O8Si2.23) (00-052-1344)

Mullite (3Al2O32SiO2) (01-079-1455) Measured patterns

Diopside (Ca1Mg1O6Si2) (98-015-9054) Mg

Magnetite (Fe3O4) (98-015-8743) Reference patterns of identified phases

10 20 30 40 50 60

2 Theta degree (Cu-Ka1) Amorphous

hump

Mg Mg

Mg

68

Figure 37. Particle size distributions of raw GGFBS and raw bottom ash with their mean particle sizes.

The GGBFS and bottom ash were studied using a field emission scanning electron microscope (SEM) (Quanta200; FEI, Eindhoven, Netherlands) in the secondary electron (SE) mode. The powdered raw materials were placed on double-faced carbon tape with a platinum (Pt) coating. Figure 38 shows SEM SE images of the raw GGBFS and bottom ash.

(a) (b)

Figure 38. SEM images of (a) raw GGBFS and (b) raw bottom ash.

The mixture proportions are given in Table 9. The bottom ash was about 10 times larger in particle size than GGBFS and so it is likely that it acts as a fine aggregate; the mixtures will be referred to as

“mortar” in this study although no sand was included in the mixture proportions.

Densitydistribution (log.)

Particle size (μm) 0

0.2 0.4 0.6 0.8 1 1.2

0.1 1 10 100 1000

Bottom ash GGBFS

Mean particle size of GGBFS = 27.00μm Mean particle size of Bottom ash = 276.31 μm

Raw GGBFS

100 μm

Raw bottom ash

100 μm

69

Table 9. Mixture proportions of mortar samples in relative weight.

Label

Binder (b)

Bottom Ash (B)

Binder Fraction in Mixture (Fr) (= S / (S + B))

Water (w) GGBFS CaO CaCl2

Total (S) B/S0.0

94 4 2 100

0 1.00

40 (w/b = 0.4)

B/S0.2 20 0.83

B/S0.4 40 0.71

B/S0.6 60 0.63

B/S0.8 80 0.56

B/S1.0 100 0.50

Note: For inductively coupled plasma-optical emission spectrometry (ICP-OES) and ion chromatography (IC) testing, the solution samples at w/b = 2.0 were used while the samples at w/b = 0.4 were prepared for all other testing.

The mortar mixture samples were prepared by adding bottom ash in relative weight ratios of 0, 20, 40, 60, 80, and 100 to the binder weight (GGBFS + CaO + CaCl2 = 100) (i.e., B/S0.0, B/S0.2, B/S0.4, B/S0.6, B/S0.8, and B/S1.0 in Table 2, respectively). As the quantity of added bottom ash increased, the relative weight of the binder to the total weight of mixture, which is the binder fraction in mixture (Fr) in Table 2 decreased from 1.0 to 0.5 (e.g., 1.0 for B/S0.0, and 0.5 for B/S1.0). Although the presence of CaCl2 could result in the corrosion of the embedded steel bars in concrete, CaCl2 was used in the mixture to increase the overall compressive strength of the binder used in this study [141]. The previous study reported that the use of CaCl2 in the CaO-activated GGBFS binder system promoted the early dissolution of the amorphous phase of GGBFS considerably [141].

Two values of water-to-binder weight ratio (w/b) were used in the sample preparation: w/b = 2 for diluted samples for ICP-OES and IC and w/b = 0.4 for all other testing samples; the relative weight of water to the total weight of the mixture decreased as the weight of the added bottom ash increased.

Thereby, the flowability of the freshly mixed mortar decreased.

The bottom ash was prepared under the surface-dry (SSD) condition before mixing. The GGBFS and CaO powders were dry-mixed with CaCl2 and then mixed with varying contents of bottom ash for two minutes in a mechanical mixer. The mixtures were then mixed with de-ionized water for three minutes. The fresh mortars were cast in brass cubic molds (50 × 50 × 50 mm) and then cured for 3, 7, 14, and 28 days. All samples were stored in a humidity chamber at 23 °C with 99% relative humidity for all curing periods.

Compressive strength tests were performed on the triplicate cubic samples for each mixture after 3, 7, 14, and 28 days. After testing, the fractured specimens were collected and finely ground with an

70

agate mortar and pestle. The powdered samples were subjected to a solvent exchange process using isopropyl alcohol to stop further hydration and carbonation for the XRD and TG tests [144].

The XRD patterns for the ground samples were collected and analyzed using the same XRD instrument and analysis program with the database used for analyzing raw materials.

Thermogravimetry (TG) (Q500; TA Instruments, Newcastle, DE, USA) was performed for the ground hardened samples with an alumina pan and nitrogen gas. The heating temperature ranged from room temperature to 1000 °C with a heating rate of 30 °C/min.

