The impact of alumina availability on sulfate resistance of slag composite cements

• Al availability is lower in slag cements, with more bound to stable hydrates.
• Ca demand for ettringite formation is lower in slag cements.
• AFt levels plateau after 90 days of attack. Only neat cement damaged.
• Damage in slag blends occur after Ca leaching. Damage mechanism differs from neat cement.
• Insufficient Ca and Al in unreacted slag for future AFt growth (years).

Slag cement pastes prepared with either 40% or 70% of slags by weight were prepared (W/B = 0.5) and subsequently exposed to a 3 g L−1 Na2SO4 solution. The slag cements were shown to be more resistant. Initially, ettringite levels rose, then plateaued upon carbonate AFm consumption, although monosulfate was also found to be in equilibrium with ettringite when using an Al-rich slag. Portlandite was initially consumed, to form ettringite, but leached out after prolonged attack until it was fully depleted, or nearly, with subsequently C-A-S-H being decalcified. Any excess aluminium released by the slag was bound to a calcium deficient C-A-S-H phase and hydrotalcite, sheltering the aluminates from ingressing sulfates. Mass balance further revealed that, if the slag in the blends were to fully dissolve to form C-A-S-H and hydrotalcite, there would have been insufficient aluminium and calcium to combine with sulfates to form ettringite. Rather, calcium from C-A-S-H would have continuously leached, leaving behind a silicate skeleton.