Use of fly ash as an admixture for electromagnetic interference shielding
Introduction
Electrical utilities in the United States generate 80 million tons of fly ash as a by-product each year, primarily from coal combustion [1]. Fly ash is typically disposed in landfills, but it is preferred to convert fly ash to a construction material [1], [2]. The use of fly ash as an admixture or as a cement replacement results in improved resistance to alkali–silica reaction, sulfate attack and corrosion of steel reinforcement, partly due to the refinement of the pore structure, the higher retention of alkalis, the formation of secondary calcium silicate hydrate, and the lower content of free Ca(OH)2[3], [4], [5], [6]. Moreover, it results in increased abrasion resistance [7], decreased carbonation depth [8], reduced heat evolution during curing [9], and enhanced chemical resistance [10], [11]. This article provides a new application for fly ash, namely, the use of fly ash as an admixture for enhancing the electromagnetic interference (EMI) shielding.
EMI shielding [12], [13], [14], [15] is in critical demand due to the interference of wireless (particularly radio frequency) devices with digital devices and the increasing sensitivity and importance of electronic devices. Shielding is particularly needed for underground vaults containing power transformers and other electronics that are relevant to electric power and telecommunication. It is also needed for deterring electromagnetic forms of spying.
Polymer–matrix composites containing electrically conductive fillers are widely used for the shielding of electronics. In contrast to a typical polymer matrix, which is electrically insulating, the cement matrix is slightly conductive. Therefore, the use of a cement matrix allows some degree of electrical connectivity among the conductive filler units, even when the filler volume fraction is below the percolation threshold. As electrical connectivity helps shielding [16], the cement matrix is attractive for the development of composite materials for shielding.
Electrically conductive admixtures in the form of discontinuous fibers [17], [18], [19] have been previously used in cement for providing EMI shielding. Although the fibers are effective, they are expensive. In contrast to carbon and steel fibers, fly ash is not conductive, but it is inexpensive and contains various oxide components, such as Fe2O3, which may enhance shielding by absorption. Furthermore, the particulate nature of fly ash facilitates dispersion, in contrast to the fibers, which tend to cling together. To confirm that iron oxide is a component that enhances shielding, this article includes a study of the effects of Fe2O3 and Fe3O4 as admixtures on the shielding effectiveness of cement.
Silica fume [20], [21], [22], [23] is very fine noncrystalline silica produced by electric arc furnaces as a by-product of the production of metallic silicon or ferrosilicon alloys. It is a powder with particles having diameters 100 times smaller than those of anhydrous Portland cement particles, i.e., mean particle size between 0.1 and 0.2 μm. The SiO2 content ranges from 85% to 98%. Both silica fume and fly ash are pozzolanic. For the sake of comparison, this article includes a study of the effect of silica fume on the shielding effectiveness of cement.
Because the electrical resistivity is a basic quantity that describes the electrical conduction behavior, this article includes measurement of the DC electrical resistivity.
Section snippets
Experimental methods
The cement used was Portland cement (Type I). The ratio of water to the total cementitious material (i.e., cement+fly ash+silica fume) was 0.35. A water-reducing agent (a sodium salt of a condensed naphthalenesulfonic acid, TAMOL SN, Rohm and Haas, Philadelphia, PA) was used in the amount of 1.00% by mass of cement for all specimens. No aggregate was used, whether fine or coarse.
The fly ash (Class F) has the composition shown in Table 1. It is supplied by the Hatfield Power Station of Niagara
Results and discussion
Table 3 shows that the EMI shielding effectiveness (i.e., attenuation upon transmission) is increased monotonically by increasing the fly ash proportion, while the attenuation upon reflection is decreased slightly and monotonically, whether the frequency is 1.0 or 1.5 GHz. This means that the fly ash enhances the shielding through enhancing the reflectivity. On the other hand, silica fume improves the shielding effectiveness only marginally at either frequency, while it increases the
Conclusion
The use of fly ash as an admixture results in increase of the effectiveness of the cement paste for EMI shielding, due to enhancement of the absorption and reflection of the radiation, as tested at 1.0 and 1.5 GHz. The Fe2O3 in the fly ash (15.4 wt.%, compared to <0.5 wt.% in silica fume) contributes to the shielding. In contrast, silica fume has little effect on the shielding effectiveness, although it decreases the resistivity slightly.
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