Understanding the mechanism controlling the sulfate requirement of blended cements is crucial for a successful deployment of these technologies towards a more sustainable industry. Particularly, clarification regarding the role of alumina content on the sulfate requirement is essential to establish suitable procedures to achieve proper sulfation in practice. In this study, the role of alumina content on the sulfate requirement of cementitious systems is investigated in PC, blends of PC and slag and LC3-type systems. It is observed that adsorption of sulfate on surfaces, mainly C-(A)-S-H, is the overwhelming factor controlling the sulfate demand of these systems. Disso-lution of alumina from slag and metakaolin and adsorption of sulfate in other surfaces seem to have only a minor effect on the overall sulfate requirement. Furthermore, these effects are not correlated with the total amount of alumina present in the SCM used.