We have attempted to establish a new process for producing a concrete chemical admixture prepared with by-product lignin of bioethanol production. In our pretreatment process of bioethanol production, soda-anthraquinone (AQ) cooking was applied to remove lignin from softwood chips. Soda-AQ lignin was recovered as precipitate by acidifying the black liquor. Five kinds of soda-AQ lignin were isolated by selecting the pH value of the precipitation. Each of the isolated soda-AQ lignins was used in the mortar fluidity test as concrete admixture. With the exception of the soda-AQ lignin at higher pH (pH 10.5) precipitation, the isolated lignin gave about two times' higher mortar flow values than that of the widely used commercial lignosulfonate admixture. Size exclusion chromatography (SEC) data showed that the isolated lignin at higher pH value gave higher molecular weight. Functional group chemical analysis suggested that the isolated lignin of lower molecular weight had a higher phenolic hydroxyl group and higher alpha-carbonyl group content.