Behaviour of a starch as a viscosity modifier for aerial lime-based mortars

Different dosages of a commercialized potato starch were added to aerial lime-based mortars in order to check its efficiency as a rheological modifier. Several fresh state properties of the mortars were studied: consistency, density, air content, water retention capacity, setting time and evolution when applied on support. The effect of the starch on ζ-potential of the lime particle surface as well as the particle size distribution and viscosity changes in lime pastes were also assessed in order to elucidate the action mechanism of the polymer. The behaviour of this starch polymer was found to be strongly dosage-dependent: it acted as a thickener when the incorporated dosage was up to 0.30% of lime weight; conversely, above that dosage, it behaved as a plasticizer. The thickening effect took place because polymer molecules were adsorbed onto lime particles acting as a flocculant, as confirmed by zeta-potential and particle size distribution results. For large amounts of polymer, steric hindrance and electrostatic repulsive forces appeared, leading to a dispersion mechanism which explained the plasticizing effect as well as the fresh mortar behaviour.