Copyright © 2022 by ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959.Class F fly ash from both fresh and stored sources, along with finely milled sand and limestone were tested by conventional strength index approaches based on ASTM C618-2019, Standard Specification for Coal Fly Ash and Raw or Calcined Natural Pozzolan for Use in Concrete and EN 450-1:2012 - Fly ash for concrete. Definition, specifications and conformity criteria for pozzolan activity. Mortars were made with both European and U.S. standard sands. Pozzolanic activity was also measured directly by Ca(OH)2 uptake via thermogravimetric analysis and electrical resistivity. The EN 450 method proved a more reliable measure of pozzolanic activity than the ASTM equivalent, which provided false positives, probably due to the narrow gradation of the U.S. standard sand and the rheologic benefit of the fly ash on the mortar. A four-point Wenner probe was used to measure the electrical resistivity on the 40 × 40 × 160 mm mortar prisms. The resistivity correlated with the measure of strength index and pozzolanic activity by other methods. The pozzolanic reaction of the Class F fly ash was found to be relativity slow, with a latency period of at least 21 days and only small indications of activity at 28 days. Unlike compressive strength, the resistivity of the control mortars did not change significantly after 28 days and remained essentially flat over one year. The resistivity of the fly ash test prisms showed continuous increase over the same period, achieving 150 % to >500 % of control at 90 days. This may be contrasted with the compressive strength data of 90 % to 110 % for the EN-450 data. The measurement of resistivity is rapid, simple, precise, and nondestructive and made with inexpensive equipment that is relatable to pozzolanic activity. It is found to be more sensitive and selective than that provided by conventional strength index methods.