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28The effect of two non-ionic chitosan derivatives (hydroxypropyl (HPCH) and hydroxyethyl 29(HECH) chitosans) and one ionic derivative (carboxymethylchitosan, CMCH) on the fresh-state 30 properties of cement mortars was studied. Zeta potential measurements and particle size 31 distribution were carried out in order to elucidate the action mechanism of the admixtures. 32Results were seen to be strongly dependent on substituents of the chitosan. Non-ionic 33 derivatives had a weak dosage-related influence on the fresh-state properties. The ionic CMCH 34 showed the more marked effect: it was found to act as a powerful thickener and to reduce the

The performance of an etherified chitosan, carboxymethyl chitosan (CMCH), when added to cement mortars doped with heavy metals, was assessed. In the presence of heavy metals (Cr, Pb, Zn) strong modifications of the fresh-state properties were evaluated. The addition of the polymer was seen to be useful in minimising some of these modifications, as those related to the setting time. A competitive mechanism for adsorption between the oxoanionic form of the metals and the carboxylate groups of the chitosan derivative was established. Studies on the metal chelating ability of the polymer and leaching from the hardened specimens showed scarce complexation under alkaline conditions, pointing to physical entrapment based on metal adsorption. However, significant chelation of metals was proved at near-neutral pH, suggesting the potential usefulness of the polymer as a agent for removing heavy metals from polluted waters and subsequently immobilizing them in cement mortars. Leaching tests carried out on polymer-metal complex-bearing samples showed a significant reduction in the amount of released Pb and Zn.

2Studies on chitosan as an admixture for cement-based materials: assessment of its viscosity enhancing effect and complexing ability for heavy metals AbstractChitosans of different molecular weights were assessed as cement admixtures in order to modify the rheological properties of a fresh cement paste. Their sequestering capability with respect to several heavy and toxic metals was checked and the complexation processes were investigated by different approaches based on electrochemical measurements. The aim was to conduct research on a potentially useful polymer to be added to cementitious materials acting as a viscosity modifier as well as a heavy metal immobilising agent. Pb, Cu, Zn, Cd, Mo and Cr were used as target metals.The highest complexation affinity was found towards Zn, with stoichiometries up to 1:40 having been calculated. Chitosan was proved to act as a thickener in cement mixtures. The highest molecular weight chitosan showed the greatest effectiveness taking into account its combined role as heavy metal retainer and thickener. In addition, a clear interaction between this high molecular weight chitosan and cement particles was also demonstrated by means of zeta potential measurements.