Proceedings of the 2018 World Transport Convention Beijing, China, June 18-21, 2018

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Effect of multi-factors on Class F Fly Ash based Geopolymer

Synthesis

Haoyu Sun School of Civil and Safety Engineering, Dalian Jiaotong University

No.794 Huanghe Road, Shahekou District, Dalian, China 414861630@qq.com

Mo Zhang School of Civil and Safety Engineering, Dalian Jiaotong University

No.794 Huanghe Road, Shahekou District, Dalian, China mozhang1985@163.com

Haitao Wang School of Civil and Safety Engineering, Dalian Jiaotong University

No.794 Huanghe Road, Shahekou District, Dalian, China wht_dalian@163.com

ABSTRACT

The influence of various factors on the synthesis of Class F fly ash based geopolymer was explored in this study. Three sets of experiments were performed to investigate the effect of Si/Al and Na/Al molar ratio, addition of metakaolin (30 wt%,40 wt% and 50 wt%), and curing temperature (ambient temperature and 80℃) on the mechanical properties and microstructure. The compressive strength,micromorphology and mineralogy of the geopolymers were characterized with unconfined compression testing, scanning electron microscopy (SEM) and X-ray diffractometry (XRD). The geopolymers synthesized with appropriate Si/Al and Na/Al molar ratios, relatively high metakaolin content and cured at the elevated temperature presented relatively higher compressive strength. The SEM and XRD results showed that more geopolymer gel matrix and more intense representative XRD pattern of geopolymer gel were observed in the geopolymer samples with improved strength, which well explained the influence of different factors on the early and long-term strength. However, certain synthesis problems, such as flash set and relatively low strength, are necessary to be further investigated for desirable mechanical properties of Class F fly ash based geopolymers.

KEYWORDS: Chemical composition; Metakaolin; Class F fly ash based geopolymer; SEM; XRD

1 INTRODUCTION

Geopolymers are a type of inorganic polymer materials of which the term was firstly coined by the French researcher Davidovits[1] in 1970s. Geopolymer is formed via the chemical reaction between aluminosilicate raw materials and alkali solutions, yielding amorphous to semi-crystalline three-dimensional structures, which consist of -Si-O-Al-O- bonds. Although a lot of researches have been carried out, the exact reaction mechanism still remained unclear due to the complexity of geopolymerization process. Based on the conceptual model proposed by Provis et al.[2], the reaction process of geopolymer can be generally divided into three stages: dissolution of raw materials, dimerization of reactive silicate and aluminate monomers, oligomerization and condensation.

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Geopolymers possess many excellent properties, including high strength, high temperature stability, good fire and acid resistance, and high stability of toxic elements encapsulation[3,4]. As a promising cementitious material, geopolymers have attracted increasing interests. Geopolymer materials have been successfully synthesized with metakaolin (MK), feldspar and other Si-Al minerals in nature and applied in practices[5,6]. Besides, the industrial wastes rich in reactive alumina and silica can be reused for geopolymer synthesis, such as fly ash[7].

Fly ash is an industrial solid by-product of pulverized coal combustion. It can generate serious environmental issues such as dust, air, soils and water pollution, and has been partially reused in construction. It is estimated that the annual output of fly ash in China will reach 500 million tons by the year 2020[8], of which a large proportion is class F fly ash (calcium content < 10%). Therefore it is essential to solve the negative impact of fly ash on resources and environment. More attention has paid to the utilization of fly ash[9]. Compared to other raw materials used in the development of geopolymers, class F fly ash has the advantages of low cost and high inventory for the application in large-scale construction industries. However, due to the low reactivity of Class F fly ash, it is difficult to synthesze fly ash based geopolymer with high mechanical properties at room temperature. However, the high purity and reaction rate of metakaolin make it an excellent type of raw material for synthesizing geopolymer with desired mechanical performance at room temperature.

To investigate the effect of chemical composition of starting materials, curing condition and types of raw materials on the mechanical properties of Class F fly ash based geopolymer, different Na/Si molar ratios, Si/Al molar ratios, fraction of metakaolin, curing temperatures were used to synthesize geopolymer with Class F fly ash. The unconfined compressive strength of the geopolymers was obtained and compared. In addition, the differences in the micromorphology and mineralogy of the geopolymers were characterized with Scanning Electron Microscopy (SEM) and X-ray Diffractometer (XRD) to study the influences of curing temperature and raw materials on the microstructures of geopolymer, and further explore the relationship between the microstructure and mechanical properties of Class F fly ash based geopolymer.

