Influence of Curing Regimes on Engineering and Microstructural Properties of Geopolymer-Based Materials from Water Treatment Residue and Fly Ash

Geopolymerization is a new method for treating water treatment residue (WTR) from water purification plants to reduce the amount of stored land in urban areas. Polymeric bond formation depends on the curing conditions. In this study, the curing conditions suitable for subsequent treatment to save energy consumption and production costs in the future application were investigated. The WTR had a high aluminosilicate content with low alkaline activity, so fly ash (FA) was added to FA and WTR mixtures in the ratio of 40 and 60 weight percent (% in wt.), respectively. The moisture content of the mixtures ranged in 12–15%, suitable for semi-dry pressing to form pellets. After this formation, the geopolymer samples were cured under different conditions (room temperature, microwave oven, in dryer at 110°C, and in autoclave with hydrothermal condition). The experimental results showed that the hydrothermal samples had better properties, such as pH<9, high stability of mechanical strength over 3.5 MPa, and soft coefficient over 0.75. The microstructural properties were investigated using modern analytical tools, such as XRD, SEM, FTIR, and NMR, to detect the chemical functional groups of the aluminosilicate networks in the geopolymer matrix and the close relationship among the properties and its microstructure.

Dryer; Fly ash; Geopolymer-based materials; Hydrothermal; Microwave oven; Water treatment residues

The geopolymerization process involves chemical reactions of aluminosilicates (Al³⁺ with coordinate number 4) with alkaline polysilicate networks for the formation of Si–O–Al bonding (Davidovits, 2020). The geopolymerization products have three-dimensional structural frameworks with the chemical formula as follows:

M_n[-(SiO₂)_z-AlO₂]_n.wH₂O

where M is the metal cations of K⁺, Na⁺, Ca²⁺, and others, n is the degree of condensation, and z = 1, 2, 3, or >> 3. The cations of Na⁺, K⁺, Ca²⁺, and others equalize the anions of [AlO₄]^5? and [SiO₄]^4?. The chain and ring structures are formed and networked together based on Si–O–Al sialate bridges (Davidovits, 2020). The geopolymer-based materials were developed based on the new bonding circuits of Q3 and Q4, as known from the reports of nuclear magnetic resonance (NMR) analyses (Giannopoulou and Panias, 2007; Davidovits, 2020; Rim et al. 2020).

The geopolymer structures are the tetrahedra [SiO4]^4? surrounded by four, three, two, one, or non-tetrahedra [AlO4]^5? (Davidovits, 2020; Rim et al., 2020). The bonding circuit forming units are denoted as Q4(nAl) with n = 0, 1, 2, 3, and 4 corresponding to the number

of tetrahedra of [AlO4]^5? or Q represents the [SiO4]^4? tetrahedra. In the ²⁹Si nuclear magnetic resonance spectrum (²⁹Si NMR), Q4(4Al) peaked at 83–88 ppm, Q4(3Al) at 88–94 ppm, Q4(2Al) at 91–98 ppm, Q4(1Al) at 96–105 ppm, and Q4(0Al) at 103–113 ppm (Rim et al. 2020).

Geopolymer-based materials with amorphous phases can transform into crystals known as zeolites when they are supplied suitable energy under known conditions of high temperature and pressure (Zhang et al., 2008). The XRD patterns of geopolymer-based materials with amorphous structural frameworks changed at similar diffraction peaks of zeolite crystals (Davidovits, 2020; Sudibandriyo and Putri, 2020).

Rapid urbanization impedes the use of large land areas to contain water treatment residues (WTR) from water purification plants. Finding new solutions to manage and treat this waste is imperative. Geopolymerization is a new method developed in the last decade to treat waste, such as WTR, from water purification plants (Luukkonen et al., 2019; Numanuddin et al., 2021).

The products based on WTR have very low strength (Waijarean et al., 2013; Geraldo et al., 2017) due to the weak alkaline activity of WTR. Therefore, WTR treatment should be combined with materials of high activity, such as FA (Suksiripattanapong et al., 2015 Horpibulsuk et al., 2016; Janani and Santhi, 2018; Susanto et al., 2020), metakaolin (Geraldo et al., 2017), rice husk ash (Waijarean et al., 2014; Poowancum et al., 2015), and others (Nguyen, 2020a; Nguyen, 2020b; Do et al., 2020a; Do et al., 2020b; Nguyen, 2021, Petrus et al., 2021).

