Ionically Conductive Mortar for Electrical Heating
Authors: Ruohong Zhao, Christopher Y. Tuan, Daobo Fan, An Xu, and Bao Luo
ACI Materials Journal November/December 2017
Newswise — This paper describes how the development of an innovative conductive composite, ionically conductive mortar (ICM), is produced, and how research and testing demonstrate in what ways the resistivity of ICM is influenced by the curing time, type, and concentration of electrolyte solution and materials of electrodes. The effectiveness of applying an epoxy resin coating to sustain ICM low resistivity is also evaluated.
Electronically Conductive Concrete
Electrically conductive concrete is a composite material made by adding electrical conductive materials to the traditional cement-based mixture. It has multiple applications, such as snow melting and deicing, indoor heating, health monitoring, and electromagnetic shielding.
The most commonly used conductive concrete materials include steel fibers, steel shavings, carbon fibers, and carbon black. Research indicates that ICM has some advantages over the traditional conductive concrete including uniform dispersion and less degradation of conductive materials.
Testing and Results
Previous research showed that ions could directionally migrate in cement-based composites, making the composites electrically conductive. Most of these studies focused on reducing the amount and sizes of the voids inside the composites to reduce permeability and increase compactness. Ionically conductive mortar, however, is designed to harness the free moisture available in the mortar by increasing the amount and sizes of the voids to enhance electrical conductivity. For this purpose, studies included the use of aluminum powder to generate tiny gas bubbles to increase the porosity of specimens. After curing, the specimens were soaked in electrolyte solutions for 96 hours to saturation. The saturated specimens were coated with a thin layer of epoxy resin, as shown in Fig. 1.
To ensure conductivity of ICM, the number of free ions and moisture content in the specimen must stay relatively constant. Different types and concentrations of electrolyte solution were shown to affect the number of free ions. Test results showed that the conductivity of ICM would increase with the increase of ion concentration in the electrolyte solution. However, the ionic conduction is limited by the solubility of the chemicals; different electrolyte solutions have varying ranges of mass fractions for achieving minimum resistivity of the conductive mortar. Moisture content in the specimens must be sustainable to achieve low resistivity of the conductive mortar. A thin epoxy coating is effective in preventing moisture evaporation from the surface. Test results showed that the resistivity of the coated specimens was much lower than that of the uncoated specimens, as illustrated in Fig. 2.
Heating tests showed that the ICM had a satisfactory electrical heating performance, and that it could achieve a heating rate of 19.7oC (35.5oF) in 120 minutes under 30V AC. Also, the heating performance of ICM improved with increasing applied voltage.
The research can be found in a paper titled “Ionically Conductive Mortar for Electrical Heating” published by ACI Materials Journal.
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