Geopolymer-based composites

Geopolymers (GPs) are a class of inorganic polymeric, X-ray amorphous materials consisting of alumina, silica, and alkali metal oxides. They are typically synthesized by mixing an aluminosilicate source (i.e., such as amorphous metakaolin or fly-ash) with amorphous silica dissolved in highly caustic alkaline (MOH) solutions, where M is an alkali cation.^1–11 GPs can be produced over a wide range of chemical compositions, centered on the molar ratios of M₂O . Al₂O₃ . xSiO₂ . yH₂O, where M is an alkali metal cation (typically, Na ^þ , K ^þ , Cs ^þ ), x ¼ 2–6, and y ¼ 7.5–13.^6,11 GPs are inorganic polymers that undergo (1) dissolution of the aluminosilicate source, (2) polycondensation, and (3) precipitation under ambient temperature. The reaction is mildly exothermic at B651C. Stoichiometric geopolymer can be fabricated as schematically shown in Fig. 1. When crystalline clay of composition (Al₂O₃ . 2SiO₂ . 2H₂O) is heated to 7001C for 2 h it decomposes to amorphous metakaolin (Al₂O₃ . 2SiO₂) which is ideally in the form of a powder. “Water glass” or alkali metasilicate solution is highly caustic containing 1 M NaOH, KOH, or CsOH dissolved in 10 M H₂O to which are added 2 M amorphous SiO₂ which dissolves completely forming a clear solution. The solution is highly viscous for Na metasilicate, but less viscous for K metasilicate and even less for Cs metasilicate solution. The amount of water varies depending on particle size and specific surface area of the aluminosilicate source (metakaolin). Too....