• International Journal of Technology (IJTech)
  • Vol 12, No 4 (2021)

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

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

Title: Influence of Curing Regimes on Engineering and Microstructural Properties of Geopolymer-Based Materials from Water Treatment Residue and Fly Ash
Nguyen Hoc Thang, Bui Khac Thach, Do Quang Minh

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Cite this article as:
Thang, N.H., Thach, B.K., Minh, D.Q., 2021. Influence of Curing Regimes on Engineering and Microstructural Properties of Geopolymer-Based Materials from Water Treatment Residue and Fly Ash. International Journal of Technology. Volume 12(4), pp. 700-710

Nguyen Hoc Thang Faculty of Chemical Technology, Ho Chi Minh City University of Food Industry, 140 Le Trong Tan Street, Tay Thanh Ward, Tan Phu District, Ho Chi Minh City, 700000, Viet Nam
Bui Khac Thach Faculty of Materials Technology, Ho Chi Minh City University of Technology (HCMUT) – Vietnam National University (VNU), 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, 700000, Viet
Do Quang Minh Faculty of Materials Technology, Ho Chi Minh City University of Technology (HCMUT) – Vietnam National University (VNU), 268 Ly Thuong Kiet Street, Ward 14, District 10, Ho Chi Minh City, 700000, Viet
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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:


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 21st 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.


Adam, A.A., Horianto, A., 2014. The Effect of Temperature and Duration of Curing on the Strength of Fly Ash Based Geopolymer Mortar. Procedia Engineering, Volume 95, pp. 410–414 

Alas, M., Ali, S.I.A., 2019. Prediction of the High-Temperature Performance of a Geopolymer Modified Asphalt Binder using Artificial Neural Networks. International Journal of Technology, Volume 10(2), pp. 417–427

Alehyen, S., Achouri, M.E., Taibi, M., 2017. Characterization, Microstructure and Properties of Fly Ash-Based Geopolymer. Journal of Materials and Environmental Science, Volume 8(5), pp. 1783–1796

Alonso, S., Palomo, A., 2001. Alkaline Activation of Metakaolin and Calcium Hydroxide Mixtures: Influence of Temperature, Activator Concentration and Solids Ratio. Materials Letter, Volume 47(1–2), pp. 55–62

Ascensão, G., Seabra, M.P., Aguiar, J.B., Labrincha, J.A., 2017. Red Mud-Based Geopolymers with Tailored Alkali Diffusion Properties and pH Buffering Ability. Journal of Cleaner Production, Volume 148, pp. 23–30

Davidovits, J., 2020. Geopolymer Chemistry and Application. 5th Edition, Institut Géopolymère, France     

Do, Q.M., Ngo, P.M., Nguyen, H.T., 2020a. Characteristics of a Fly Ash-Based Geopolymer Cured in Microwave Oven. Key Engineering Materials, Volume 850, pp. 63–69

Do, Q.M., Nguyen, V.U.N., Nguyen, H.T., 2020b. Development of Refractory Synthesized from Waste Ceramic Fiber and Chamotte. Journal of Polymer & Composites, Volume 8(2), pp. 101–109

Duxson, P., Fernández-Jiménez, A., Provis, J.L., Lukey, G.C., Palomo, A., van Deventer, J.S.J., 2007. Geopolymer Technology: The Current State of the Art. Journal of Materials Science, Volume 42(9), pp. 2917–2933

Duxson, P., Provis, J.L., Lukey, G.C., Separovic, F., van Deventer, J.S.J., 2005. 29Si NMR Study of Structural Ordering in Aluminosilicate Geopolymer Gels. Langmuir, Volume 21(7), pp. 3028–3036

Geraldo, R.H., Fernandes, L.F.R., Camarini, G., 2017. Water Treatment Sludge and Rice Husk Ash to Sustainable Geopolymer Production. Journal of Cleaner Production, Volume 149, pp. 146–155

Giannopoulou, I., Panias, D., 2007. Structure, Design and Applications of Geopolymeric Materials. In: 3rd International Conference on Deformation Processing and Structure of Materials, Belgrade, Serbia, pp. 5–15

