Effects of the concentrated NH4NO3 solution on mechanical properties and structure of the fly ash based geopolymers

The geopolymers based on fly ash were exposed to the concentrated (6 M) NH4NO3 solution for 540 days. The highest compressive strength decrease occurred in the initial 28 days testing period. Study of the effects of the NH4NO3 solution on the geopolymer structure indicated leaching of alkali and alkaline earth cations and breaking of –Si–O–Al– bonds in aluminosilicate gel. The reduction of compressive strength of the geopolymers was associated with the structural defects in the aluminosilicate gel created by breaking of –Si–O–Al– bonds. Subsequent fluctuation in compressive strength was a result of the structural defects reparation by silicon.

► Effects of 6 M NH4NO3 solution on geopolymers strength and structure were studied.
► Initial compressive strength decrease was followed by stagnation or slight increase.
► Leaching of alkali and alkali earth cations from the geopolymer structure.
► Breaking of –Si–O–Al– bonds in aluminosilicate gel.
► Defects in the structure were re-occupied by silicon.