3.3.1 Mechanical Property of Geopolymers
In order to investigate and figure out the mechanical property of the geopolymer specimens, the effect of the curing time, sodium hydroxide (NaOH), Fly Ash (F.A.), and gypsum (Gy) on the Unconfined Compressive Strength (UCS) were performed based on some selected geopolymers matrix. Furthermore, Table 4 summarizes the samples that have the higher values of UCS of the different series.
188.8.131.52. Effect of Curing Time on Mechanical Strength
The role of curing time on the compressive strength (UCS) has been investigated with the proportion of F.A. (0%, 25%, 50%, 75%, 100%). Fig. 5 shows the result of curing time on the compressive strength for FMS-0xa geopolymer specimens at 10M NaOH. These mixtures were cured for different curing times, which are 7, 14, and 28 days. Only the FMS-05a mixture with 0% of F.A. decreased with the increase of curing time. The trends of FMS-04a with 25% of F.A. which to pass the days will decrease gradually with the increase of curing time but not completely to zero compared to the FMS-05a mixture. that showed the F.A. particles have high reactivity and play a great role in the mechanical properties. The lack of data in compressive strength of the geopolymer specimens with 100% mine tailing (M.T.) was reported in many research that there is no geopolymerization. Additives like fly ash, gypsum were utilized in geopolymerization processes to consolidate mine tailings (Rao and Liu 2015).
Figure 5 summarizes the specimens cured at room temperature whose compressive strength values increase with the cured time. A longer curing period increases the polymerization rate resulting in higher compressive strength. The results show that longer curing periods did not lower the compressive strength of geopolymer concrete as declared (Jaarsveld et al. 2002). These will be the best candidates to perform the toxicity study to determine the encapsulation capacity of toxic metals.
184.108.40.206 Effect of Fly Ash and NaOH Concentration on Mechanical Strength
Figure 12(a-b) displays the result of Unconfined Compressive Strength (UCS) of fly ash-based geopolymer specimens cured at room temperature for seven days with various F.A. proportions (0%, 25%, 50%, 75%, 100%) and NaOH concentration at 5M and 10M. This figure shows that both F.A. and NaOH contribute strongly to the increase of strength. Higher the amount of F.A. and NaOH is greater than the UCS value, as demonstrated in another study (Zhang et al. 2011). This is due to higher O.H. or sodium oxide content during the geopolymerization reaction, as claimed by Zhuang et al. There are reactions and condensation between fly ash and alkaline reagents. The outcome, Si4+, and Al3+, combined with complex crystallization, oligomerization, and polymerization, yields a new aluminosilicate-based polymer with a novel amorphous three-dimensional network structure (Zhuang et al. 2016). The Si/Al ratio played a great role in the geopolymerization as the main precursor. Furthermore, as demonstrated by Zhang et al. (L. Zhang et al., 2011), the elevated of the Unconfined Compressive Strength (UCS) with the proportion of F.A. is owing to the Si/Al ratio of the MT/FA mixture and its reactivity. Generally, a low Si/Al ratio is preferable for a good geopolymerization (Rangan et al., 2014) and should be within 1-3 (Xu et al., 2003; Zhang et al. 2011; Rangan et al., 2014).
220.127.116.11. Effect of Gypsum (G) on Mechanical Strength
In order to investigate the effect of gypsum (G) on the Unconfined Compressive Strength (UCS), three series of geopolymers specimens (FMS-0x, FGMS-0x, and FGMS-0xa) were performed with different proportions of F.A. (0%, 25%, 50%, 75%, and 100%) cured for seven days at 5M NaOH with Gypsum that range at 0g, 10g, and 20g. Fig. 12 (c) shows the role of gypsum with different proportions of fly ash on the compressive strength. It can be seen that the strength of the geopolymers with gypsum increase compare with those without G, which means adding gypsum can improve the geopolymerization as claimed (Boonserm et al., 2012). The highest peak of strength was gained by adding 10g of G. This increase happened because of the entering of Ca2+ in the bond Si-O-Al-O and equilibrating the charge Al ions (Fernández-Jiménez et al. 2006), that contributes strongly to the formation of CSH, aluminosilicate structure and lead to the improvement of compressive strength (Boonserm et al. 2012). Further, the trend drops down with the increased amount of G (Rattanasak et al., 2011).
3.3.2 Microstructure and Microchemistry of Geopolymers
In order to characterize and evaluate the microstructure and microchemistry of fly ash-based-geopolymer matrix and figure out the connection between the microstructure and compressive strength of geopolymers, SEM imaging, XRD diffractometer, and FTIR spectra of selected geopolymers specimens were evaluated.
