Abstract: A composition for use in construction materials, which may be substituted for high performance concrete, patching materials, joint compounds, and the like, such as backer boards or panels, which includes a settable calcium sulfate, preferably a hemihydrate, Portland cement, a finely divided pozzolanic material, lime, and an aggregate, optionally including other additives. The volume ratio of the aggregate to the combined calcium sulfate, Portland cement, pozzolanic material, and lime (a cementitious binder) is equal to or greater than 2/1. Panels made from this composition are useful, particularly when exposed to water since they have good dimensional stability.