2 EXPERIMENTS

2.1 Materials

The fly ash from Dalian Huaxiang Fly Ash Co. Ltd was used in the geopolymer synthesis. The chemical composition of fly ash is shown in Table 1. The proportion of the fly ash smaller than 45μm is 26%. According to the content of calcium oxide, it is classified as Class F fly ash. The sodium silicate solution (SiO2/Na2O molar ratio = 2.00, ρ = 1.45g/cm3, SiO2 = 26.60%, Na2O = 13.50% and H2O = 60.00%) and the 50 wt% sodium hydroxide solution were mixed as the alkaline activator. The metakaolin from Jiaozuo, Henan province, was added into the fly ash, of which the chemical composition is listed in Table 1.

Table 1 Chemical composition of the fly ash and metakaolin (wt.%)

Chemical composition SiO2 Fe2O3 Al2O3 CaO Na2O K2O SO3 MgO TiO2 MaO LOl

Fly ash 50.44 7.44 31.43 3.47 0.49 0.60 0.39 1.79 0 0 1.17 Metakaolin 49.67 1.32 42.54 0.19 0.68 0.18 0 0 2.14 0.14 2.74 2.2 Experimental procedure

The alkaline activator was prepared by mixing the sodium silicate and sodium hydroxide solutions according to the predetermined proportion corresponding to the different Si/Al and Na/Al molar ratios. The activator was mixed 24 hrs in advance to ensure that the solution reach room temperature and full uniformity. The raw materials and activator were mixed for specific periods shown in Table 2. The pastes were then cast into the 15mm(diameter)×37.5mm(height) cylindrical molds.The samples were stored in the seal bags until they were demolded after 3 days. For the

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samples cured at 80°C, they were cured in the oven at 80°C for 24h and then continued to cure at ambient temperature for the rest of curing periods. The sample matrix is shown in Table 2.

The unconfined compression tests were performed after 3, 7 and 28 days. Three replicates of each sample sets were tested. The microstructure of the geopolymers was obtained with XRD and SEM to understand the relationship among the synthesis conditions, microstructure and mechanical properties.

Table 2 The sample matrix of geopolymers

Sample NO.

Si/Al Molar Ratio

Na/Al Molar Ratio

Content of Metakaolin

(wt%) Curing temperature Mixing

time(min) Objective

1 2.0 1.2 30 Ambient temperature 30 Effect of metakaolin 2 2.0 1.2 40 Ambient temperature 30

3 2.0 1.2 50 Ambient temperature 30 4 2.0 0.9 0 Ambient temperature 3

Effect of Si/Al and

Na/Al Molar ratio

5 2.0 1.0 0 Ambient temperature 3 6 2.0 1.1 0 Ambient temperature 3 7 2.4 1.2 0 80°C 3 8 2.5 1.2 0 80°C 3 9 2.6 1.2 0 80°C 3

10 2.0 1.1 0 Ambient temperature 3 Effect of curing

temperature

11 2.0 1.1 0 80°C 3 12 2.2 1.3 0 Ambient temperature 3 13 2.2 1.3 0 80°C 3

3 RESULTS AND DISCUSSION

3.1 Effect of metakaolin

The compressive strength of the geopolymer samples with different proportions of metakaolin was plotted with the curing time. As shown in Figure 1, except for the 28-day compressive strength of Sample 1, the compressive strength of the geopolymers increased with the increase of metakaolin content. It is reported that the metakaolin mainly consists of amorphous phases that are readily dissolved in alkaline activators and can improve the reaction rate and degree. With the increase of metakaolin content, the gel phase increases in the early stage of geopolymerization, which contributes much to the increase of compressive strength [10].

