Abstract: A process for making a hardening agent for weak soil or sludge from steelmaking slag. About 5-30 wt. % of specified reforming agent selected from the group consisting of one or more of silicate rock, mineral, calamine, glass waste, foundry waste sand, waste brick, red mud, volcanic slag, iron oxide and mixtures thereof is added to a molten steelmaking slag, causing a reaction forming a reacted steelmaking slag. The reacted steelmaking slag is then rapidly cooled to form a powder. Iron is then removed from the powder and the powder is then mixed with from about 3-5% of a powder selected from the group consisting of one or more of lime, plaster and cement to form a hardening agent for weak soil or sludge.