The present invention concerns a process for oxidizing lignin at alkaline conditions. This oxidized lignin can be used as a dispersant in various end-products, for example as a renewable plasticizer in cement-containing products, such as concrete, where the ability of the oxidized lignin to endure alkaline conditions is an advantage. Further, the invention concerns a concrete mixture containing aggregate, cement, water and a plasticizer formed of oxidized lignin, and a process for producing such a concrete mixture. The oxidized lignin can be manufactured using a process, where the lignin starting material is reacted with an oxidizing agent in solution in alkaline conditions.