Comparing Sample 2 with Sample 3, the compressive strength of Sample 2 increased more significantly in the later period. This might be due to the protecting vitreous coating on the surface of fly ash, which was difficult to be activated and resulted in low gelation activity of fly ash and slow reaction in the early stage [10]. However, with the continuous depolymerization-polymerization of vitreous phases in fly ash, the compressive strength increases gradually. Consistently, Sample 2, synthesized from the raw material with a greater fraction of fly ash, obtained a more significant increase of compressive strength in the later curing process. Therefore, the geopolymer with higher metakaolin content (Sample 3) has a higher early strength but lower growth of strength in the longer period. Note that the 3-day compressive strength of Sample 1 is zero, since the samples did not set after the curing of 3 days.

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Figure 1: The compressive strength of the geopolymer samples with metakaolin

3.2 Effect of Na/Al and Si/Al Molar Ratio

Figure 2 presents the compressive strength of the geopolymer samples with the same Si/Al molar ratio but different Na/Al molar ratios after different curing periods. With the increase of the Na/Al molar ratio, the compressive strength at different ages increased. In other words, within a certain range, higher concentrations of sodium hydroxide were beneficial for the strength gain. It was consistent with the finding by Panias D et al.[11]. They reported that the increase of surface protonation (the combination of hydroxyl group on the colloidal surface and hydrogen ion by adsorption) accelerates the dissolution rates of oxides and aluminosilicate phases. In this way, the vitreous silicate and aluminate phases of the raw material would dissolve faster in the solution with a higher concentration of sodium hydroxide, which is the essential procedure for the polycondensation and further development of strength. The similar conclusion was drawn in the study by Steveson M et al.[12], which stated that the Si-O-Si bonds of the raw materials were depolymerized in the alkali circumstances during the reaction process and thus the solubility of the aluminosilicate phases increases, as depicted in the Equation 1.

Si-O-Si + 2NaOH = Si-O-Na+ + Na+-O-Si + H2O (1)

Figure 2: Compressive strength of different Na/Al molar ratios

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The compressive strengths of the three sample sets with different Si/Al ratios cured for different periods are compared in Figure 3. With the increase of Si/Al molar ratio, the overall trend of the compressive strength at every age declined. However, higher content of silicon is theoretically favorable for compressive strength due to the three-dimensional polymeric framework formed with more Si-O-Si bonds, which has a major contribution to mechanical strength [13]. The main reason to the contrary trend in Figure 3 might be that the content of silicon was extremely high, leading to the restriction of the reaction [14]. Moreover, the samples were cured at 80℃ and the reaction conditions were more complicated due to the high temperature, which may give rise to the dimness of the variables, such as the extent of reaction and the moisture evaporation.

Figure 3: Compressive strength of different Si/Al molar ratios

3.3 Effect of curing temperature

According to previous studies, the elevated curing temperature is generally beneficial to Class F fly ash based geopolymers. Consistently, the compressive strength of geopolymer samples cured at the elevated temperature was higher than that cured at ambient temperature at early stages, as demonstrated in Figure 4 and Figure 5. The fly ash dissolved faster at high temperatures, which accelerated the destruction of the vitreous aluminosilicate structures and improved the reactivity of fly ash, and thus the geopolymerization rate.

As shown in Figure 4 and Figure 5, the compressive strength of the geopolymer samples cured at 80℃ slightly decreased in the later period (28 days), which was even lower than that cured at ambient temperature. This might be due to that the elevated temperature accelerated the water evaporation, resulting in higher porosity and micro-cracks in the samples. The rapid curing may also result in incomplete reaction that prevent the formation of more compact structure and the continuous strength increase. Therefore, the elevated temperature is beneficial for the early strength gain, yet might be unfavorable for long-term strength. This agrees well with the results of metakaolin based geopolymers in the study by Rovnaník [15].

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Figure 4: The compressive strength of the geopolymers with Na/Al=2.2 and Si/Al=1.3 cured at

ambient temperature and 80℃

Figure 5: The compressive strength of the geopolymers with Na/Al=2.0 and Si/Al=1.1 cured at

ambient temperature and 80℃

3.4 Effect of fly ash

According to many experiments performed by researchers, the reaction rate and degree of the Class F fly ash for geopolymer synthesis are both low. In this study, various Si/Al and Na/Al molar ratios of geopolymer precursors were attempted. The precursors prepared with several mix proportions, such as Si/Al=2.0 and Na/Al=1.2, flash set within five minutes. The samples listed in Table 2 set in appropriate time period, about 3 minutes. The Class F fly ash based geopolymer set shortly after mixing. However, after the addition of metakaolin, the geopolymers set relatively slower. The major difference of the setting time between fly ash based and fly ash-metakaolin based geopolymers was partially resulted by the nature of raw materials. Due to the layered structure of metakaolin, it might separate the fly ash spheres and thus decelerated the polymerization on the surfaces of fly ash spheres. Additionally, the alkaline activation of metakaolin might be faster than that of Class F fly ash, since the former is more reactive. Therefore, the reaction of fly ash spheres

