The Mechanism of Early Stage Compressive Strength Attainment in Low-calcium Ground

Coal Bottom Ash Geopolymer Hyo Kim1 and Sungil Hong1 1Department of Chemical Engineering, University of Seoul, 163 Seoulsiripdae-ro, Dongdaemun-gu, Seoul 02504, Korea CONFERENCE: 2017 World of Coal Ash – (www.worldofcoalash.org) KEYWORDS: Geopolymer, Coal Bottom Ash, Early Stage, Compressive Strength, Thermal Conductivity, FTIR, SEM ABSTRACT We investigated the development of compressive strength and thermal conductivity of coal bottom ash geopolymer. NaOH solutions of 12, 13 and 14 M are used as alkali activators and waterglass is added to adjust the SiO2/Na2O value. The mass ratio of H2O/total mixture and the molar ratio of SiO2/Na2O are kept constant at 0.2 and 5, respectively. The geopolymer specimens are cured in 75 °C dry oven for 24 hours and then rested in 25 °C oven until their physical properties are tested at 1, 3, 7, 14 and 28 day(s). The compressive strength of higher than 30 MPa is obtained with 13 and 14 M NaOH solutions at the age of 14 days. The thermal conductivity of bottom ash geopolymer was lower than 0.6 W/mK after 7 days. The development of geopolymer structure is observed by FTIR and SEM analysis. The results of physical properties and structural analysis show that the bottom ash geopolymer attains high compressive strength in early age. Therefore, the bottom ash geopolymer has a great potential to replace the conventional bricks and blocks with avoiding the environmental problems arising from its current landfill-treatment dependence. 1. INTRODUCTION Geopolymer, composed of three-dimensional alumino-silicate cementitious materials, has been widely studied since the first report published by Davidovits in 19791. Geopolymer uses boiler slag, mine tailings, clay and fly ash as reagents, which benefits construction and environment2, 3. Geopolymer is synthesized from alumina- and silica-rich resources without emitting too much CO2 like Ordinary Portland Cement3. For building structure, geopolymer has an advantage of attaining high compressive strength within several hours to few days4, 5. Resistance to fire and acids and immobility to toxic metals and nuclear waste are additional benefits that make geopolymer attractive for civil construction2, 6-8.

2017 World of Coal Ash (WOCA) Conference in Lexington, KY - May 9-11, 2017http://www.flyash.info/

Unlike the fly ash, the coal bottom ash is not widely used as a source material of geopolymer because it is a mixture of irregular and rough particles, ranging from millimeters to centimeters. Thus, a large proportion of the bottom ash produced has been landfilled. However, the bottom ash contains some hazardous elements such as lead (Pb), zinc (Zn), cadmium (Cd), and copper (Cu) that contaminate soil and ground water9, 10. Therefore, using bottom ash as a raw material of geopolymer is a better way to reduce the environmental problem. The chemical composition of bottom ash is almost identical to that of fly ash. Thus, after milling and sieving properly, the bottom ash geopolymer can be synthesized in a similar way as the fly ash geopolymer. The fly ash has been activated under an alkaline condition to form geopolymer gel at room temperature. The high concentration of NaOH or KOH premixed with waterglass (sodium silicate solution) is normally used in the alkali activators. This geopolymer gel is cured and hardened in a thermal condition ranging from a room temperature to 80 °C for several hours4. Accordingly, the bottom ash geopolymer is synthesized by following the fly ash synthesis steps. However, in our previous study a dark stained streak always appeared around on the top of hardening sample prepared in a gel state. The dark stained streak originated from the floating of dissolved impurities in the bottom ash during the curing process. This streak separated the top part as a crust which ultimately deteriorated the compressive strength of geopolymer specimen by interfering with the drying process of samples. Here, therefore we investigate the development of compressive strength and thermal conductivity of bottom ash geopolymer with a lower mass ratio of water and total mixture without any visible gel state. 2. EXPERIMENTAL 2.1 Materials The coal bottom ash was provided from the Yeongheung power plant, Korea. Because of its large size, bottom ash was crushed using hammer mill and sieved with a No. 200 standard sieve (75 μm). Sodium hydroxide (NaOH) pellets were dissolved by tap water to make an alkali activator. The concentrations of NaOH solution were 12, 13 and 14 M, and a waterglass (Na2O = 9.4 wt.%, SiO2 = 38.4 wt.%, H2O = 62.2 wt.%) was premixed at least 24 hours for stabilization before this alkali activator was added to the ground coal bottom ash. 2.2 Geopolymer synthesis In our preliminary experiments, it was found that the mass ratio of water and total mixture (H2O/total) determined whether the mixture of bottom ash and alkali activator showed a visible gel state or not. Therefore, it is necessary for the mixture of bottom ash and activator to be not-a-gel-state with low values of H2O/total to avoid the occurrence of dark strained streak on the specimen. The mass ratio of activator/ground bottom ash (liquid/solid) was set up to keep the H2O/total constant at 0.2. The mass ratio of waterglass to NaOH pellets (waterglass/NaOH) was determined to hold the

