PVA- and PEG-assisted sol-gel synthesis of aluminosilicate precursors for N-A-S-H geopolymer cements

This study investigates critical factors affecting polymer-assisted sol-gel synthesis of synthetic aluminosilicate powders that can be alkali-activated to produce a sodium-stabilized aluminosilicate hydrate (N-A-S-H) geopolymer cement. More specifically, a 2² factorial experiment was conducted to elucidate the influence of polymer architecture (ie, poly(ethylene glycol) (PEG) vs poly(vinyl alcohol) (PVA)), polymer content (ie, low vs high ion-to-polymer-oxide (I/O) atomic ratio), and sol-gel aging pH conditions (ie, low vs high) on the atomic structure of resultant synthetic aluminosilicate powders and geopolymer cements. Molecular structure was investigated using solid-state (single-pulse and ¹H cross-polarization) ²⁹Si and ²⁷Al nuclear magnetic resonance (NMR) and Fourier-transform infrared (FTIR) spectroscopy. The mineralogy of geopolymer cements was assessed with X-ray diffraction and compared to alkali-activated metakaolin-based cements of equivalent stoichiometry. Results demonstrate that polymer architecture (PEG vs PVA) is a key factor in producing (a) undesirable phase segregation (ie, γ-alumina) and (b) incomplete dehydroxylation (ie, vicinal silanol) in synthetic aluminosilicate powders. More specifically, PEG-derived aluminosilicate powders yield partial dissolution and produce geopolymer cements with variable silicate incorporation. Contrastingly, PVA-derived aluminosilicate powders produce geopolymer cements with identical mineralogy to that of metakaolin-based geopolymer cements and exhibit both Brønsted-acid sites near the aluminum nuclei and geminal silanol groups. Sol-gel aging pH conditions reveal the ability to influence the hydroxyl group content, which is an important factor affecting the durability of cementitious materials. Lastly, three plausible mechanisms of metal complexation are hypothesized to permit incorporation of solubilized metal ions via a polymer-assisted sol-gel process.