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Cement pastes containing short steel fibers, which contribute to electron conduction, exhibit positive values (up to 68 ␮V/°C) of the absolute thermoelectric power. Cement pastes containing short carbon fibers, which contribute to hole conduction while the cement matrix contributes to electron conduction, exhibit negative or slightly positive values of the absolute thermoelectric power. The hole and electron contributions in carbon fiber reinforced cement paste are equal at the percolation threshold. Addition of either steel or carbon fibers to cement paste yields more reversibility and linearity in the variation of the Seebeck voltage with temperature difference (up to 65 °C).

I. INTRODUCTION

Thermoelectricity refers to the phenomenon in which electricity (i.e., a voltage) is generated by a temperature gradient. This phenomenon is known as the Seebeck effect. It also refers to the reverse phenomenon (known as the Peltier effect), in which electricity (i.e., a current through a junction between two dissimilar materials) causes heating or cooling. The Seebeck effect is the basis of thermocouples for temperature sensing. The Peltier effect is used for heating and cooling. This paper is focused on the Seebeck effect, which is scientifically the most basic aspect of thermoelectricity, as its occurrence does not require the presence of a junction. Thermoelectric materials are conventionally metals and semiconductors, as they cannot be electrical insulators. Cement, such as Portland cement, is conducting, although it is far less conducting than metals. Cement reacts with water through hydration (curing) and forms cement paste (a solid), which is the matrix material in concrete (with fine and coarse aggregates) and mortar (with fine aggregate only). Since concrete is a widely used material for buildings and construction, and energy saving and thermal control are important for most structures, it is desirable to exploit the thermoelectric behavior of concrete. Therefore, this paper addresses the thermoelectric behavior of cement paste. The electrical resistivity of cement paste can be decreased by orders of magnitude by the addition of short conducting fibers (such as steel and carbon fibers) to the cement mix.1–9 In the same vein, the thermoelectic power of cement paste can be greatly changed by the addition of short conducting fibers.10,11 As shown in this paper, plain cement paste is n-type. The addition of carbon fibers contributes to hole conduction10,11; the addition of steel fibers contributes to electron conduction (this work). 2844

http://journals.cambridge.org

J. Mater. Res., Vol. 15, No. 12, Dec 2000 Downloaded: 15 Mar 2015

Hence, the addition of carbon fibers causes the thermoelectric power to be less positive or more negative (depending on the amount of carbon fibers added), whereas the addition of steel fibers causes the thermoelectric power to be more positive (as high as 68 ␮V/°C), as reported in this paper. The high value of the thermoelectric power attained by adding steel fibers makes steel-fiber-re