Calcium sulfates take an important role in the reaction of pozzolanic materials by forming ettringite with aluminum ions from the pozzolan. 1 Sulfate carriers like gypsum, bassanite, or anhydrite are used to retard the tricalcium aluminate (C 3 A) reaction in ordinary portland cement (OPC). 2 The latter mechanism is used to avoid flash set and to optimize strength development by preventing a superposition of the silicate and aluminate clinker reaction. 3 As calcined clay blended cements 4 or limestone calcined clay cements (LC 3 ) 5 are currently among the most promising solutions to address CO 2 emissions in cement industry 6 , their influence on cement hydration in general and on the aluminate reaction in particular is gaining attention. In calcined clay blended cements, where additional aluminum sources are present, awareness has been raised for extra sulfate demand, as the

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