Horpibulsuk, S., Suksiripattanapong, C., Samingthong, W., Rachan, R., Arulrajah, A., 2016. Durability Against Wetting–Drying Cycles of Water Treatment Sludge–Fly Ash Geopolymer and Water Treatment Sludge–Cement and Silty Clay-Cement Systems. Journal of Material Civil Engineering, Volume 28(1), doi.org/10.1061/(ASCE)MT.1943-5533.0001351

Janani, S., Santhi, A.S., 2018. Multiple Linear Regression Model for Mechanical Properties and Impact Resistance of Concrete with Fly Ash and Hooked-End Steel Fibers. International Journal of Technology, Volume 9(3), pp.526–536

Lee, M.G., Yi, G., Ahn, B.J., Roddick, F., 2000. Conversion of Coal Fly Ash into Zeolite and Heavy Metal Removal Characteristics of the Products. Korean Journal of Chemical Engineering, Volume 17(3), pp. 325–331

Luukkonen, T., Heponiemi, A., Runtti, H., Pesonen, J., Yliniemi, J., Lassi, U., 2019. Application of Alkali-Activated Materials for Water and Wastewater Treatment: A Review. Review in Environmental Science and Biotechnology, Volume 18, 271–297

Mustafa, A.B.A.M., Kamarudin, H., Hussain, M.B., Rafiza, A.R., Zarina, Y., 2012. Effect of Na2SiO3/NaOH Ratios and NaOH Molarities on Compressive Strength of Fly-Ash-Based Geopolymer. ACI Materials Journal, Volume 109(5), pp. 503–508

Nguyen, H.T., 2020a. Geopolymerization: A Review on Physico-Chemical Factors Influence to the Reaction Process. Journal of Polymer & Composites, Volume 8(2), pp. 128–137

Nguyen, H.T., 2020b. Novel Porous Refractory Synthesized from Diatomaceous Earth and Rice Husk Ash. Journal of Polymer & Composites, Volume 8(2), pp. 128–137

Nguyen, H.T., 2021. Microstructure Stability and Thermal Resistance of Ash-Based Geopolymer with Sodium Silicate Solution at High Temperature. International Journal of Engineering Research in Africa, Volume 53, pp. 101–111

Nguyen, H.T., Dang, T.P., 2021. Fly Ash-Based Geopolymer: Green Material in Carbon-Constrained Society. Solid State Phenomena, Volume 321, pp. 65–71

Nguyen, H.T., Dang, T.P., Pham, M.T., Le, V.Q., 2021a. Engineering Properties of Alkali-Activated Materials Produced from Thu Duc Water Plant Waste Sludge, Fly Ash, and Alkaline Activator by Semi-dry Pressing Method. Journal of Polymer & Composites, Volume 9(1), pp. 39–48

Nguyen, H.T., Nguyen, V.P., Do, Q.M., 2021b. Effects of Curing Time to Engineering Properties of Alkaline-Activated Materials Synthesized from Thu Duc Water Plant Waste Sludge, Fly Ash, and Geopolymer Aggregate. Materials Science Forum, Volume 1029, pp. 111–117

Novais, R.M., Buruberri, L.H., Seabra, M.P., Labrincha, J.A., 2016. Novel Porous Fly-Ash Containing Geopolymer monoliths for Lead Adsorption from Wastewaters. Journal of Hazardous Materials, Volume 318, pp. 631–640

Numanuddin M.A., Samarakoon S.M.S.M.K., 2021. Utilization of Industrial By-Products/Waste to Manufacture Geopolymer Cement/Concrete. Sustainability, Volume 13(2), ID 873

Palomo, A., Alonso, S., Fernandez-Jiménez, A., Sobrados, I., Sanz, J., 2004. Alkaline Activation of Fly Ashes: NMR Study of the Reaction Products. Journal of American Ceramic Society, Volume 87(6), pp. 1141–1145

Panias, D., Giannopoulou, I.P., and Perraki, T., 2006. Effect of Synthesis Parameters on the Mechanical Properties of Fly Ash-Based Geopolymers. Colloids and Surfaces A: Physicochemical and Engineering Aspects, Volume 301(1–3), pp. 246–254

