The dielectric response of compacted cementitious composites T. H. Panzera*, P. H. R. Borges†, J. C. Rubio‡, C. R. Bowen§, K. Strecker* and A. L. R. Sabariz* University of Sa ˜o Joa ˜o del-Rei, Brazil; Monier Technical Centre Ltd, UK; University of Minas Gerais, Brazil; University of Bath, UK This paper investigates the influence of processing parameters on the dielectric constant of compacted cementitious composites based on ordinary Portland cement and silica particles. A full factorial design was carried out to study the effects of size and shape of silica particles and uniaxial compaction pressures of 10 and 30 MPa on the dielectric constant and the apparent porosity of the composites. Silica particle size and compaction pressure were determined to be significant factors affecting the apparent porosity. The dielectric constant of the cementitious composite is significantly influenced by a combination of particle size, shape and compaction pressure. Introduction Alternating current (AC) impedance spectroscopy (IS) is a powerful non-destructive technique used for studying the hardening of cement-based materials. De- spite being used for ceramic materials since the 1950s (MacDonald, 1987), it was in the early 1990s that McCarter and Brousseau (1990) first used the AC impedance arc at high frequency for the characterisa- tion of cement pastes and established the correlation between AC response and hydration of hardened ce- ment paste. Later studies (Gu et al., 1993; Xu et al., 1993a) showed that the main factors determining the impedance of cement-based systems are the concentra- tion of ions in the pore solution, total porosity and pore size distribution of the material. For cement paste with a given water to cement (w/c) ratio, the ionic concen- tration governs the AC response in the first hours of hydration, when the impedance is low due to high porosity combined with a high concentration of ions. Later, it is expected that the impedance increases with decreasing porosity and pore size. For pastes with dif- ferent w/c ratios, the dielectric constant decreases as the w/c ratio increases, confirming its relation to the variation in the solid phase fraction by volume (Andrade et al., 1999). Over recent years there has been some interest in the evaluation of the AC impedance and determination of the dielectric constant for low-porosity cement-based composites. Xu et al. (1993b) studied low-porosity cement pastes containing silica fume and concluded that the appearance of the high-frequency impedance arc is, as anticipated, a consequence of the high pres- sure and low porosity associated with silica fume addi- tion. Hu et al. (1999) studied the dielectric properties of dense systems (so called DSP cements) made with a fixed w/c ratio of 0 . 4 and concluded that the humidity of the sample (i.e. the drying conditions) strongly af- fects the dielectric constant. MacPhee et al. (1996, 2000) studied the impedance of pore reduced cements (PRCs); these are cement pastes prepared with a w/c ratio of 0 . 35, which are pressed to 200 MPa in a pore fluid squeezer after 3 h of hydration. The main advan- tage of those systems is the high electrical resistance (compared with that of conventional Portland cement pastes) within the accessible frequency range of stan- dard laboratory instrumentation. Recent research has proposed industrial and commer- cial applications of high-dielectric cement composites Advances in Cement Research, 2010, 22, No. 3, July, 127–134 doi: 10.1680/adcr.2010.22.3.127 127 www.cement-research.com 1751-7605 (Online) 0951-7197 (Print) # 2010 Thomas Telford Ltd * Department of Mechanical Engineering, University of Sa ˜o Joa ˜o del-Rei (UFSJ), Campus Sto Anto ˆnio, Prac ¸a Frei Orlando 170, 36 . 307 . 352, Sa ˜o Joa ˜o del-Rei, Minas Gerais, Brazil † Monier Technical Centre Ltd, Sussex Manor Business Park, Gatwick Road, Crawley, West Sussex, RH10 9NZ, UK ‡ Department of Mechanical Engineering, University of Minas Gerais (UFMG), Av. Anto ˆnio Carlos 6627, Campus Pampulha, 31 . 270 . 901, Belo Horizonte, Minas Gerais, Brazil § Materials Research Centre, Department of Mechanical Enginee- ring, University of Bath, Bath, BA2 7AY, UK (ACR 800034) Paper received 22 August 2008; last revised 6 July 2009; accepted 23 November 2009