Dispersing performance of superplasticizers admixed to aged cement

•Ageing of cement and binary mixtures of C3A and gypsum was studied.•Effect of aged cement on dispersing performance of different PCEs was investigated.•Dispersing effectiveness of PCEs can decrease or improve in aged cement.•Effect is controlled by agglomeration and progressing ettringite formation.•Concentration and kind of polymorph of C3A determines the effect of ageing on PCE. A mechanistic study using fresh and aged binary mixtures of C3A (cubic and orthorhombic) and gypsum revealed that the interaction and hence dispersing performance of superplasticizers in aged cement is controlled by two competing processes, one being agglomeration and caking of the binder particles resulting in a decreased specific surface area, and the other process being continuous formation of hydration products, especially of ettringite, which leads to an increase of surface area. The results suggest that the interaction of superplasticizers with aged cement can be positive or negative, depending on the composition of the cement relative to C3A, with negative effects being most prevalent.