One-Part Geopolymers and Aluminosilicate Gel-Zeolite Composites Based On Silica: Factors Influencing Microstructure and Engineering Properties

Acid resistance of alkali-activated materials: recent advances and research needs

“…It has been described in the previous section that leaching of non-framework cations (mainly alkali ions) precedes the dissolution of the aluminosilicate framework; the term 'congruent' is used here to denote a mechanism in which breaking of Si-O-Al (and potentially Si-O-Si) bonds in the AAM releases monomeric and oligomeric silica species together with the Al into solution, as opposed to a mechanism where leaching of Al leaves behind a coherent silica framework. Such a dissolution-reprecipitation mechanism is not only indicated by the above analytical results, but would also be predicted for the following reasons (see also the more detailed discussions in [16,22,28]): (1) The removal of Al from an aluminosilicate network with Si/Al ≤ 2, which is typical of the N-A-S-H and K-A-S-H gels of low-Ca AAMs, releases silica monomers and small silica oligomers, i.e., it leads to the almost complete destruction of the silicate network [13]. However, larger silica polymers may form as well, particularly at low pH and in the presence of bivalent cations [29].…”