Ascertained chemical properties and application of Volcanic Ash (VA) in concrete would help a researcher’s judgement in exploring a viable source of pozzolana to be used in cement replacement for concrete production. This study was aimed at investigating the pozzolanic potential of volcanic ash of the Jos Plateau sourced from Pidong along Bokkos – Mangu Road. Mix ratio of 1:2:4 and 0.6 water cement ratio (w/c) was used for the production of the test concrete cubes. Twenty-one concrete specimens, each for 7- and 28-days curing, were produced with three test cubes for control specimens and three each for cement replacement of 5%, 10%, 15%, 20%, 25% and 30% respectively. The concrete specimens were produced by mixing the required weight of cement, volcanic ash, aggregate and water, after which it was poured into an oiled mould of size 150mmx150mmx150mm and compacted in three layers of 50mm each by ramming using steel rod, after which they were vibrated using vibrating machine. The test cubes were demoulded after 24 hours, weight and immersed in a curing tank filled with water for 28days at room temperature. After 7 and 28 days the test cubes were removed, weight and air dried. Failure load was obtained by crushing the test cubes using the compression machine in the Building Laboratory of the Department of Building, University of Jos, Plateau State. The results show that the sum of SiO2, Al2O3 and Fe2O3 is equal to 77.95% which is greater than the minimum requirement of 70%. It is found that the compressive strength of the modified concrete decreases as the percentage cement replacement with VA increases from 5 to 30 %. Also, the early age (7 and 28 days) compressive strength of modified concrete of all replacement level are significantly lower than the plain concrete. It is also found that the pozzolanic activity index (PAI) of modified concrete of all replacement level decreases as the percentage cement replacement with VA increases. Moreover, at 20% cement replacement with VA, 7 days curing gave PAI of 83.44% while 28 days curing gave PAI of 85.24%; both of which are greater than the minimum requirement of 75%. Therefore, the study concluded that based on the higher sum of acidic oxide (SiO2 + Al2O3 + Fe2O3) and PAI values than that which is prescribed in ASTM C 618 for Class N pozzolana, qualified the material suitable as a pozzolana.
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