Influence of chemical composition and inorganic admixtures on the electrical conductivity of hydrating cement pastes
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F. D. Tamas University ofVeszprem, Institute of Silicate Chemistry and Technology, H-8201 Veszprem, P.O.B. 158, Hungary (Received 22 June 1988; accepted 29 September 1988)
Electrical conductivity of cementitious materials with a wide range of chemical compositions has been studied during the first 24 h of hydration. Relationships between heat of hydration and electrical conductivity curves are discussed. Results are tentatively explained in terms of some physical and chemical parameters. Further evidence that the electrical conductivity of hydrating cement pastes is related to the hydration mechanisms operating in these systems was obtained. The order in which the cations of inorganic admixtures (chlorides and hydroxides) were found to increase the peak and rate of development of the electrical conductivity is the same order that they have been found to increase the heat liberated upon hydration of systems containing these admixtures.
I. INTRODUCTION Conductometric analysis was first applied by Akashi1 to study cement and concrete materials. There has been an increased interest in using this type of analysis due to development of sophisticated impedance analyzers as well as the fact that work by recent authors indicates that it may be a particularly promising technique to help explain the hydration mechanisms of cementitious systems due to the ionic nature of these systems. The measurement of electrical conductance/resistance of cementitious materials was also introduced by Boast,2 Calleja,3'4 and Monfore.5 Recently Vernet et al.,6 Tamas et a/., 7813 McCarter and Curan,9 and Malik et al.10 provided data on electrical conductivity measurements of cement pastes in the setting and early hardening period. These findings can be related to the chemical reactions occurring upon hydrolysis of the various compounds as well as to physical properties of cement-based materials.15"17 The majority of researchers found only one maximum on the conductograms; however, Tamas7 observed a second maximum on the conductance vs time plot: the regular one, shortly after mixing, and a second, transient maximum after 3-24 h (depending on temperature, the presence of admixtures, etc.). Later it was shown813 that this transient maximum occurs only if the gypsum (or SO3) content of the cement is within certain limits. A part of Tamas' results7 has been reproduced later by Malik et al.10 as well. Studies done by Calleja4 suggested that there is a relationship between the electrical resistance development and the temperature change in cement paste samples. Also hydration reaction mechanisms and setting processes have recently been related to electrical response of cement pastes by Afshar and McCarter12 and by Vernet et al6 J. Mater. Res., Vol. 4, No. 1, Jan/Feb 1989
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In the present work heat of hydration curves for various cement pastes prepared with hydraulic cements with different chemical composition have been determined in an attempt to relate the electrical conductivity curves to the
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