molar ratio of SiO2/Na2O at 5. It has been known that the molar ratio of SiO2/Na2O is a key factor for determining the mechanical properties of geopolymer11-13. The details of the mix design are shown in Table 1 where BA12, BA13 and BA14 refer to the samples prepared with 12, 13 and 14 M NaOH, respectively. A waterglass was not added in BA12, and BA13 contained a small amount of waterglass (waterglass/NaOH = 0.39). On the contrary, the amount of waterglass used in BA14 was much larger than others to keep the ratio of SiO2/Na2O =5 by supplying additional SiO2 from the waterglass. At ambient temperature, ground bottom ash and alkali activator were mixed by Hobart mixer for 5 minutes and then this mixture was poured into a 50 mm cubic mold and compressed with a hand press at the same time. After that, samples were demolded and cured in a 75 °C dry oven for 24 hours in a sealed plastic bag. They were then unsealed and rested in an ambient temperature (25 °C) oven to the designated test periods (1, 3, 7, 14 and 28 day(s)).

Table 1. Mix design details of samples Sample name

NaOH molarity (M)

Waterglass/ NaOH mass ratio

Liquid/solidmass ratio

SiO2/Na2O molar ratio

H2O/total mass ratio

Na/Al molar ratio

BA12 12 0 0.37 5.00 0.20 0.87 BA13 13 0.39 0.39 5.00 0.20 0.90 BA14 14 0.71 0.41 5.00 0.20 0.91 2.3 Test of physical properties

The particle size distribution of ground bottom ash was measured by Particle Size Distribution Analyzer (LA-960, Horiba, Japan) and its chemical composition was analyzed by XRF (ZSX Primus Ⅱ, Rigaku, Japan). The compressive strength of samples was measured by using Compression Testing Machine (PL-9700H, Woojin Precision Co., Korea) at 1, 3, 7, 14 and 28 days after demolding. At the same time points, thermal conductivity was evaluated by using Thermal Constants Analyzer (TPS-500s, Hot Disk®, Sweden). The compressive strength and thermal conductivity values are the means of three samples. The morphology of geopolymer samples was examined by SEM micrographs (SNE-3000M, SEC, Korea) and the structural characteristics was analyzed by Fourier Transform Infrared Spectroscopy (Frontier FT-IR Spectrometer, Perkinelmer, USA). The bulk density of samples was calculated by dividing the measured weight by a volume of 50 × 50 × 50 mm3 and the powder density was measured by gas pycnometer (AccuPyc Ⅱ 1340, Micromeritics, U.S.A.). The porosity of samples was calculated using the following equation:

𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃 (%) = �1 − 𝐵𝐵𝐵𝐵𝐵𝐵𝐵𝐵 𝑑𝑑𝑑𝑑𝑑𝑑𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃

𝑃𝑃𝑃𝑃𝑃𝑃𝑑𝑑𝑑𝑑𝑃𝑃 𝑑𝑑𝑑𝑑𝑑𝑑𝑃𝑃𝑃𝑃𝑃𝑃𝑃𝑃� × 100.