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was impeded. A larger portion of fly ash would act as lubricant in fly ash-metakaolin based geopolymer precursor than in the fly ash based counterpart. This might also extend the setting time.

Excessive fast set is unfavorable for further development of geopolymeric structures due to the lack of free water and incomplete reaction of raw materials. Once the precursor sets, the dissolution of raw materials was terminated before complete. Therefore, the reactive aluminate and silicate monomers were short for the subsequent polymerization. This is also might be the reason for the decreased long-term strength of the samples cured at elevated temperatures. Further investigation on the reaction mechanism of Class F fly ash based geopolymer remains to be performed.

3.5 SEM analysis

The microstructure of the geopolymer samples was characterized with SEM to reveal the development of geopolymeric microstructure. The SEM images of the geopolymer samples at 7-days age are presented in Figure 6 to Figure 8.

As shown in Figure 6, more microcracks were observed in the samples with high content of metakaolin. However, the microcracks seemed to have little impact on mechanical properties, as shown in Figure 1. A small amount of crystals and many small-sized particles were observed in Sample 1 (30 wt% metakaolin), which might be generated by unreacted activator or incomplete reaction. The geopolymer matrix of Sample 2 (40 wt% metakaolin) and Sample 3 (50 wt% metakaolin) was more homogeneous compared to Sample 1. This is consistent to the higher compressive strength of Sample 2 and Sample 3.

Figure 6: SEM images of fly ash-metakaolin based geopolymers with (a) 30% metakaolin, (b) 40%

metakaolin and (c) 50% metakaolin

The influence of the curing temperature on geopolymer formation is also reflected distinctly in

the SEM images. Figure 7 and 8 showed that the microstructure of ambient temperature cured geopolymer was looser than that cured at 80℃, which should be resulted by the low reaction extent at

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relatively low temperature. This confirmed the conclusion drawn in Section 3.3 that higher curing temperature accelerated the reaction and played a crucial role on the development of the early strength.

Besides, the development of geopolymer seemed more preferable with the raw material composition of Si/Al=2.2 and Na/Al=1.3, based on the comparison between Figure 7 and 8 and that between Figure 4 and 5. Regardless of the curing temperature, the microstructure of Sample 12 and Sample 13 was more compact; and the sample sets with Si/Al=2.2 and Na/Al=1.3 showed higher compressive strength.

Figure 7: SEM images of geopolymers samples (Si/Al=2.0, Na/Al=1.1) cured at (a) ambient

temperature and (b) 80℃

Figure 8: SEM images of geopolymers samples (Si/Al=2.2, Na/Al=1.3) cured at (a) ambient

temperature and (b) 80℃

3.6 XRD analysis

The XRD spectrums of geopolymer samples cured for 7 days are shown in Figure 9 to Figure 12.

As shown in Figure 9, the XRD patterns of the samples with different Si/Al molar ratios were similar. The intensity of the hump between 27°~30° 2θ, representing the formation of geopolymer, slightly decreased with the increase of Si/Al ratios. This indicated that excessive Si/Al ratio may hinder the geopolymerization process, and verify the relatively low strength of the samples with excessively high Si/Al ratios.

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Figure 9: XRD of samples with different Si/Al molar ratio and constant Na/Al molar at 80°C:

(1)quartz, (2)mullite, (3)moganite(SiO2), and (4)gismondine(CaAl2Si2O8•4H2O)

Figure 10 presents the influence of the metakaolin content on the mineralogy of geopolymer. It

is observed that with the increase of metakaolin content, (i) the intensity of the mineral peaks decreased; and (ii) the height of the hump between 27°~30° 2θ increased. It confirms the perspective in the Section 3.1 that since metakaolin was easily dissolved in the alkali activator in the early stage, more gel phase would be formed due to the increased content of metakaolin. The XRD results of the three samples were in consistent with the SEM analysis, indicating that the geopolymeric matrix would be more readily to form in the geopolymers with higher content of metakaolin.

