Polysaccharides are set-retarding admixtures which contribute to water retention and workability of mortars. In order to rationalize the use of these admixtures in mortars, we studied the effect of polysaccharides on cement hydration with the intention to elucidate interaction mechanisms.In front of the lack of information concerning these admixtures and the effect of polysaccharides mortars properties, we realized first a study on raw materials. Hence we gathered structural data on admixtures such as substitute nature and the average molecular weight.Cement hydration in dilute media (W/C=20) allows us to propose mechanisms. Polysaccharides neither act on cement dissolution nor on first hydrates nucleation. The predominant mechanism seems to be adsorption on first hydrates hence slowing down or delaying cement hydration.The study in concentrated media (W/C<0.5) is closer to real conditions of use. Hydrates quantification shows that among the admixtures studied starch either (EA) and dextrin (AM2) are the more powerful set-retarding admixtures, With these admixtures on one hand ettringite phase is stabilized and on the other hand CSH and portlandite formations are delayed.A discussion of the different possible mechanisms of interaction is proposed.
