In hyperalkaline en~cironments such as concretes and alkaline wastes, the aqueous geochemistry of arsenate should be controlled by reactions of arsenate ions with alkaline phases such as portlandite, calcite and ettringite. This should result in the formation of surface complexes, and/or solid solutions of metalarsenate precipitates. At present, insufficient thermodynamic data is available to facilitate prediction of Ca-arsenate precipitates. In addition, little spectroscopic data is available for As(V) interactions with Ca mineral phases or for Ca-arsentate precipitates. The present study examined the sorption of arsenate by CaO. Examination of reaction products with direct physical methods (XRD, SEM, FTIR, Raman and XAS) indicated the formation of johnbaumite (Cas(AsO4)3(OH)).