Adsorption of superplasticizers in fly ash blended cement pastes and its rheological effects

The adsorption of superplasticizers in fly ash blended cement paste and its rheological effects were investigated. It is shown that the absorption of superplasticizer on portland cement particles is very different from that on fly ash particles. The fly ash particles have smooth surfaces and are negatively charged, so its adsorption capacity is weaker than the portland cement particles. The amount of adsorbed SP in the fly ash blended cement paste depends highly on the replacement proportion of portland cement with fly ash, and to a much less extent on the nature of the fly ash. However, the amount of adsorbed superplasticizer does not correspond well the ζ-potential of the solid particles, due the strong adsorbing capacities of the Portland cement particles. When fly ash replaces portland cement in the paste, the rheological behavior is radically changed, which is closely related to the fineness and density of the ash. The packing and agglomeration of the solid particles are the controlling factors on the rheological parameters of the fresh paste, instead of the amount and type of adsorbed superplasticizer.