The utilization of citro- and desulphogypsum in the cement industry as set retarders was tested and compared with natural gypsum. Chemical gypsums were used in two different forms, powder and agglomerated under pressure. Setting times and volume stability of cement pastes, as well as compressive and flexural strengths of cement mortars were evaluated. A Student's t statistic was applied to test the equality of strength results at the ages of 3, 7, and 28 days. (C) 2000 Elsevier Science Ltd. All rights reserved.