Variations of Yield Stress of Fresh Cement Pastes in the Presence of Superplasticizer Highlighted by a Hydration Index

摘要

The Theological properties of fresh cement pastes (FCPs) in the presence of superplasticizer were systematically investigated in this study where influential factors such as mix proportion including the water-to-cement (W/C) ratio and superplasticizer-to-cement (Sp/C) ratio, temperature, and time were discussed. The initial yield stress of FCPs with varied mix proportions and its development over time were tested at 0, 20, and 40℃ to characterize the rheological properties of the pastes. Measurements on the amount of adsorbed superplasticizer, zeta potential, particle size, microscopic observation, and calorimetry were performed to quantify the influences of superplasticizer on the microstructure of FCPs under varied mix proportions and temperatures. Results indicate that the initial yield stress of the pastes and its development over time evidently descend due to the plasticizing effect and retardation effect of superplasticizer. Higher temperature facilitates a sharper rise in the initial yield stress and the growth of yield stress with time while the effects of temperature are weakened in the case of high W/C ratio or Sp/C ratio. Higher temperature results in a larger adsorbed amount and a higher zeta potential, which is believed to result from the accelerated dissolution and hydration rate. The effects of W/C ratio, superplasticizer dosage, and temperature on the development of yield stress over time were integrated into the parameter of relative hydration degree. A generalized model to describe the evolution of yield stress was deduced by introducing the hydration index of relative hydration degree α', which was verified by the experimental results that the yield stress ascended in a roughly linear fashion with 1/(1-α').