Electrical properties of fluidified Portland cement mixes in the early stage of hydration

The electrical conductivity and the dielectric constant of cement mixes hav e been studied in the initial stages of setting to correlate the time evolu tion of the electrical parameters with the chemical and microstructural mod ifications. The formulations, based on a mix of Portland cement and microsi lica, were fluidified with various amounts of an acrylic polyelectrolite an d had a w/c ratio of 0.24. Measurements were carried out at ambient tempera ture for times of up to 40 h, in the frequency interval 30 Hz-200 kHz. The conductivity showed a marked decrease correlated with the loss of connectiv ity of the macroscopic porosity (depercolation). The time to the drop of co nductivity depended linearly on the concentration of superplasticizer whose retarding effect was clearly evidenced. A dielectric amplification phenome non was observed which vanished after the porosity depercolated. The temper ature profiles of mixtures with different concentrations of the superplasti cizer were drawn and a simple model was developed to compute the specific t hermal power as a function of time. A substantial coincidence was found, on the time axis, between depercolation and maximum rate of setting. Vicat te sts were also carried out. (C) 2000 Elsevier Science Ltd. All rights reserv ed.