3. RESULTS AND DISCUSSION 3.1 Raw material

The particle size distribution of ground bottom ash was examined before it is used. As shown in Fig. 1, the mean particle size is 46.53 μm and 90% of ground bottom ash is below 88.50 μm. The chemical composition of ground bottom ash is analyzed by XRF and tabulated in Table 2. The proportion of calcium is low (5.75%) and sorted to the Class F fly ash (ASTM C 618) because a sum of SiO2, Al2O3 and Fe2O3 is above 70%.

0.01 0.1 1 10 100 10000123456789

Volume percent Cumulative volume percent

Diameter (µm)

Volu

me

perc

ent (

%)

Mean size: 46.53 µmD10: 12.00 µmD50: 36.19 µmD90: 88.50 µm

0

20

40

60

80

100

Cum

ulat

ive v

olum

e pe

rcen

t (%

)

Fig. 1. Particle size distribution of ground bottom ash.

Table 2. Composition of bottom ash determined by XRF analysis

Component Weight % SiO2 53.7 Al2O3 20.9 Fe2O3 12.0 CaO 5.75 K2O 1.77 MgO 1.43 TiO2 1.28 Na2O 1.02 C 0.88

3.2 Compressive strength

We sought to investigate the development of compressive strength of bottom ash geopolymer samples over time. Fig. 2 shows the compressive strength of samples according to each measurement date. The compressive strength of geopolymer samples increases over time as the geopolymerisation proceeds further. The reaction resultant water in samples is evaporated in drying environment and this process also contributes to the improvement of compressive strength as well as the stronger alkaline solution. The compressive strength of higher molarity samples (BA13 and BA14) is beyond that of lower molarity sample (BA12) for all days. Except for BA12, BA13 and BA14 show the great increase in the compressive strength after the age of 7 days. This development is due to the higher alkaline condition. The increase in alkalinity extends the activation of bottom ash and produces more geopolymer. However, the difference of compressive strengths between BA13 and BA14 is not as large enough as the difference between BA12 and BA13 or BA12 and BA14. It is because, as shown in Fig. 3, the large cracks longer than 2 cm occur in the higher molarity samples (BA14) after 14 days. The high Na/Al molar ratio in high molarity samples (Table 2) could cause these cracks14. BA13 results in higher compressive strength than BA14 at 28 days because the cracks are more grown for BA14 (Fig. 3 (i)). The deterioration of compressive strength caused by cracks means, on the other hand, that much higher compressive strength can be achieved if the occurrence of cracks is prevented with proper curing process or some additives in the strong alkaline condition. The difference of compressive strengths between 14 days and 28 days is less than 1.2 MPa for all samples. Therefore, geopolymerisation is shown to be almost completed within 14 days.

1 3 7 14 28

9

18

27

36

45

Com

pres

sive

stre

ngth

(MPa

)

Test day(s)

BA12 BA13 BA14

Fig. 2. Development of compressive strength over the curing ages (The values are averaged with 3 specimens cured in 75 °C for a day and then in 25 °C oven for the rest of days).

Fig. 3. Side-view photographs of samples at the age of 1 day ((a)-(c)), 14 days ((d)-(f)) and 28 days ((g)-(i)). 3.3 Densities, porosity and thermal conductivity

The bulk density, powder density and porosity of each sample are listed in Table 3. BA13 has the greatest bulk density, while BA12 has the lowest bulk density. A small addition of waterglass in BA13 sample increases the bulk density. However, the abundant waterglass makes the mixture of bottom ash and alkali activator subject to pellet forms15, which keep the mixture from being compressed easily into the mold. Therefore, the bulk density of BA14 is found lower than BA13. For all samples, the bulk density decreases with time because the water content is reduced by evaporation. Similarly, the powder density also diminishes until 7 days. However, the powder density increases after 7 days and it is accounted the more dense microstructures are formed as they are observed by SEM images (Fig. 6). All samples became more porous as the age went on.

