Application of Geopolymer Composites in Wastewater Treatment: Trends, Opportunities, and Challenges

Geopolymer Chemistry

Geopolymer is a class of largely X-ray amorphous aluminosilicate materials, generally synthesised at ambient or slightly elevated temperature by reaction of a solid aluminosilicate powder with a concentrated alkali metal silicate or hydroxide solution (Provis et al., 2005). This inorganic material was created by the French scientist Joseph Davidovits in 1970s. The aim of its creation was to develop a fire-resistant alternative to organic polymers in the aftermath of various catastrophic fires in France between 1970 and 1972.Since the period following the work carried out by Wastiels et al. (1994), the application of geopolymer shifted to uses in construction as green alternative material to ordinary Portland cement. Nowadays, its plethora applications include: fire resistant materials, decorative stone artefacts, thermal insulation, low-tech building materials, low energy ceramic tiles, refractory items, thermal shock refractories, bio-technologies (materials for medicinal applications),foundry industry, cements and concretes, composites for infrastructures repair and strengthening, high-tech composites for aircraft interior and automobile, high-tech resin systems, radioactive and toxic waste containment, arts and decoration, cultural heritage, archaeology, history of sciences and adsorbent material for water treatment. The wide variety of potential application gives rise to referred name. In literature, geopolymer is described by different terminologies such as mineral polymer, inorganic polymer, inorganic polymer glasses, alkali ash material, soil cement etc.

Mechanism of Formation of Geopolymer

The formation of geopolymer involves the mixing of solid precursors (aluminosilicates sources) such as metakaolin, fly ash, volcanic ash, slag etc with alkaline activator solution (alkali hydroxide and metasilicate). After the mixing, multi steps of reactions occur namely:

• 1.

  Dissolution of glassy aluminosilicates;

• 2.

  Reorganization and diffusion of dissolved ions with formation of small coagulated structures;

• 3.

  Polycondensation to form aluminosilicate gel phases; and

• 4.

  Solid state transformation and hardening to form hard solid.

The aforementioned geopolymerization reaction steps are summarized in Figure1 (van Deventer et al., 2007).

Figure 1.

Schematic outline of the reaction processes involved in geopolymerisation

Structure of Geopolymer

The structure of geopolymer materials are similar to zeolites, but the microstructure of a geopolymeric material is a three-dimensional silico-aluminate amorphous or semi-amorphous structure instead of crystalline (Provis et al., 2005; Xu, 2002). The geopolymer network consists of SiO₄ and AlO₄ tetrahedra linked alternately by sharing all the oxygens. Positive ions (Na⁺, K⁺, Li⁺, Ca²⁺, Ba²⁺, NH₄⁺, H₃O⁺) must be present in the framework cavities to balance the negative charge of Al³⁺ in IV-fold coordination. The poly(sialate) is designation of geopolymers based on silico-aluminates where sialate is an abbreviation for silicon-oxo-aluminate. The empirical formula of poly(sialates) is Mn{-(SiO₂)z-AlO₂}n, wH₂O which M is a cation such as potassium, sodium or calcium, and «n» is a degree of polycondensation; «z» is an integer 1, 2, 3 (Davidovits, 2017). Geopolymers comprise the following molecular units:siloxo (-Si-O-Si-O-), sialate (-Si-O-Al-O-), sialate-siloxo (-Si-O-Al-O-Si-O-), sialate-disiloxo (-Si-O-Al-O-Si-O-Si-O-),ferro-sialate(-Fe-O-Si-O-Al-O-Si-O-). Figure 2 is a representation of the structure of geopolymer as proposed by Davidovict in 1994 (Davidovits, 1994).

Figure 2.

Structure of geopolymer proposed by Davidovits