When in contact with water, calcined gypsum rehydrates through dissolution, nucleation and crystallization steps. Additives may be used to improve the paste workability and the mechanical properties of the hardened piece. This study investigated the effect of addition of polysaccharides on the setting time, hydration heat, consistency and compressive strength. The changes in the gypsum microstructure were observed by scanning electron microscopy (SEM). The results showed that dextrin, starch and cellulose did not affect the gypsum properties. However, the addition of only 0.025% of carboxymethylcellulose (CMC) results in an increase in the initial and final setting time of 12 and 52%, respectively, without significantly affecting the mechanical strength of the hardened piece. The effect of CMC is attributed to the formation of calcium carboxylate, which reduces the availability of calcium in the crystallization step, slowing down the setting time. (C) 2013 Elsevier B.V. All rights reserved.