When the samples are cured in an autoclave under high pressure and temperature conditions, the alkaline activators dissolve SiO₂ and Al₂O₃ faster. The H₂O molecules supply [OH]^? to enter into the structures of Na₂O, SiO₂, and Al₂O₃; hence, this is called hydrothermal process. Finally, at atmospheric pressure, the water is removed from the structures to form polymer bonds in the geopolymer-based material by drying (Alas and Ali, 2019; Nguyen, 2021; Nguyen and Dang, 2021; Nguyen et al., 2021a; Nguyen et al., 2021b).

Swanepoel and Strydom (2002) suggested that the optimal condition for strength development of geopolymer samples is at 60°C for 48 h. Palomo et al. (2004) concluded that geopolymer samples based on fly ash (FA) were cured at 65°C for one week or 85°C for 24 h, which had enough energy to achieve levels of reactions and strength responding to the engineering requirements. However, when the temperature was over 60°C, the strength of specimens decreased. Adam and Horianto (2014) revealed that the curing temperature range of 80–120°C for 20 h had better strength than that of curing for 4 and 6 h. The geopolymer-based materials cured in microwave conditions are a new method in a short time from 1 to 7 min. The microwave conditions accelerate the reaction and hardening processes while participating in water elimination reactions (Do et al., 2020a).

The mechanical strength of geopolymer-based materials increases with an increase in alkali concentration from 4 to 10 M (Prasanphan et al., 2019). In the concentration range from 10 to 18 M, the strength of the geopolymer gradually decreased (Alonso and Palomo, 2001). This parallels the experimental results of Mustafa et al. (2012), which revealed increased strength of the geopolymer specimens that used alkali solution from 6–12 M, while the geopolymer samples that used alkali concentration from 12–16 M decreased in strength. The pH values in 13–14 are best for forming geopolymeric networks with good mechanical properties. The ratios of M₂O/ SiO₂, SiO₂/Al₂O₃, H₂O/M₂O, and M₂O/ Al₂O₃ (M is Na or K) range from 0.2 to 0.48, 3.3 to 4.5, 10 to 25, and 0.8 to 1.6, respectively (Nguyen, 2020a).

Geopolymers from WTR have many potential applications in building materials, water and wastewater treatment, including adsorbents/ion exchangers, membranes and filter, catalysts, stabilizers of water and WTR (Luukkonen et al., 2019), pH regulators (Novais et al., 2016; Ascensão et al., 2017), and others. The geopolymer materials are considered the materials of the 21^st century due to their zero CO₂ emissions in the manufacturing process and their high mechanical strength.

Therefore, in this study, experiments were conducted to produce geopolymer-based materials using industrial solid wastes of WTR and FA. The products were cured under various conditions to determine their engineering properties. Also, the pH values of the geopolymer were tested to assess their impact on the environment. The microstructural properties of the materials were characterized and evaluated using the analytical tools of X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FTIR), scanning electron microscope (SEM), and nuclear magnetic resonance spectroscopy (MNR) to confirm polymeric bond formation in the geopolymer-based material.

       The geopolymer samples were formed through the pressing process of the roller press and cured in various conditions of the drier, microwave, autoclave (hydrothermal), and at room temperature. The results showed that the compressive strength of all samples exceeded 3.5 MPa, responding to the requirements of TCVN 6477-2012 Vietnamese standard. The various curing conditions affected the engineering properties of the geopolymer samples, such as volumetric weight and mechanical strength. The samples cured at room conditions had slow geopolymerization efficiency after 28 days, and they failed for the softening coefficient. The geopolymer samples cured at 110°C using drier and microwave significantly increased in mechanical strength. The samples cured by hydrothermals using autoclaves had high stability on engineering properties and a high-softening coefficient of up to 90%. This demonstrates that geopolymer-based materials have high water resistance when they are cured using the hydrothermal method using autoclave. Moreover, the geopolymer samples cured using the hydrothermal method also had pH values less than 9 because the group of [AlO₄]^5? kept the Na⁺ cations better than the [SiO₄]^4? group based on the results of the ²⁹Si NMR spectrum. Thus, the more Q₄(Al) bonds are, the higher the ability to hold Na⁺ cations in the aluminosilicate structures of geopolymer. However, the reactions of Na⁺ ions with CO₂ filled open pores, increasing the volumetric weight and mechanical strength of the geopolymer samples cured in different conditions. Further research is required to investigate the roles of [SiO₄]^4? and [AlO₄]^5? in reactions with alkaline activators using the analytical tools of NMR, SEM, FTIR, and XRD.

     The authors would like to thank Ho Chi Minh City University of Technology (HCMUT), VNU-HCM for their timely support and facilities for conducting this study.

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