Inductively coupled plasma-optical emission spectrometry (ICP-OES) and ion chromatography (IC) were conducted to examine the dissolution behaviors of the GGBFS and bottom ash using a spectrometer (700-ES; Varian, Palo Alto, CA, USA) and a reagent-free ion chromatography system (ICS-3000; Thermo Scientific, Waltham, MA, USA), respectively. In this study, the target elements were calcium (Ca), silicon (Si), aluminum (Al), magnesium (Mg), sulfur (S), and iron (Fe) because the raw materials, GGBFS and bottom ash, were mainly composed of these elements. This was determined by XRF (Table 1).

The sample preparation for ICP-OES and IC analyses was conducted with a water-to-binder weight ratio of 2.0 (w/b = 2.0) at 23 °C for all diluted samples as mentioned earlier. Two hundred grams of water and 100 g of the binder were mixed and then the quantity of bottom ash was increased in increments of 20 g from 0 to 100 g. It is important to note that bottom ash was not included in the binder weight and the bottom ash in SSD was used. Then, the diluted mixtures were consistently agitated using a magnetic stirrer for 24 h at room temperature. After agitation, the filtrated liquid phases from the mixtures were tested for ICP-OES and IC measurement [145].

Pilot samples of the artificial fine aggregates were manufactured for all mixture proportions in Table 9 through cold-bonded pelletization using a disk pelletizer. First, the binder was dry-mixed with varying quantities of bottom ash using the same mixture proportions as in Table 9, except for the fact that the bottom ash was used in a dry state because wet mixing is not necessary for pelletization.

The mixed powders were put in a disk pelletizer with a diameter of 80 cm and then pelletized by spraying water. Previous studies noted that disk inclination angle and disk revolution speed affect the efficiency of pelletization [146, 147] and after several adjustments were made, the best disk inclination angle and revolution speed were found to be 34 RPM and 42°, respectively, as illustrated in Figure 39.

71

Figure 39. Pelletizing parameters for maximum pelletization efficiency in this study.

The freshly pelletized aggregates were cured in a humidity chamber for 28 days at room temperature. Finally, cured aggregates smaller than 4.75 mm were separated via sieving for use as fine aggregates.

After considering the results of the strength test on the mortar samples and the pelletizing formability after the cold-bonded pelletization, the best mixture proportion, B/S0.4, was selected from Table 9 for the production of artificial fine aggregates. The produced aggregates using B/S0.4 were cured in a humidity chamber at 23 °C with 99% relative humidity for 28 days. Aggregates were then used for a water absorption test and a leaching test for heavy metals. Although it is best to conduct the compressive strength and the instrumental analyses for the aggregate sample as well, since some mixture proportions could not be well manufactured into aggregate samples and it is difficult to set the same w/b between the aggregate samples, the identification of reaction products, which is largely influenced by w/b, was conducted only for the samples after the compressive strength test.

The water absorption was measured in the developed 28-day-cured fine aggregate sample according to the ASTM C128-15 [148]. After the fine aggregates of 500 g were submerged in water for 24 h at room temperature, the samples were slowly and equally dried until they reached the SSD condition, as determined by a cone test. The weight of the saturated samples (S) was measured. The samples were dried in an oven at 100 °C for 24 h to determine the oven dry mass (A). The value of the water absorption of the fine aggregates was then calculated following the formula from the ASTM C128-15: Absorption, % = [(S − A) / A] × 100.

The leaching test was performed for both the raw bottom ash and the manufactured fine aggregates [64]. The sample preparation for the leaching test was conducted by following the toxicity characteristic leaching procedure (TCLP) for aggregates of the US Environmental Protection Agency (EPA) to detect

(3) Disk inclination angle = 42 ° (1) Disk revolution speed

= 34 RPM

(2) Disk diameter

= 80 cm

Pelletizing parameters in this study

72

concentrations of arsenic (As), cadmium (Cd), barium (Ba), lead (Pb), and chromium (Cr); a solution of pH = 5, which was diluted from 0.5 N acetic acid, was prepared for the extraction medium. The ratio of solution to solid for the samples was set as 20:1 and the diluted samples were agitated with a magnetic stirrer for 18 h. The liquid was tested using ICP-OES after filtrating. Additionally, although the TCLP regulation does not include copper (Cu), zinc (Zn), and nickel (Ni), the concentrations of these elements were also measured using the ICP-OES because a previous study [30] reported that bottom ash contained them in large quantities.

5.3. Compressive Strength of the Use Of Coal Bottom Ash And GGBFS In The Manufacturing