The authors mention that “It is also known that the compressive strength of geopolymers decreases sharply when the alkali concentration exceeds an optimum value… due to the presence of residual alkalis in the mass of geopolymers and the formation of water-soluble salts. Some of these alkali ions may also migrate to the surface of the specimens, react with atmospheric CO2 and cause the formation of efflorescence”. But the truth is that happens for any alkali concentration value as it was demonstrated by Skvara et al. (2012) who stated that Na, K is bound only weakly in the nanostructure of the (N,K)–A–S–H gel and is leachable almost completely. This was also reported by Zheng et al (2007) who stated that the alkaline and soluble silicates that are added during the processing cannot be totally consumed during geopolymerization due to the existence of dissolution equilibrium of raw aluminosilicate in alkaline silicate solutions and this causes severe efflorescence of the final geopolymer products and high porosity and permeability due to the movement of alkali together with water to the geopolymer surfaces.

Škvára, F.; Šmilauer, V.; Hlaváček, P.; Kopecký, L.; Cílová, Z. (2012) A weak alkali bond in (N, K)–A–S–H gels: evidence from leaching and modelling. Ceramics – Silikáty 56 (4) 374-382. Zheng D., Van Deventer J.S.J., Duxson P., “The dry mix cement composition, methods and systems involving same”, International Patent WO 2007/109862 A1, 2007.