Upcycling Spent Waterglass Foundry Sand into Reactive Fine Aggregates for Alkali-Activated Slag to Achieve Enhanced Compressive Strength

Spent waterglass foundry sand (SWFS) is a major waste in foundry industries and should be disposed of preferentially. Two SWFS subjected to working temperatures of 100 degrees C (SWFS1) and 800 degrees C (SWFS8) were prepared to investigate their effects on the flow, compressive strength, micromechanical properties, and pore structure of alkali-activated slag (AAS). Experimental results confirmed that the overall performance of AAS was determined by the dissolved properties of the dried waterglass coating and the content of SWFS. The flow values of AAS in the first 1 h were increased by adding two SWFS. SWFS1 can release abundant silica tetrahedra into the activation system, contributing to micromechanical and macromechanical properties. The compressive strength of SWFS1-added AAS mortars was increased by 28.8% to 49.0% at 28 days. In contrast, SWFS8 exerted weaker dissolved properties. The compressive strength of AAS mortars was slightly enhanced and the maximum increment at 28 days was only 8.5%. In summary, this paper confirmed the feasibility of using SWFS as reactive fine aggregates for AAS owing to the dried waterglass coating.