Effect of Different Molecular Weights and Chemical Composition of Superplasticizers on the Structural Build-up of Cement Paste Using Dynamic Oscillatory Rheology

Two polycarboxylate ether (PCE) superplasticizers with different molar masses and functionalities were studied in relation to their strain and time-dependant rheological properties. The effect of dosage on cement paste fluidity and structural-build up was of particular interest. Through size exclusion chromatography (SEC) the relative molar mass and polydispersity index (PDI) was determined. Superplasticizer A (SP-A) was found to be three times the molar mass of superplasticizer B (SP-B). The dosage limitations were then determined through a spread flow test. The SP-B, with a lower molar mass, and a backbone functionality of a methacrylate - ester, began to flow at higher dosages and with relatively small changes in dosage, a large impact on the fluidity was notable. Dynamic oscillatory rheology was used to determine structural build-up, as a non-destructive method. The structural build-up of cement paste is a time-dependent phenomenon therefore a time sweep was done. A constant strain and angular frequency, within the linear viscoelastic regime (LVE), was used. The LVE was determined through a series of oscillatory strain sweeps for cement pastes with and without SP-A and B. Thereafter the rate of rigidification (Gridge) and percolation time (tperc) as a function of hydration time was investigated.