Dielectric behaviour of hardened cement paste systems

Abstract

The hydration of Portland cement involves initial dissolution of CaO, CaSO4.1H20 or CaSO4-2H20, alkaline contents and aluminate phases etc. This is followed by the reaction of C3S and C2S with free water and formation of C-S-H, calcium hydroxide (CH), ettringite and other minor compounds, which bridge the individual particles and fill up the capillary pores. These reactions promote formation of a rigid microstructure and strength development. The stability of C-S-H depends on the variation of environmental conditions as evidenced by changes in morphology, interfacial regions, pore structure and intrinsic strength. These characteristics can be studied by means of impedance or conductance measurements. Electrical methods are considered as a nondestructive, rapid way to determine the physical properties, such as strength, porosity and permeability. Electrical conductivity measurement (ECM) [14] and a.c. impedance spectroscopy (ACIS) [5-15] have been used in cement and concrete hydration investigations. Information on hydration behaviour and microstructural development was obtained. These studies indicated that certain parameters obtained from ECM and ACIS are very sensitive to microstructural characteristics of cement paste and concrete. Dielectric constant measurement (DCM) is one of the most popular methods of characterizing solid materials because it is easier to apply than chemical analysis techniques. The electrical, as well as physical and structural, characteristics of hydrated cement systems can be evaluated. There is a paucity of data [16-20] reported on the dielectric properties of cement and concrete, especially at RF frequencies. In this study, the dielectric properties of hardened cement paste at various water/cement ratios were investigated over the frequency range from 1 MHz to 1 GHz. This preliminary work revealed that the dielectric constant value of hydrated cement paste varies indirectly with original water/cement ratio. Type 10 Portland cement was used. The chemical composition (wt%) was as follows: SiO 2 19.83; CaO 61.21; Fe203 3.20; A1203 4.18; MgO 4.09; SO3 3.93; Na20 0.45; K20 0.82. Fresh cement was mixed at various water/cement ratios from 0.3 to 0.7 and cast in cylindrical moulds, 3 cm diameter × 6 cm. The samples were cured in 100% relative humidity environment for 24 h and subsequently immersed in saturated lime solution for 1 year. Cement paste samples were sliced to form 3 mm thick discs and