Innovative one-part geopolymerisation for efficient recovery of highly reactive alum sludge waste

Alum sludge (AS), a by-product of drinking water treatment, is difficult to use in cementitious systems because of its high Al content, low reactive silica availability and associated compatibility issues. Although alum-sludge-based binders and one-part geopolymers have both been studied, the feasibility of incorporating Al-rich AS into a one-part geopolymer remains insufficiently understood. In this study, calcined AS was evaluated as a reactive precursor, blended with ground granulated blast furnace slag (GGBS) and activated with solid sodium silicate and sodium carbonate. The effects of sodium silicate modulus (Ms), Na 2 O/Al 2 O 3 , SiO 2 /Al 2 O 3 ratio and AS content on flowability, mechanical performance, and microstructure were systematically investigated. The results show that a workable and mechanically effective AS/GGBS, one-part geopolymer can be achieved when the activator composition is appropriately designed. Within the investigated range, the optimum parameters are Ms = 2.0, Na 2 O/Al 2 O 3 = 1.0 and SiO 2 /Al 2 O 3 = 2.5, which deliver a 28-day compressive strength of 66.5 MPa. Mixtures activated solely by sodium silicate or sodium carbonate developed little or no strength, whereas their combined use produced improved workability and strength, indicating complementary roles of the two solid activators. Microstructural analyses confirmed the formation of a predominantly amorphous binding matrix together with crystalline phases including gaylussite and calcite, consistent with C-(N)-A-S-H type reaction products. Increasing AS content reduced ultimate strength but decreased the sensitivity of compressive strength to mix-design variations. Overall, this study shows that strength development depends on maintaining a balance between AS-derived Al release, GGBS-derived Ca/Si dissolution, soluble silicate supply, and carbonate-induced phase formation.