The role of aluminium in C-A-S-H during sulfate attack on concrete
For a recent study tests with C-A-S-H phases in Na2SO4 solutions (3 and 30 g/L SO4) were carried out to study changes in the C-A-S-H nanostructure during sulfate attack on concrete.
Durung the studies, particular attention was paid to the availability of aluminium in C-A-S-H for ettringite formation. Ettringite only formed above the saturation pH of portlandite.
The removal of aluminium from C-A-S-H decreased the Al/Si ratio and mean chain length (MCL) of the C-A-S-H phases. No ettringite formed in solutions below the saturation pH of portlandite. Here, decalcification of the C-A-S-H phases resulted in an increase in MCL.
Controlled by the availability of calcium
The formation of ettringite from aluminium in C-A-S-H exposed to Na2SO4 solutions is controlled by the availability of calcium. Sodium is adsorbed on deprotonated silanol groups in C-A-S-H. At 3 g/L SO4, the SO42− ions are adsorbed with calcium on deprotonated silanol groups, but at 30 g/L SO4, they are also bonded to other sites in C-A-S-H.
The study has been published in Cement and Concrete Research, Volume 116.
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