This work deals with the relative efficiency of polysaccharides and their influence on cement hydration. Several parameters such as the structure, concentration, average molecular weight, and soluble fraction value of polysaccharides were examined. Cement hydration was monitored by isothermal calorimetry, thermogravimetry (TGA), and infra-red spectroscopy (FTIR). Results clearly show that retardation increases with higher polysaccharide-to-cement weight ratio (P/C). Low molecular weight starch showed enhanced retarding effect on the hydration of cement. The retardation effect of polysaccharides is also dependant on the composition of cement.

International audienceCellulose ethers such as hydroxyethylmethyl cellulose (HEMC) and hydroxypropylmethyl cellulose (HPMC) are common admixtures in factory made mortars. Nevertheless, their use principally remains empirical, and no cement-admixture interaction mechanism has ever been rigorously demonstrated. The main issue of this publication deals with the control of secondary effects generated by these admixtures such as the retardation of cement hydration. In this frame, a study of the impact of HEMC and HPMC molecule parameters on the modification of cement hydration was carried out. Minor influence of the molecular weight and of the hydroxypropyl or the hydroxyethyl group content was observed. On the contrary, the results emphasize that the methoxyl group content appears as the key parameter of the hydration delay mechanism

This work compares the effects induced by polysaccharides on the hydration of cement. It also brings new insights into the interaction mechanisms between these two components. Several parameters such as structure, concentration, average molecular weight, and the soluble fraction value of the polysaccharides were examined. The hydration of cement was monitored by conductivity measurement, and ionic chromatography. The influence of polysaccharide structure on the kinetics of cement hydration was revealed. The extent of retardation increases when polysaccharide concentration rises. Dextrins with lower average molecular weights compared with starches favor a higher soluble fraction value and further retard hydration. The growth of hydrates seemed to be more affected by the presence of these admixtures than did the dissolution of anhydrous particles or the nucleation of former hydrates.1 Keywords : C Ionic conductivity, Cement, B Composites, C Chemical properties, Polysaccharides , Hydration. 1.IntroductionAmong the organic admixtures widely used in the formulation of mortars and concrete, polysaccharides are polymers which can be equally classified in water-reducers, set-retarders, and water retention agents.Numerous authors have demonstrated that the properties of mortar and concrete can be significantly modified at fresh state as well as at hardened state by the addition ofSeveral mechanisms were proposed to explain the interactions between cement and set-retarders (i.e. sugars or carboxylic acids) [9][10][11][12][13][14][15]. Some authors focused their studies on interaction with anhydrous surfaces. For Hansen [16], the adsorption onto anhydrous particles could occur and protect surfaces from initial attack by water. Susuki [17] proposed that the retarding action of admixtures could be linked to the precipitation of insoluble calcium salts at the surface of anhydrous particles even though a clear correlation between solubility and retarding ability of carboxylic acids was not established.Other studies pointed out that interactions between admixtures and hydrates could also exist. According to Thomas and Birchall [18], the retarding action of sugars is explained in terms of adsorption onto and poisoning of hydrates surfaces. Young [19] suggested that an incorporation of admixtures into crystal lattices could occur. For example, in the case of C 3 A hydration, he proposed that organic compounds could enter 2 the interlayer region and stabilize hexagonal hydrates like C 4 AH 13 at the expense of the cubic form C 3 AH 6 . However, in the case of C 3 S hydration, Popova [20] showed that polymer-CSH interaction was restricted to adsorption. Indeed no significant structural modification of CSH were observed by 29 Si NMR characterization.Hence a mechanism describing the influence of set retarding admixtures on cement hydration is still not well elucidated. In order to bring new insights, the present study focuses on the hydration of cement in presence of compounds widely used in mortars i.e.polysaccharides. Th...