The influence of various polyelectrolytes on the precipitation of gypsum

Abstract In order to establish a relationship between the molecular structure of polycarboxylates and their growth retarding influence on gypsum crystals, seeded suspension growth experiments were performed at various growth conditions. Two types of polycarboxylates were studied with a molecular structure based on either polyacrylic or polymaleic acid. The molecular structure of these compounds was varied by partial substitution with monomers containing other functional groups than carboxylic acid groups, like hydroxyl, amino and sulfonic acid groups as well as hydrophobic groups. Besides the type of additional functional groups, also their position in the molecule was varied with respect to their distance from the polymer backbone. Some polymaleic and polyacrylic compounds were also studied at different pH-values. For the mechanism of growth inhibition a model is proposed, which explains why polymaleates are more effective than polyacrylates at high pH-values, and why their inhibitor effectiveness can be increased with additional hydroxyl and amino groups, but not with sulfonic acid groups. By application of various adsorption procedures to the seed crystals it became clear that the adsorption process of the polyelectrolytes onto the gypsum crystal surface is completed within a few minutes. With advancing outgrowth of the seed crystals, adsorbed polyelectrolyte inhibitor molecules are eventually overgrown.