False Positives in ASTM C618 Specifications for Natural Pozzolans

ASTM C618 is used to qualify fly ash and natural pozzolans for use in concrete and for sale to the concrete industry. This study tests the notion that ASTM C618 does not adequately qualify natural pozzolans for use in concrete, with the primary concern that ASTM C618 has the potential to provide "false positives" for inert materials. Pozzolanicity tests and the tests outlined in ASTM C618 were performed on a variety of natural materials, including those that were known to be pozzolanic or inert and those with unknown pozzolanicity. Compressive strength testing at a fixed water-cementitious materials ratio (w/cm) was also performed on the materials, and the results were compared against the results of the strength activity index (SAT) from ASTM C618, which has a variable w/cm. This study proves that inert natural minerals passed ASTM C618 for a Class N natural pozzolan for use in concrete, suggesting that the standard is inadequate. Keywords: ASTM C618; Class N; natural pozzolan; pozzolanic reaction.