The role of aluminium in C-A-S-H during sulfate attack on concrete

Storage 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. 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. 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 SO₄ ²⁻ 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.