Certain fine-grained argillaceous dolomitic limestones show an increase in solid volume when placed in strong alkali. Mineralogical analysis shows that the dolomite is replaced by magnesium hydroxide (brucite), calcium carbonate, and alkali carbonate. This reaction does not account directly for the expansion, as the volume of the solid products is less than the volume of the dolomite.Expansion is shown to be the result of an increase in solid volume that results from water uptake. The behavior is accounted for by a hypothesis that may be summarized as follows.(1) Alkaline attack on the dolomite crystals releases included clay in an "active" state with an unwetted surface.(2) Some of the Ca++ and Mg++ ions probably present on exchange sites are replaced by Na+ ions.(3) Development of a hydrated double layer made up of oriented water molecules, cations near the newly exposed clay surface, and anions further from the surface.(4) Development of the double layer and accompanying hydration processes cause the water uptake, which leads to expansion.