Preferential adsorption of superplasticizer on cement/silica fume and its effect on rheological properties of UHPC

The adsorption behavior of two comb-shaped superplasticizers (PCEs) containing only carboxyl groups and both carboxyl and phosphate groups on cement/silica fume, and their influence mechanism on rheological properties of pastes with low w/b of 0.2 were investigated. The preferential adsorption of PCEs on the surface of different powders (e.g. cement and silica fume) was observed. The PCE containing only carboxyl groups adsorbed more on the surface of cement, while phosphate-containing PCE adsorbed more on the surface of silica fume at low dosage and exhibited stronger adsorption capacity both on cement and silica fume at high dosage. For cement paste, PCE containing only carboxyl groups was suggested to improve flow spread. However, as silica fume was introduced, the blends of the two PCEs could achieve higher packing density of particles in paste due to the excellent dispersion of phosphate containing-PCE to silica fume, resulting in higher flow spread (lower yield stress) and lower viscosity.