Preparation of sulfomethylated softwood kraft lignin as a dispersant for cement admixture†
Kraft lignin is currently combusted in the kraft pulping process. However, it can be modified to produce value-added products. The modification of kraft lignin using a combination of oxidation and sulfomethylation was studied in this work in order to produce sulfomethylated lignin. Oxidized lignin with a carboxylate group content of 1.95 meq g−1 was obtained under the optimal oxidation conditions of 100 °C and 1 h of treatment with 20 wt% nitric acid concentration. Sulfomethylated lignin with a sulfonation degree of 2.05 meq g−1 was obtained under the optimal sulfomethylation conditions of 100 °C, 3 h treatment, formaldehyde to lignin molar ratio of 1 : 1, sodium metabisulfite to lignin molar ratio of 0.5 : 1, and 4.0 wt% sodium hydroxide concentration. The sulfomethylated lignin with a higher charge density adsorbed more on cement particles and improved the fluidity of a cement paste more effectively than did commercial lignosulfonates. Adding 0.5 wt% of sulfomethylated lignin to the cement paste increased the fluidity of the paste from 65 mm to 200 mm, while the addition of 0.4 wt% of unmodified kraft lignin to the paste resulted in 70 mm fluidity.