Influence of plasticizer type on hydration kinetics of self-compacting concrete

In self-compacting concrete (SCC), the role of superplasticizer is crucial for rheological reasons. Besides, the superplasticizer (SP) can influence the hydration reaction kinetics. In this study, the effect of the superplasticizer on the hydration reaction kinetics is investigated using cement pastes composed of CEM I 52.5 N from two different suppliers, two types of polycarboxylic ether (PCE) based superplasticizer and fly ash as mineral addition. To define the early-stage hydration reaction kinetics, the heat of hydration was monitored using isothermal calorimetry and the setting time was determined using automated Vicat needle measurements. The similar types of PCE superplasticizers (SPX and SPY) have different charge density of the backbone. SPY has the highest charge density and also the solid content of its dispersion is higher. Adding both SP dispersions in equal concentration (as % cement mass) to the cement pastes, either or not containing fly ash, the following findings are established for the Vicat measurements: the initial setting time increases, while increasing the concentration of each SP; the initial setting time is higher for 1% of SPY compared to 2% of SPX; the setting time interval increases disproportionally, compared to initial setting time, for 2% of SPY. Isothermal calorimetry measurements of the same pastes have lead to the following observations concerning the SP dosage: similar retardation of initial setting as in the Vicat measurements is observed; the height and the width of the hydration peaks decreased when more SP was added, suggesting a diminished formation of the AFt-phases. TGA measurements confirm the prolonged precipitation of Ca(OH)2 and therefore, reaction of the SP with Ca2+ ions in pore solution might be expected. In that way, the estimated retardation can be linked to the charge density of the SP. Other mechanisms concerning SP-ion interaction were regarded feasible to explain the observed retardation behavior. The obtained differences in retardation behavior of the same type of cement from different suppliers have been attributed to the cement characteristics: The cement with the larger fraction of fine particles has shown an earlier initial hydration. The cement with the highest calculated SO3/C3A ratio clearly showed a more explicit hydration peak, attributed to AFt formation. The addition of fly ash showed an acceleration of the hydration reactions, favoring the arguments for a more homogeneous distribution of nucleation sites provided by fly ash.