Finally, it can be noted that one of the most important factors that contribute to the mechanical strength is the water ratio used. Basically, the lower this ratio is, the greater the final UCS strength will be. The geopolymer specimen series (FMS-0xa) with 0.27 of Liquid/Solid ratio has recorded the highest values in terms of UCS strength which vary between 1.7 and 14,075 MPa. The compressive strength also depends on the curing time. The UCS test of the geopolymer showed slight changes in compressive strength after 28 days, but no changes have been recorded after 56 days(Badur and Chaudhary 2008). When sodium silicate was utilized, the presence of silica retards the zeolite formation rate. As a result, first, greater strength values are gained at lower degrees of reaction. The fly ash activated by an alkaline solution can have a greater compressive strength with greater zeolite content. (Lloyd, 2009)
18.104.22.168 SEM Imaging Analysis
The SEM imaging was carried out to figure out the effect of aging periods, NaOH concentration, and gypsum on the microstructure of geopolymers. The SEM micrographs of different geopolymer specimens are displayed in Figs. 7-10.
In order to understand the effect of aging time on the microstructure of the geopolymer specimens, SEM imaging of FGMSW-3b matrix was carried out and cured under room temperature condition after 7, 14, and 28 days with 100% F.A., 20g of Gy at 10 M of NaOH concentration (Fig. 7). Fig. 7 provides a comparative analysis of SEM micrographs of various geopolymers effectively treated at varying periods at a moderate optical zoom. According to this figure, there is very little modification in the microstructure of geopolymers after seven days, implying that curing time has little influence on the microstructure, as also stated by another study. (Zhang et al. 2011).
To figure out the effect of sodium hydroxide (NaOH) on the microstructure of geopolymer specimens (FMS-02 and FMS-02a) cured under room temperature after 14 days curing period with 100% F.A. at different concentrations of NaOH (5M and 10M), the SEM imaging was investigated. Fig. 8 depicts the various modifications shown in the micro-structural of the geopolymers. Fig. 8 (a) showed the existence of F.A. in a negligible amount as the concentration of NaOH increases, discussing the function of NaOH in polymerization. Fig. 8 (b) indicates the existence of F.A. in a massive portion as the concentration of NaOH increases, which also clarifies the function of NaOH in geopolymers. Further analysis revealed that at 15M NaOH, the particles of F.A. have been almost non-existent, indicating that the increased the NaOH concentration, the faster the geopolymerization rate. At 15M, the geopolymer gel was much more cohesive and thicker than that at 5 and 10M, yet geopolymerization was quite substantial at 10M (Zhang et al. 2011). It also confirms the compressive strength (UCS) results (Fig. 12 (a)), which show that the maximum UCS value was acquired to 10M NaOH.
The effect of sodium silicate (Na-silicate) on the microstructure was evaluated, by comparing the specimen FMS-01 activated with only NaOH and FMSW-01 activated with NaOH and Na-silicate both cured at ambient temperature, at 5M NaOH with 100% F.AAs shown in Fig. 9 (c) and (d), there is still a notable change in structural system between all these images; Fig. 9 (d) is more compact than Fig. 9 (c), attributed to the existence of sodium silicate in that sample at a 1.08:1 ratio. Once sodium hydroxide (NaOH) and sodium silicate (Na-Silicate) are mixed to make an alkali solution, the blending has superior mechanical properties than NaOH alone(Palomo et al. 1999); the very same research claimed that sample only with NaOH has a porous material compared to one provided both with NaOH and Na-Silicate, which has a higher density structure.
