Multiple techniques were applied to the investigation of the phase composition of the deteriorated surface of Portland-limestone cement concrete specimens exposed for 10 years to conditions promoting thaumasite sulfate attack. The results were interpreted in terms of the limestone content of cement and the incorporation of supplementary cementitious materials (SCMs) (natural pozzolana, blastfurnace slag, metakaolin). Thaumasite, detected in high amounts in concrete specimens made without SCMs, was associated with Al to some extent. Considerable degradation of calcium silicate hydrate occurred in all specimens, resulting in the formation of Al-associated cross-linked silicate chains, magnesium silicate hydrate and silica gel, a process most effectively prevented by metakaolin. For the highest limestone content in cement or use of natural pozzolana, thaumasite underwent extensive decomposition. Al incorporation into cross-linked silicate chains increased with increasing limestone content in cement. Longer aluminosilicate chains with larger Al fractions were observed in concrete specimens incorporating SCMs.