(a) BA12 (1 Day) (b) BA13 (1 Day) (c) BA14 (1 Day)

(g) BA12 (28 Days) (h) BA13 (28 Days) (i) BA14 (28 Days)

1 cm

(d) BA12 (14 Days) (e) BA13 (14 Days) (f) BA14 (14 Days)

1 cm

Fig. 4 shows the change of the thermal conductivity for all samples. The thermal conductivity decreases with time because of water evaporation. However, the thermal conductivity slightly rises after 14 days. This can be explained that the re-increase of powder density had been attributed to the formation of more dense structure (Fig. 6). The relationship between the molarity of alkali activator and the thermal conductivity is not clear. For all samples, the thermal conductivity values after 7 days are lower than 0.6 W/mK. It is comparable to that of the clay bricks having the same bulk density16. Table 3. Bulk density, powder density and porosity BA12 BA13 BA14

Bulk density (g/cm3)

Powder density (g/cm3)

Porosity (%)

Bulk density (g/cm3)

Powder density (g/cm3)

Porosity (%)

Bulk density (g/cm3)

Powder density (g/cm3)

Porosity (%)

1 day 1.74 2.43 28.4 1.82 2.43 25.1 1.77 2.42 26.9

3 days 1.72 2.40 28.3 1.81 2.43 25.5 1.75 2.40 27.1

7 days 1.66 2.39 30.5 1.75 2.39 26.8 1.71 2.39 28.5

14 days 1.64 2.41 32.0 1.71 2.41 29.0 1.69 2.40 29.6

28 days 1.64 2.43 32.5 1.71 2.44 29.9 1.68 2.42 30.6

1 3 7 14 280.40

0.45

0.50

0.55

0.60

0.65

0.70

0.75

0.80

Ther

mal

con

duct

ivity

(W/m

K)

Test day(s)

BA12 BA13 BA14

Fig. 4. Thermal conductivity according to test day(s).

3.4 FTIR analysis The framework structures of geopolymer were investigated by the FTIR analysis. Fig. 5 shows the result of FTIR analysis of samples at the age of 1 day and 28 days. The broad peaks around 1000 cm-1 and 750 cm-1 in the result of original bottom ash are due to (Si, Al)-O asymmetric stretching vibration17 and Al-O-Si bending vibration18, respectively. For synthesized geopolymer samples, broad peak around 3400 cm-1 and small peak around 1650 cm-1 are both associated with the hydration water18. A small peak around 1450 cm-1 is attributed to the atmospheric carbonation of sodium (Na2CO3)19. At 1 day after synthesis, the intense and wide peak is located at 960 cm-1 for all samples which is originated from the peak at 1009 cm-1 in the original bottom ash. This peak is due to Al-O and Si-O asymmetric stretching vibration – a major fingerprint of geopolymerisation20. This peak moves again to higher wavenumber (966–967 cm-1) at the age of 28 days, which is related to the development of compressive strength17. The peak at around 430 cm-1 is associated with the in-plane banding of Al-O and Si-O linkage20. 3.5 SEM analysis

The microstructure of geopolymer was observed using scanning electron microscope (SEM). The SEM images of geopolymer samples at the age of 1 day and 28 days are shown in Fig. 6. A few of geopolymer products which have spherical and tiny shape are observed in BA12 sample at 1 day (Fig. 6 (a)), and the amount of these products increased at 28 days (Fig. 6 (b)). The amount of geopolymer is larger for higher molarity sample BA13 (Fig. 6 (c)) and further more products are observed in BA14 sample which form dense geopolymer matrix. This dense geopolymer matrix contributes to higher compressive strength21. At the age of 1 day, a lot of microcracks which may be caused by evaporation of water in the drying process are observed for all samples (Fig. 6 (a), (c) and (e)). These cracks lower the compressive strength at the early age. However, the more produced geopolymer in later stage covers the microcracks (Fig. 6 (b), (d) and (f)) and so the compressive strength is improved. BA13 and BA14 samples at 28 days (Fig. 6 (d) and (f)) show the most dense and smooth geopolymer matrix.

