Geopolymers amorphous

Na MAS NMR has also been used to study the structure and sodium environment in amorphous sodium aluminosilicate geopolymers, showing that the charge-balancing Na" " is present in a highly hydrated form (Barbosa et al. 2000). When the geopolymer is heated to > 1200°C, the sodium ions lose their hydration water, as evidenced by a shift in the position of the Na resonance from- 5.5 ppm to — 19 ppm, but the amorphous nature of the material is retained. 

The aluminum atom in this coordination is negatively charged and charge compensated by cations from the alkaline activation solution, such as Na, K, or by artificially introduced cations such as Li ", Ca, Ba ", NH4, or H30 [9,10]. According to the original nomenclature [3], the amorphous to semicrystaUine structure of geopolymers further consists of poly(sialate) (-Si-O-Al-O- PS), poly(sialate-siloxo) (-Si-O-Al-O-Si-0- PSS), and poly(sialate-disiloxo) (-Si-O-Al-O-Si-O-Si-O- PSDS) building blocks (Fig. 2). 

Figure 3 Graphical representation of the synthesis of amorphous (Am-Geo) and nanocrystalline (Nano-Geo) geopolymers [26].

Rational design and optimal synthesis of aluminosilicate systems with controlled physicochemical properties (sorption, separation and/or catalytic activity, etc.) require profound knowledge of the framework structure, consisting of extra-framework cations and surface structural defects [46,47]. However, the comprehensive description of the amorphous and semicrystalline structures of geopolymers is inherently nontrivial therefore, advanced techniques of structural and physicochemical analysis must be applied in combination with the traditional approaches. 

Figure 31 MAS NMR (black lines), Si H CP/MAS NMR (red (light gray in the print version) lines) and Si H REDOR (blue (gray in the print version) lines) NMR spectra recorded for amorphous and semicrystalline geopolymers, left and right, respectively.

Figure 35 H- H MAS NMR correlation spectra of amorphous and semicrystalline geopolymers measured with a mixing period of 25 ms. The spectra were measured before A and C) and after hydrothermal treatment (B and D), respectively. Reprinted with permission from [i 16], Copyright, institute of Chemicai Technoiogy, 201 h...

The A1 H REDOR-reference and REDOR-dephased 3Q/MAS NMR spectra of a typical amorphous geopolymer [26] are depicted in Fig. 36. The main Al i resonance is nearly unaffected by Al dipolar... 

Figure 36 The ISO-sheared AI H REDOR-dephased and REDOR-reference 3Q/MAS NMR spectra of amorphous geopolymer (black and red (light gray in the print version) contours, respectively). The REDOR spectra were recorded using 21 dephasing cycles. Reprinted with permission from [26], Copyright American Chemical Society, 2012.

Recently, much has been done in the design and optimization of reaction protocols that allow for the controlled synthesis of chemically identical geopolymers that differ in the tendency to form crystalline fractions in the amorphous matrix [20,26,117]. The amount of crystalline phases, protocrystalline domains, and various structural defects, however, must be strictly controlled because their excess can lead to a loss of the required mechanical and physicochemical properties. This requirement is particularly important if waste products, such as colored kaoHn, slag, or fly-ash, are considered for use as input material [118-125]. 

Figure 37 Overlay of solid-state Al and Si MAS NMR spectra of X-ray amorphous geopolymers. Reprinted with permission from [31]. Copyright, John Wiley and Sons Ltd, 2013.

M. Urban ova, L. Kobera,J. Brus, Factor analysis ofAl-27 MAS NMR spectra for identifying nanocrystalline phases in amorphous geopolymers, Magn. Reson. Chem. 51 (2013) 734-742. 

Inorganic polymers, also known as geopolymers, are aluminosilicates, conventionally prepared by condensation of a solid aluminosilicate such as the dehydroxylated clay mineral metakaolinite with an alkali silicate solution under highly alkaline conditions. At ambient temperatures these materials set and harden to an X-ray amorphous product containing solely tetrahedral A1 and tetrahedral Si characterised by a broad Si MAS NMR resonance at about -92 ppm. The most recent Na MAS NMR studies have provided more details about the way in which the charge-balancing alkali ions are incorporated in the structure. 

However, grinding itself requires an energy input, making chemical pre-treatment a more energetically attractive option. Pre-treatment of the clay with IM NaOH converts a significant proportion of the 6-fold coordinated Al to 4-coordinated, and when subjected to the conventional geopolymer synthesis, produces a material that sets this is, however, not X-ray amorphous, but contains crystalline carbonated phases. ... 

In an alternative synthesis method that does not involve a solid aluminosilicate reactant as the Al source, sodium aluminate solution is reacted with alkali silicate, formed in situ by reaction of SiO with the appropriate alkali hydroxide . The Na product sets at 40°C with a crushing strength of 26 MPa, is X-ray amorphous and contains solely 4-coordinated Al. Its Si MAS NMR spectrum indicates the presence of some unreacted Si02 in addition to the typical Si resonance of a geopolymer . This method has been applied to the synthesis of other analogues of aluminosilicate geopolymers such as the galliogermanates, where suitable solid precursors are unavailable. 

This method has been suceessfully used to synthesise a lithium aluminosilicate geopolymer, which is difScult to prepare by the conventional method because of the poor solubility and weak alkalinity of LiOH solution. In this solid stale synthesis , halloysite clay was reacted with LiOH at S50°C. When wetted, the product of this solid state reaction sets at 40°C to a compound containing solely 4-fold coordinated A1 and a typical Si MAS NMR spectrum (Figure 5), but is not fully X-ray amorphous . When heated at 900 C, this geopolymer crystallises to o-eucryptite, LiAISi04 but if additional quartz is added, heating at 1300 C for 8hr causes the crystallization of p-eucryptite and P-spodumene (LiAlSi206) . 

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