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The production of nonferrous metals, including aluminium, copper, manganese, zinc, lead, nickel, and tin, is forecasted to increase in the mid-21st century, and consequently, the amount of slags derived from their processing will significantly rise. Nonferrous metal slags are industrial waste streams produced in large volumes, whose disposal poses environmental hazards due to potential release of toxic elements. New technologies to enable their sustainable management are urgently needed, and alkali-activation technology offers a route to transform nonferrous slags into value-added cementitious products for construction. This chapter provides an overview of recent advances in the understanding of relationships between slag composition and hardened cement properties, as well as their long-term safety and durability. Beyond purely material aspects, this chapter puts these developments into context of the circular economy and discusses the opportunities and barriers for greater adoption of alkali-activation technology to valorize nonferrous slags.

To future-proof alkali-activation technology, there is a need to look beyond well-established precursors such as fly ash and blast furnace slag, due to resource competition, geographical distribution and technical limitations. Clay minerals are abundant and diverse aluminosilicate resources available around the world. However, due to the mineralogical complexity amongst the most common 1:1 (kaolinite, halloysite) and 2:1 (montmorillonite, illite) clay minerals, and practical issues such as workability, their use has been more limited. Recent advances have improved understanding both of pre-activation treatments (thermal, mechanical, chemical), and of the factors influencing clay reactivity, phase assemblages and properties of final products. This opens new opportunities for the exploitation of these resources to produce sustainable cements. A one-size-fits-all approach for processing and activating clay minerals is not viable. Instead, activation routes need to be tailored according to the clay mineralogy to achieve the binder properties required for key applications. Published with golden open access ispartof: Cement And Concrete Research vol:132 status: published

The ability of Nirex Reference Vault Backfill (NRVB), a cement backfill material, to capture carbon dioxide from Intermediate Level Radioactive waste packages after repository backfilling, has been assessed. Large-scale trials assessed the physical and chemical reaction of carbon dioxide with the hardened backfill grout. A carbonation front, radial in nature, was observed extending into the grout and three distinct regions were identified in the hardened grouts. A carbonated region, a carbonation front, and a partially carbonated zone were discerned. Potassium, and to a lesser extent sodium, were concentrated in the carbonated region just behind of the main reaction front. The area just ahead of the carbonation front was enriched in both sulphur and aluminium, while sulphur was found to be depleted from the carbonated material behind the main reaction front. Within the main carbonated region, virtually all of the hydrated cement phases were found to be carbonated, and carbonation extended throughout the grout, even within material indicated by phenolphthalein solution to be uncarbonated. Importantly, carbonation was observed to impact both the mineral assemblage and porosity of the cement backfill; it is therefore important to understand these characteristics in terms of the long term evolution of NRVB and its groundwater buffering safety function within the geological disposal facility near-field.

Phase diagrams for alkali-activated slag (AAS) binders are simulated at (metastable) thermodynamic equilibrium, spanning the relevant compositional envelopes for these materials. The phase diagrams are generally consistent with experimental observations in the literature, dominated by calcium (alkali) aluminosilicate hydrate (C-(N-)A-S-H) gels and Mg-Al layered double hydroxides. Relationships between the stabilities of the predicted solid phase assemblages, pore solution compositions, and the bulk chemical composition are identified, yielding an improved understanding of AAS binder chemistry. Strätlingite is predicted at low to intermediate Si concentrations and at high Al content, while zeolites (and thus most likely also disordered alkali-aluminosilicate (hydrate) gels) tend to precipitate at higher concentrations of both Si and Al; katoite and AFm-type phases are stabilised at intermediate levels of CaO + Al2O3 + MgO. The application of these results in designing AAS binders can enable the phase assemblages and chemical properties of these materials to be more precisely controlled.

Alkali-activation technology has been used to produce inorganic cements for over a century, as a means of valorizing wastes or industrial by-products derived from different commercial activities. Granulated blast furnace slags (GBFS), derived from the iron-making industry, have been widely utilized for the production of alkali-activated cements. Significant advances in understanding the roles of different factors which govern the properties of alkali-activated GBFS cements have been made in recent decades, to the point that concretes based on alkali-activated GBFS are commercially deployed in several parts of the world. However, GBFS is not the only slag that can be used as a raw material for producing Portland clinker-free inorganic binders. Various other metallurgical slags, which currently have little or no commercial value, can also be utilized as raw materials for producing inorganic cements. The main difficulty to overcome in this area is the generally lower hydraulic reactivity of these slags compared with GBFS, and the high content of heavy metals which can limit the utilization of some such slags as building materials. Alkali-activation can thus, in some instances, also be seen as a means for the consolidation and safe disposal of such materials, which may otherwise pose an environmental hazard. This chapter provides an overview of inorganic cements produced via alkali-activation, particularly those which utilize nonblast furnace metallurgical slags including steel, ferronickel, titaniferous, stainless steel, lead, copper, zinc, nickel, manganese, silicomanganese, and phosphorus slags.

In this study, Na 2 CO 3 -activated slag cements were produced from four different blast furnace slags, each blended with a calcined layered double hydroxide (CLDH) derived from thermally treated hydrotalcite. The aim was to expedite the reaction kinetics of these cements, which would otherwise react and harden very slowly. The inclusion of CLDH in these Na 2 CO 3 -activated cements accelerates the reaction, and promotes hardening within 24 h. The MgO content of the slag also defines the reaction kinetics, associated with the formation of hydrotalcite-type LDH as a reaction product. The effectiveness of the CLDH is associated with removal of dissolved CO 3 2 − from the fresh cement, yielding a significant rise in the pH, and also potential seeding effects. The key factor controlling the reaction kinetics of Na 2 CO 3 -activated slag cements is the activator functional group, and therefore these cements can be designed to react more rapidly by controlling the slag chemistry and/or including reactive additives.

Carbonation is a degradation process that negatively affects alkali-activated materials, and represents one of the main perceived limitations for their adoption in structural applications. In this chapter the protocols followed to assess the carbonation susceptibility via accelerated methods are presented, along with a critical overview of the role of testing conditions and chemistry of the binding phases in defining the results acquired from those tests. It is highlighted that there is a need to develop testing protocols that better replicate the natural carbonation mechanism of alkali-activated materials, to enable accurate prediction of their long-term performance.

This paper assesses the effect of the use of an alternative activator based on nanosilica/MOH (M = K+ or Na+) blended solutions on the performance of alkali-activated fly ash binders. Binders produced with commercial silicate activators display a greater degree of reaction, associated with increased contents of geopolymer gel; however, mortars produced with the alternative nanosilica-based activators exhibited lower water demand and reduced permeability, independent of the alkali cation used. Na-based activators promote higher compressive strength compared with K-based activators, along with a refined pore structure, although K-activated samples exhibit reduced water demand. Zeolite type products are the major crystalline phases formed within these binders. A wider range of zeolites is formed when using commercial silicate solutions compared with the alternative activators. These results suggest that there are variations in the availability of Si in the system, and consequently in the alkalinity, depending on the silicate source in the activator, which is important in determining the nanostructure of the geopolymer gel., This study was sponsored by the Ministerio de Ciencia e Innovacion of Spain (Project GEORES MAT2010-19934 and research scholarship BES-2008-002440), European regional development fund (FEDER), and the Universitat Politecnica de Valencia (Spain). The participation of SAB and JLP was funded by the Australian Research Council (ARC), including partial funding through the Particulate Fluids Processing Centre, a Special Research Centre of the ARC. A special acknowledgement is also due to the Centre of Electron Microscopy of the Universitat Politecnica de Valencia and Pedro Garces from the Universidad de Alicante for support in some experiments.