To understand the effect of gypsum on the microstructure of the geopolymer matrixes (FMS-02, FGMS-02, and FGMS-02a) at low magnification, the SEM analysis was performed with 75% F.A. at 5M NaOH at different content of gypsum: (a) 0g, (b) and (c) 20g 10 g of gypsum cured for seven days curing under ambient air condition. The scanning electron microscopy (SEM) geopolymer matrixes (FMS-02, FGMS-02, and FGMS-02a) are displayed in Fig. 10. This figure reveals that the opacity of these three geopolymers varies significantly. The sample of FGMS-02 with 10g of Gy in Fig. 10 (b) is much more condensed with low permeability than that of the other geopolymers; nevertheless, there is much more unreacted F.A. in the FMS-02 geopolymer with 0g of Gy in Fig. 10 (a) than the other geopolymers, which clarified the function which gypsum performed in terms of microstructures and mechanical properties by boosting the polymerization rate. In addition, these SEM imaging results justified the Unconfined Compressive Strength results (Fig. 12. (c)) that the UCS values increased with the addition of 10g of Gy. This performance could be attributed to the combination of mine tailings (low reactivity) to fly ash (high reactivity) and gypsum, which concurred with the research finding of Xiaolong et al., which claimed that mine tailings are often crystalline, leading to low reactivity throughout geopolymerization therefore; as a result, products with minimal compressive performance. As a result, adding extra elements with higher reactivity to mine tailings-based geopolymers can efficiently tune and optimize their characteristics. (Xiaolong et al. 2021). Furthermore, because most of the chemicals included in this function are manufacturing wastes, its use has additional benefits for the environment. Strong-containing calcium compounds have such a higher favorable impact on geopolymer durability than lower-containing calcium. This is due to the development of extra CSH gels, which, when combined with NASH, increases structural integrity, as previously reported by Xiaolong et al.,.(Xiaolong et al. 2021)
22.214.171.124 XRD Analysis
X-ray fluorescence spectrometry has been used to determine the elemental composition of the geopolymers using the XRD-6100 diffractometer and the XRD patterns have been analysed through JADE 6.0 Software. Because of its amorphous or nanocrystalline nature, the N-A-S-H gel formed during polymerization is difficult to characterize with XRD. Nonetheless, the XRD patterns of the specimens were used to determine the crystalline formation in the different mixture designs of the fly ash-based geopolymer presented with high compressive strength. (FMS-01a (100% F.A.), FGMS-02 (75% F.A.), FGMSW-02 (75% F.A.), FGMSW-01a (100% F.A.), FGMSW-03b (50% F.A.), synthesized under room temperature conditions cured at 7, 14 and 28 days is shown in Fig. 6, where different phases have been obtained. Where C= Corundum Al2O3, Cc= Calcite CaCO3, G=Gypsum CaSO4 2H2O, M= Mullite Al6Si2O13, Q= quartz SiO2, Ss=Sodium silicate Na2(SiO3), Sh=Calcium Silicate hydritade Ca1.5Si0.5 xH2O, Ch= Chabazite, J= Jadeite, A=Anhydrite CaSO4.
Mullite and quartz, which have been discovered in raw fly ash, were observed throughout all samples. All of the enabled samples exhibited amorphous ridges focused around 220° to 30°, including all samples, confirmed the formation of a geopolymer gel. (Keyte et al., 2009a). Except for the FMSW-05 sample, that does not show the geopolymerization since mine tailing is the only element in its composition. Singh et al., in their research, found a similar observation (Singh et al., 2018). Numerous crystalline structures, including quartz and mullite, have been regarded as non-reactive, even though their reaction speed in alkali-silicate solutions is remarkably slower when compared to inorganic materials.(Keyte et al., 2009b) in some of the samples as FGMS-02 and FGMSW-03, could identify de chabazite, which is one of the crystal that can encapsulate the heavy metals such as Cu and Pb (Jun et al., 2015).
Calcite, CaCO3, is formed once calcium hydroxide reacts to carbon dioxide in the atmosphere; calcium solubility at elevated pHs has been well recognized to decrease due to the instability of calcium hydroxide forming(Komnitsas and Zaharaki 2009), his mineral was found in the majority of the samples.
Crystallization amorphous gels were the subtler shown in fly ash-derived geopolymers. Compared to the fly ash instance, much less of the binder is gradually morphed into zeolite stages. Furthermore, variables that promote zeolite forming, like increasing the alkalis of the binder, increasing strength, and reducing any proclivity for strength loss, at least for the duration considered. The creation of zeolites, including chabazite, was recognized through XRD analysis in most specimens. Other minerals discovered included calcite and quartz, which were linked to the presence of zeolites.
126.96.36.199 FTIR Analysis
The FTIR of the five best specimens in terms of compressive strength with various materials was investigated. The FTIR spectra of the five (5) geopolymers (FMS-01a (100% F.A.), FGMS-02a (75% F.A.), FGMSW-02(75% F.A.), FGMSW-02(75% F.A.), and FGMSW-03b (50% F.A.), containing a different proportion of Fly Ash and sodium hydroxide concentration (5M, 10M) cured at 7, 14 and 28 days are shown in Fig. 11. Table 5 shows the Infrared characteristic bands identified in F.A. and geopolymers specimens. In-plane and bending vibrations of Al-O / Si-O, 460cm−1, and 550 cm-1 are assigned. The existence of a 1456cm-1 band may be due to C= O vibrations, which indicate the presence of carbonate bands. In addition, the I.R. spectra studies show bands close to 1016 cm−1 and 1143 cm−1 due to asymmetric Si-O stretching while banding at 458 cm−1 to SiO4 bending in-plane Si-O. However, bands between 772, 579, 537, and 439 cm−1 correlate with Al-Si minerals. In addition, new bands emerging at 3423, 1638, and 1540 cm−1 are consistent with-OH, H2O bending and asymmetric carbonate stretching, respectively (Ismaiel Saraya and El-Fadaly 2017). The peak appeared at 1640 cm−1 as a result of bending H-O-H vibration, and the intensity of this peak increased with a rise in NaOH concentration, suggesting a rise in geopolymerization degree (Devi and Saroha 2016).