Aspects of Particle-Dispersant Interaction - Mode of Attachment of Dispersants to Particles

Dispersing chemicals are often used to improve the properties of concrete. In the cement field these chemicals are known as superplasticizers. Basically, the superplasticizers can be divided in to two classes: mainly negatively charged ionic and mainly non-ionic. The backbones of both types of superplasticizers are hydrophobic aliphatic and/or aromatic chains. To function as superplasticizers, these chemicals have to attach themselves to the solid particles of cement pastes. Ionic superplasticizers are generally assumed to attach themselves to positively charged solid particles by electrostatic bonds. The mode of attachment of non-ionic superplasticizers is not well specified. In this paper, the modes of attachments of both the types of superplasticizers have been reexamined. It is known that silica powder is positively charged below pH 3 and negatively charged above pH 9. Advantage has been taken of this charge reversal of silica powder to study the modes of attachment of both types of superplasticizers. It is observed that negatively charged -SNFC functions equally well both with pastes of positively and negatively charged silica particles.This means that -SNFC can attach itself to silica particles irrespective of sign of charge on the particles. Function of a non-ionic superplasticizer is found to be very sensitive to the number density of the solid particles. At high number density this type of dispersant can act as flocculants irrespective of the sign of charge on silica particles. At low number density of solid, this type performs in alkaline medium. From the above results it has been postulated that all types of superplasticizers attach themselves by depositing their hydrophobic part on to the solid particles. The effects of this type of attachment have been discussed.