In South Dakota during the last decade, alkali-silica reactivity (ASR) has changed from a localized occurrence to a statewide problem. ASTM P214 was chosen as the preferred test for determining potential reactivity of sands used for concrete in the state. The test was modified to use different NaOH concentrations to explore the feasibility of using two different base strengths to better predict reactivity. Of 30 sands tested, over 40% gave results that were inconclusive (expansion values in the 0.1 to 0.2% range), and only 10% of the sands were nonreactive. A modified interpretation of the test results to include a best fit for expansion at 3, 7, 11, and 14 days, using the general equation Y= A sub 2 X-squared+ A sub 1 X+ A sub 0 (where X= Time to the 1/2 power) gave a much clearer pattern of projected reactivity when the A sub 1 coefficients were plotted against the A sub 2 coefficients for all 30 sands. The sands break out into two linear" families" of coefficients: one reactive and one nonreactive. It appears the method also can be used to determine the effectiveness of pozzolans at reducing ASR.