Sulfate Attack
Q Why does sulfate attack (ettringite and thaumasite formation) primarily affect cement or cement/lime mortars rather than hydraulic lime mortars?
A The aluminate phases in natural hydraulic lime are different to those in Portland cement due to the lower burning temperature. Lime is burnt with minimal liquid phase which would generally require a temperature approaching 1300oC. At this lower temperature any aluminate forms gehlenite, C2AS, which has low reactivity to hydration or further reaction. Very little of the more reactive tri-calcium aluminate, C3A, forms below 1300oC, certainly less than would be present in sulfate-resisting Portland cement. In the cement kiln the alumina goes into the melt above 1300oC and C3A then forms as the liquid phase solidifies on cooling.
Ettringite, calcium-aluminate-sulfate-hydrate, forms from reactive C3A, hence the minimal quantity in hydrating hydraulic lime systems exposed to sulfate ions.
Thaumasite, calcium-silicate-sulfate-carbonate-hydrate, although not containing aluminate, has a structure almost identical to that of ettringite and it is thought that ettringite is an intermediary, or at least has a seeding effect, in the formation of thaumasite (Taylor, Cement Chemistry). Hence little thaumasite will be found in hydrating hydraulic lime systems