Kraft lignin biorefinery: from pulping side streams to concrete plasticizers

Abstract

Kraft lignin is a residue of the kraft pulping process that is normally burnt to produce steam and energy. However, given its high availability as the second most abundant biopolymer, a lignin biorefinery is a promising approach to reach a sustainable bioeconomy. Nevertheless, because of lignin's complex structure, low reactivity and heterogeneity, this is still challenging. The modification of lignin to produce lignosulfonates is a possible use once they have already been validated by the market as plasticizers in concrete mixtures, surfactants, etc. The aim of this work is to produce lignosulfonates from commercial eucalypt kraft lignin and heat-treated lignin by a sulfomethylation reaction and analyze their use as a concrete plasticizer, comparing with a commercial admixture. The produced lignosulfonates presented a higher concentration in sulfur content, once no purification step was employed, but still a significant increase in water solubility from 7.2% to 98.8% when compared to kraft lignin. Cement pastes, fresh and hardened concretes were produced for the evaluation of the plasticizing performance. Fresh concretes were analyzed as to initial consistency (slump test), while the hardened concretes regarding to axial compression strength and the 1 cement pastes as to viscosity. In concentrations of around 2.2% both kraft lignosulfonate and heat-treated lignosulfonate were found to display an increase in the concrete workability of 75 to 95%. Both lignosulfonates were therefore found to be promising once they present the potential to not only reduce water consumption in concrete preparation while maintaining similar resistance, but also improve waste management in kraft mills.