Admixtures used to enhance placing characteristics of concrete

Abstract

Placing characteristics of concrete can be enhanced by using plasticizing and superplasticizing admixtures without any change in the water-cement ratio with respect to the plain mixture. The main ingredients used in superplasticizers are based on sulfonated melamine formaldehyde (SMF) condensate or naphthalene formaldehyde (SNF) condensate. More recently, a new family of products, based on acrylic polymers (AP), has been proposed. These polymers are more effective than those based on SMF or SNF in terms of lower base water-cement ratio at a given slump and lower slump loss. Moreover, the effectiveness of the AP-based superplasticizers does not depend significantly on the mode of addition, whereas the slump level of concrete mixes with SMF- and SNF-based admixtures is much higher with a delayed addition of superplasticizer with respect to that of mixing water. The dispersion of cement particles responsible for the fluidity increase, caused by the superplasticizer addition, is not necessarily related to the electrostatic repulsion associated with zeta potential measurements, as it was found for SMF- and SNF-based admixtures. For the AP-based superplasticizers, the polymer adsorption, rather than the electrostatic repulsion, is responsible for the dispersion of large agglomerates of cement particles into smaller ones, which results in a remarkable increase in the fluidity of cement mixes.