Figure 10: XRD of fly ash-based geopolymers added metakaolin: (1)quartz, (2)mullite,

(3)moganite(SiO2), and (4)gismondine(CaAl2Si2O8•4H2O)

The effect of temperature on the mineralogy of geopolymers was also illustrated with XRD

patterns shown in Figure 11 and 12. Compared to the samples cured at 80℃, many irregular peaks were characterized in the samples cured at ambient temperature, while the hump between 27°~30° 2θ was less observable. This implied the low reaction degree and incomplete dissolution of raw materials of the geopolymer precursors cured at ambient temperature. Consistently, the compressive strength of the geopolymers cured at ambient temperature was lower than the 80℃ cured counterpart (Figure 4 and 5). From this perspective, it is reasonable that the mechanical strengths of Samples

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12(Si/Al=2.2, Na/Al=1.3) were higher than Samples 10 (Si/Al=2.0, Na/Al=1.1), owing to the more complete dissolution of raw materials and higher reaction degree, as compared between Figure 11 and 12.

Figure 11: XRD of the geopolymers (Si/Al=2.0, Na/Al=1.1) cured at different temperature: (1)quartz,

(2)mullite, (3)sodium silicate hydrate(Na2SiO3•9H2O), and (4)moganite(SiO2)

Figure 12: XRD of the geopolymers (Si/Al=2.2, Na/Al=1.3) cured at different temperature: (1)quartz,

(2)mullite, (3)sodium silicate hydrate(Na2SiO3•9H2O), (4)moganite(SiO2), and (5)gismondine(CaAl2Si2O8•4H2O)

4 CONCLUSIONS

The reaction of Class F fly ash based geopolymer can be affected by many factors, including curing temperature, chemical composition (including Si/Al and Na/Al molar ratios) and addition of metakaolin. In order to better understand the complicated reaction process and obtain a desirable mix design, the influence of these multiple factors on the mechanical strength, reaction rate and microstructure of Class F fly ash based geopolymer were investigated in this study.

Based on the comparison among the fly ash-metakaolin based geopolymers with the raw materials containing 30 wt%, 40 wt% and 50 wt% metakaolin, it was found that high content of metakaolin can enhance the early compressive strength, which, however, declined the increase of

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long-term strength. Consistent with the improved early strength, the geopolymers synthesized with higher content of metakaolin showed that their representative XRD pattern was more significant.

The effect of Na/Al and Si/Al molar ratio on the mechanical properties of geopolymers found in this study agreed well with the previous researches. Moderate increase in the Na/Al ratio favored the development of strength. However, excessive Si/Al hindered the reaction, which led to the decrease of the compressive strength.

It was found that elevated temperature has positive effects on the early strength gain of Class F fly ash based geopolymer due to the accelerated reaction, which was confirmed with the XRD patterns of the geopolymers cured at ambient temperature and 80℃. However, the fast water evaporation at the excessive elevated temperature might lead to the formation of more micropores and microcracks in the microstructures, and impeded continuous reaction. This was confirmed with the SEM images of the 80℃ cured geopolymer that contained more microcracks than the ambient temperature counterparts. The microstructures also reached a consensus with the relatively low long-term strength of the samples cured at elevated temperature.

For all the curing conditions, the geopolymers with relatively high compressive strength showed certain consistent characteristics in the micromorphology and mineralogy: (i) the microstructures characterized with SEM were more homogeneous and contained more geopolymeric matrix; and (ii) the representative XRD pattern of geopolymer was more significant and the minerals’ peaks were less intense.

In this study, some problems were still remained in geopolymer synthesis, such as flash set, low strength, and low reaction rate and degree. Further studies are still needed to thoroughly and systematically investigate the reaction mechanism and improve the early and long-term strength Class F fly ash based geopolymers.

5 ACKNOWLEDGEMENTS

Supports from the staffs of mechanical and microstructural laboratories in Dalian Jiaotong University for this research are gratefully acknowledged. The author also would like to express thanks to the graduate student Wenrui Bian of Dalian Jiaotong University for his generous assistance in the experiments.

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