Electromagnetic skin depth of cement paste and its thickness dependence

The skin depth δ describes the ability of electromagnetic radiation (particularly microwave) to penetrate a material. This ability is relevant to radar-related applications, electromagnetic interference shielding and telecommunication. However, the values of δ have not been adequately reported for cement-based materials and the effect of the absorption distance (thickness) on δ has not been previously addressed. This work reports and clarifies δ (3.5 mm–12 mm) of cured cement paste. The previously and scantily reported δ values of cement-based materials are mostly incorrect, with discrepancy as large as three orders of magnitude, due to the flaws in the method of δ determination. In addition, this work reports the thickness dependence of δ of a cement-based material for the first time. Due to the Skin effect, δ increases with increasing thickness, with the fractional increase in δ per unit thickness 1.15, which is high because δ (3.5–12 mm) exceeds the thickness (2.61–8.26 mm), so that the electric field decays precipitously as it traverses through the thickness. In contrast, for aluminum, the fractional increase in δ per unit thickness is 0.080, which is low because the thickness exceeds δ, so that the electric field decays gradually. The δ of cement paste tends to decrease slightly with increasing frequency (8.5–11.5 GHz).