When combined with portland cement and compacted, fly ash is a high-strength material. In some instances, it may also be desirable to control the hydraulic conductivity (k) of the compacted mixture. Therefore, a study was performed to assess the effects of water content (w), cement content, curing time, and compaction effort on the hydraulic conductivity of compacted cement-stabilized fly ash. When compacting relatively dry mixtures (w < 20%), k is independent of compaction effort, and is on the order of 10(-5) cm/s. When compacting between w of 20% and optimum water content (w(opt)), compaction effort affects k, and, at a given w, k decreases by about an order of magnitude when increasing from standard to modified proctor effort. When wet of w(opt), k is on the order of 10(-6) cm/s regardless of compaction effort or water content. With respect to curing time, extended curing time has relatively little effect on k within a 60-day time frame. Based on the results of this study, an approach to construction quality assurance testing can be applied to estimate k based on in situ measurement of dry density (rho(d)) and w. (c) 2006 Elsevier Ltd. All rights reserved.