From the associated paper:
"Measurements were taken using an Agilent E4990A impedance analyser. Each mortar cube was removed from the water and surface dried using a cloth before measurements were taken. At this point the length of the cube was verified to reduce the risk of mistaken identity. Surface temperature was not controlled when out of the water, but was recorded before measurement. The surface temperatures range from 16 to 24C with the average being 20C. These temperatures were not used in the analysis but are included in the online data for the study.
Measurements were taken at 7, 14, 21, 28, and 35 days after casting the mortar. Stainless steel electrodes were clamped to the cubes and connected to the impedance analyser using the four-terminal pair probe configuration described by the Keysight Technologies impedance measurement handbook section 3.1.4. The initial set of measurements was taken at 200 frequencies within the 20Hz-20MHz range, using a linear frequency sweep. For subsequent measurements the scheme was changed to use 1000 frequencies in a logarithmic sweep to better represent the response at lower frequencies. Measurements about 10MHz were found to be significantly affected by the measurement probes in use. These were discarded."