Abstract: Expansive reactions between lime and sulfate-bearing clay soils have attracted little attention until relatively recently. Lime treatment of Stewart Avenue in Las Vegas, Nevada, had induced heave in excess of 12 in. Heaved areas are found to contain abundant thaumasite, a complex calcium-silicate-hydroxide-sulfate-carbonate-hyd rate mineral. Thaumasite forms a solid solution series with ettringite, a calcium-aluminum-hydroxide-sulfate-hy drate mineral. In the presence of aluminum, ettringite forms first and is replaced by thaumasite only at temperatures below 15°C. The mechanism of heave is a complex function of available water, the percentage of soil clay, and ion mobility. Only the long-term possolanic chemistry of normal lime-soil reactions is disrupted. Cation exchange, agglomeration, and carbonation are unaffected. With the present state of knowledge, lime-induced heave is difficult to predict for all but most obvious conditions.