4000 3500 3000 2500 2000 1500 1000 500

3453

A.U.

wavenumber (cm-1)

1 day

3453

3450

1653

1653

1647

1448

1449

1449

960

960

960

425

423

422

1009779

428

Bottom ash

BA12

BA13

BA14

4000 3500 3000 2500 2000 1500 1000 500

28 day

A.U.

wavenumber (cm-1)

1009779

428

3356

3357

3356

1648

1650

1655

1453

1453

1456

967

967

966

881

881

881

424

423

420

Bottom ash

BA12

BA13

BA14

Fig. 5. FTIR spectra of bottom ash and geopolymer at the age of 1 day (above) and 28 days (below).

Fig. 6. SEM images of geopolymer samples at the age of 1 day ((a), (c) and (e)) and 28 days ((b), (d) and (f)) at 1.0 K magnification.

(a) BA12 (1 Day)

(c) BA13 (1 Day)

(e) BA14 (1 Day)

(b) BA12 (28 Days)

(d) BA13 (28 Days)

(f) BA14 (28 Days)

30μm 30μm

30μm 30μm

30μm 30μm

4. CONCLUSION In this study, we investigate the development of compressive strength and thermal conductivity of ground bottom ash geopolymer. The bottom ash geopolymer with low amount of alkali activator exhibited high compressive strength at the early age. The compressive strength above 30 MPa was obtained in only 14 days with alkali activator of 13 or 14 M NaOH added by waterglass to keep the ratio of SiO2/Na2O at 5. The thermal conductivity of all samples was lower than 0.6 W/mK after 7 days. It is comparable to that of ordinary clay bricks having the same bulk density16. The structural characteristics is analyzed by FTIR spectroscopy and the peaks related to the formation of geopolymer are detected. The shift of peaks attributed to the geopolymer formation was also observed. The SEM images indicate a high presence of microcracks at an initial stage and geopolymer products at a later stage. This change of microstructure contributes to the development of compressive strength. The side-view photographs of geopolymer samples show the occurrence of large cracks in high molarity samples (BA14). These cracks could deteriorate the mechanical properties. In other words, if the occurrence of cracks could be prevented with proper curing process or some another additives, the geopolymer with enhanced mechanical properties can be synthesized. Therefore, the bottom ash geopolymer has a great potential to replace the conventional bricks and blocks with avoiding the environmental problems arising from its current landfill-treatment dependence. REFERENCES [1] Davidovits, J. Synthesis of new high-temperature geo-polymers for reinforced plastics/composites, Society of Plastic Engineers, 1979, pp.151-154. [2] Van Jaarsveld, J.G.S., and Van Deventer J.S.J. The potential use of geopolymeric materials to immobilise toxic metals Part Ⅰ. theory and applications, Minerals Engineering, 1996, 10, pp.659-669. [3] Bondar, D. Geo-polymer concrete as a new type of sustainable construction materials, Proceedings of the 3rd International Conference on Sustainable Construction Materials and Technologies, 2013, pp.780-786. [4] Davidovits, J. Geopolymers: man-made rock geosynthesis and the resulting development of very early high strength cement, Materials Education, 1994, 16, pp.91-139. [5] Palomo, A., Macias, A., Blanco, M.T., and Puertas, F. Physical, chemical and mechanical characterization of geopolymers, Proceedings of the 9th International Congress on the Chemistry of Cement, 1992, pp.505-511.

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