In the 1940s, the US Bureau of Reclamation (USBR) began a long-term, nonaccelerated laboratory test programme to determine the influence of a variety of concrete-mix parameters on resistance to severe sulphate exposure conditions. The time of failure of these samples as influenced by their water-to-cement (w/c) ratio, cement composition, and percent replacement of cement with fly ash are reported. The analysis indicates that there is a "safe zone" for concrete made with w/c ratio lower than 0.45 and cement with unhydrated tricalcium aluminate (C3A) content lower than 8% where failure did not occur within the 40-year exposure period. As expected, concrete samples cast with high amount of C3A failed after a relatively short time of sulphate exposure. Expansion tests indicated that cements containing high amounts of C3S may lead to premature failure of concrete, even when moderate w/c ratios are used. Samples prepared with 25% and 45% replacement of cement with fly ash showed significantly less expansion than comparable mixtures containing no pozzolans. 9 refs.