Interplay of Anionic Functionality in Polymer-grafted Lignin Superplasticizers for Portland Cement

PEGylation of lignin derivatives has been shown to enhance emulsifying and dispersant activities. Here, the effects of anionic grafts were explored for dispersant activity within Portland cement. Kraft lignin (KL) and lignosulfonate (LS) are two important forms of purified lignin whose chemistries are characterized by low concentrations of carboxylate and high concentrations of sulfonate groups, respectively. These lignin derivatives were grafted with poly(methacrylic acid) (PMAA) or poly(3-sulfopropyl methacrylate), prepared via atom transfer radical polymerization, to introduce an anionic polymer corona on the lignin core. The dispersion of cement paste by these hybrid polymers was compared with the PEGylated lignin analogues as well as a leading cement superplasticizer, poly(carboxylate ether) (PCE). Slump values significantly increased for both the PMAA-grafted lignins compared to other analogues, allowing for significant reductions in the cement water content, with PMAA-grafted LS approaching the performance of commercial PCE and suggesting that graft chemistry has a strong effect on the dispersant function. Adsorption, zeta potential, and intrinsic viscosity were measured for the lignopolymer analogues to explore the interplay between lignin and graft chemistries in the mechanism of cement dispersion. The KL analogues all adsorbed to a greater extent but had zeta potential and intrinsic viscosity that demonstrated a strong function of the graft chemistry. In contrast, the LS analogues displayed a complex interplay with the graft chemistry that ultimately led to the largest slump values measured in this series. These results suggest that anionic polymer grafts can enhance the superplasticizer functionality of lignin derivatives, but the effects of the polymer graft are a complex function of both lignin and polymer-graft chemistries.