Ceramics-Silikáty 43, (2) 48 - 51 (1999)

The aim of the present paper is to demonstrate that by using frequencies > 1MHz, it is possible to get information about phenomena at the interface between cement and water, and therefore to follow hydration in a cement based material at the early stages. Disks of Portland type cement was prepared with water to cement weight ratio, w/c, equal to 0.4. The samples cured for 10 hours at 20 °C and 50 % relative humidity were tested electrically at 10, 14, 16, and 18 hours after mixing. The experimental technique was based on high frequency measurements (1 MHz to 1.8 GHz) with a 4291 A Hewlett-Packard apparatus. This frequency range is useful to follow phenomena at the cement / water interfaces during hydration or, equivalently, hardening of the cement based material. The complex permittivity as a function of frequency was studied in relation to the time after mixing. The progressive passage of water from an electrolytic form to hydrated phases was identified. It corresponds to the relaxation of the real part of the permittivity at 2.2 MHz and approximately 1 MHz, respectively. A Cole-Cole type distribution, taking into consideration the degree of heterogeneity, α (with 0 < α < 1), of the material, was proposed to fit the experimental data points. It is shown that a increases from 0.37 to 0.48 as the hardening time goes from 10 to 18 hours.