Petrus, H.T.B.M., Olvianas, M., Astuti, W., Nurpratama, M.I., 2021. Valorization of Geothermal Silica and Natural Bentonite Through Geopolymerization: A Characterization Study and Response Surface Design. International Journal of Technology, Volume 12(1), pp. 195–206

Poowancum, A., Nimwinya, E., Horpibulsuk, S., 2015. Development of Room Temperature Curing Geopolymer from Calcined Water-Treatment-Sludge and Rice Husk Ash. Calcined Clays for Sustainable Concrete, Volume 10, pp. 291–297

Prasanphan, S., Wannagon, A., Kobayashi, T., Jiemsirilers, S., 2019. Reaction Mechanisms of Calcined Kaolin Processing Waste-Based Geopolymers in the Presence of Low Alkali Activator Solution. Construction and Building Materials, Volume 221, pp. 409–420

Protsak, I.S., Morozov, Y.M., Dong, W., Le, Z., Zhang, D., Henderson, I.M., 2019. A 29Si, 1H, and 13C Solid-State NMR Study on the Surface Species of Various Depolymerized Organosiloxanes at Silica Surface. Nanoscale Research Letters, Volume 14(160), pp. 1–15

Rim, G., Marchese, A.K., Stallworth, P., Greenbaum, S.G., Park, A.H.A., 2020. 29Si Solid State MAS NMR Study on Leaching Behaviors and Chemical Stability of Different Mg-silicate Structures for CO2 Sequestration. Chemical Engineering Journal, Volume 396, doi.org/10.1016/j.cej.2020.125204

Schoonheydt, R.A., Johnston, C.T., Bergaya, F., 2018. Clay Minerals and Their Surfaces. Developments in Clay Science, Volume 9, pp. 1–21

Silva, I.C.E., Gomes, J.C., Albuquerque, A., 2010. Study of the Structural Stability, pH Variation in Water and Influence of the Curing Conditions on Mechanical Resistance of Mineral Wastes Geopolymeric Artificial Aggregates (WGA) as Alternative Materials for Wastewater Treatment Processes. Silesian University Technology, Volume 3, pp. 121–128

Sudibandriyo, M., Putri, F.A., 2020. The Effect of Various Zeolites as an Adsorbent for Bioethanol Purification using a Fixed Bed Adsorption Column. International Journal of Technology, Volume 11(7), pp. 1300–1308

Suksiripattanapong, C., Srijumpa, T., Horpibulsuk, S., Sukmak, P., Arulrajah, A., Du, Y.J., 2015. Compressive Strengths of Water Treatment Sludge-Fly Ash Geopolymer at Various Compression Energies. Lowland Technology International, Volume 17(3), pp. 147–156

Susanto, H., Fahmi, Y., Hutami, A.T., Hadi, Y.T., 2020. Effects of Fly Ash Loading on the Characteristics of PVC-Based Cation Exchange Membranes for Reverse Electrodialysis. International Journal of Technology, Volume 11(3), pp. 544–553

Swanepoel, J.C., Strydom, C.A., 2002. Utilisation of Fly Ash in a Geopolymeric Material. Applied Geochemistry, Volume 17(8), pp. 1143–1148

Waijarean, N., Asavapisit, S., Sombatsompop, K., MacKenzie, K.J.D., 2014. The Effect of the Si/Al Ratio on the Properties of Water Treatment Residue (WTR)-Based Geopolymers. Key Engineering Materials, Volume 608, pp. 289–294

Waijarean, N., Asavapisit, S., Sombatsompop, K., 2013. Strength and Microstructure of Water Treatment Residue-Based Geopolymers Containing Heavy Metals. Construction and Building Materials, Volume 50, pp. 486–491

Zhang, Y., Sun, W., Li, Z., 2008. Infrared Spectroscopy Study of Structural Nature of Geopolymeric Products. Journal of Wuhan University Technology-Materials Science, Volume 23(4), pp. 522–527