Based on SEM–EDS observations, the attack
mode of ASR can be described as follows: for the
grains with light to middle alteration degree, the
potassium presence in the depths of grains shows
that the hydroxide ions, which are associated with
the potassium (potash), penetrate inside grains
through nanopores and induce an internal attack of
grains. More grains have altered faces, and there is
a larger potassium content increase. The absence of calcium shows its difficulty to diffuse. The diameter
of the solvated calcium cation is larger than
potassium [4] and may be the cause of the
diffusivity difference. The products of the hydroxide
ion attack on the silica: the sites (SiO5/2) and the
dissolved siliceous anions are charge balanced by
the potassium ions. From the degradation level, the
grain porosity opens and the calcium can penetrate
inside grains. This phenomenon is amplified sometimes
by microcracks of the flint aggregate. The
calcium ions take the place of the potassium ions on
the most accessible sites (SiO5/2) by ionic
exchange. Indeed, the dissociation of Si–O–K
groups is higher than Si–O–Ca groups [10]. The
potassium ions are thus again available to neutralise
newly created and less accessible sites (SiO5/2).
The calcium ions also take part in the siliceous
anions precipitation of the grains core solution to
form C–S–H and/or C–K–S–H. The increase of
potassium and calcium content inside the grains
increases with degradation level.