Iowa highway concretes containing reactive dolomite, (CaMg) 2 CO 3 , aggregate, composed of fine-grained, microporous dolomite, sometimes have service lives of less than 10 years. This premature deterioration may, in part, be caused by expansive forces created by newly formed minerals such as brucite, Mg (OH) 2 , in the cement paste as a result of dedolomitization of reactive dolomite coarse aggregate. Although calcite is the most abundant secondary mineral in cements of poorly-performing concretes, the present study found no evidence that it was expansionary. Brucite is common but less abundant than calcite and occurred chiefly in and near the margins of reactive dolomite in both the aggregate and cement paste of poorly-performing concretes. Most brucite occurs in partially dedolomitized rims around dolomite coarse aggregates. This type of brucite is widely disseminated through the rims, consists of extremely small (<1 m) microcrystalline masses, and was produced by direct precipitation from pore solutions. Smaller amounts of brucite occur in the cement paste. This type is relatively coarse-grained (10 mu m-20 mu m) and most was formed primarily by crystal surface mediated (topochemical) reactions between magnesium-rich pore solutions and portlandite, Ca(OH) 2 . Numerous microcracks are present in cement paste but are not spatially associated with brucite locations. There is no direct evidence for cracking caused by brucite but this is not conclusive evidence against brucite-induced expansion. Brucite is widely disseminated so that expansion at innumerable micro-locations may cause general concrete expansion which should be relieved by cracking at weaker locations in the concretes.