Kraft lignin biorefinery: from pulping side streams to concrete plasticizers

Kraft lignin is generated as a side stream 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, this is still challenging because of lignin’s complex structure, low reactivity, and heterogeneity. 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. This work aimed to produce lignosulfonates from commercial eucalypt kraft lignin and heat-treated lignin by a sulfomethylation reaction, analyze their use as a concrete plasticizer, and compare them with a commercial admixture. The produced lignosulfonates presented a higher concentration in sulfur content, as 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, as well as fresh and hardened concrete specimens were produced for the evaluation of the plasticizing performance. Moreover, the consistency (slump test) of fresh concrete, the axial compression strength of hardened concrete, and the viscosity of cement pastes were analyzed. In concentrations of around 2.2%, both kraft lignosulfonate and heat-treated lignosulfonate were found to display an increase in concrete workability from 75 to 95%. Therefore, both lignosulfonates were found to be promising since 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.