Understanding Cationic Polymer Adsorption on Mineral Surfaces: Kaolinite in Cement Aggregates

Alain Jacquet,H Greenwell,Dawn Geatches,Stewart Clark

doi:10.3390/min8040130

Abstract

We present a joint experimental and theoretical investigation into the adsorption of polycationic quaternary ammonium polymers on the clay mineral kaolinite. Within the cement and concrete manufacturing industries such polymers are used to improve the final product by inerting the adsorption capacity of the clay minerals for more expensive additives. The adsorption of the presently used polymer (FL22) was compared with both a similar variant, but without a hydroxyl group (Fl22mod) and uncharged polyvinyl alcohol (PVA). Experimental results show that adsorption of FL22 is higher than that of FL22mod at both pH 6 and at pH > 10 and that the adsorption of PVA is the highest. Theoretical density functional theory (DFT) results and simplified models consisting of the basal surfaces of kaolinite, with monomers of FL22, FL22mod and PVA gave monomer coverage per unit surface area of kaolinite, a comparison of the configurations of the relaxed models, formation energies and Mulliken charges. These results show that the polycationic polymers interact with the basal surfaces of kaolinite electrostatically, explaining the high affinity of these polymers for kaolinite surfaces in the experimental results. The hydroxyl groups of FL22 and PVA form hydrogen bonds with the basal surfaces of kaolinite in conditions of pH 6. The joint experimental and theoretical results suggest that, due to the presence of the hydroxyl group, the conformation of FL22 changes under pH, where at neutral pH it lies relatively flat to the kaolinite surfaces, but at higher pH, conformational changes of the polymer occur, thereby increasing the adsorbed quantity of FL22.

Highlights

Cementitious materials have provided a robust construction material for hundreds, if not thousands of years
These results show that, with the exception of polyvinyl alcohol (PVA) trimers at the siloxane surface, the trimer models show the same charge trends as the monomer models, and we can state that the interactions of the polymers with the basal surfaces of kaolinite are primarily electrostatic
In order to prevent clay minerals associated with the aggregates adversely affecting the cement formulation through adsorption of plasticizers with resultant modification of properties, low cost polymers are used to “inert” the aggregate to the cement formulation

Summary

Introduction

Cementitious materials have provided a robust construction material for hundreds, if not thousands of years. Modern cements are highly technical materials, with the mixtures containing many additives to improve final properties, allow more control of mixing properties and to allow the setting time to be offset as desired. Such complex systems are very susceptible to the quality of the materials used, and the base materials of cements are natural materials, for example sand, which are heterogeneous and may contain impurity phases. In sand deposits clay minerals are near ubiquitous, usually making up less than 10% by mass fraction. Owing to their small size, clay minerals fill much of the pore space in sand deposits and form coatings on quartz grains [1].

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Conclusion