Reuse of steel slag and blast furnace slag as 100% waste-based binders or blended with metakaolin for the synthesis of one-part geopolymers

The construction industry plays a key role in the development of society, but it also has a major impact on the environment by consuming enormous quantities of non-renewable natural resources and producing emissions that greatly contribute to climate change. The great attention to environmental problems has forced the development of new eco-friendly materials. Alkali-activated materials (AAMs), such as geopolymers, are promising cementitious binder substitutes to produce mortars and concretes. AAMs help reduce energy consumption and CO₂ emissions related to cement production, and mitigate environmental risks related to waste management, thanks to the possibility of re-using waste materials as aluminosilicate precursors. AAMs are usually obtained through a reaction between a solid aluminosilicate precursor (e.g. metakaolin) with an alkaline activator in solution, with the so-called two-part method. Aim of this work is the design of one-part alkaliactivated binders prepared from industrial aluminosilicate waste precursors (blast furnace slag and steel slag) using solid alkaline activators, instead of harmful alkaline solutions. These binders are used for the development of sustainable mortars and plasters by adding sand and natural additives. FTIR and XRD characterizations were performed in order to validate and prove the geopolymerization reaction of this one-part method occurred, while the morphological analysis of the samples was obtained by means of SEM analysis. From the point of view of mechanical characterization, flexural and compression tests were performed. With respect to the corresponding two-part formulation, serving as control, one-part samples exhibited lower flexural strength, but a comparable compressive strength (around 25 MPa) Almost doubled values of compressive strength were achieved by using the slag mixed with metakaolin as precursor. The addition of metakaolin is indeed known to improve the mechanical properties and workability of the mixtures. The obtained formulations have properties suitable for use as innovative and sustainable building materials.