# Viability of Cupola Slag as an Alternative Eco-Binder and Filler in Concrete and Mortars
## 1 Introduction  
In 2017, the iron foundry sector reached a production of 76.2 Mt worldwide and 12 Mt in Europe, 35% corresponding to ductile iron [1]. Cupola slag is a by-product generated in cupola furnaces, which often ends up in landfills. The slag contains oxides coming from impurities in the load material that have not been reduced in the cupola, which are misfolded from the furnace's refractory walls, from coke ash, and from losses in the melting of the loaded metal. As the slag's density is lower than that of iron, it can be easily separated from the metal. As it is not reduced during the iron ore smelting process, it is produced in a proportion of 50-60 kg per ton of molten ore [2]. The pozzolanic properties of cupola slag are influenced by its cooling process, rapid cooling (total immersion in water) leading to greater vitrification of the slag, leaving the silica in an amorphous structure and, thus, susceptible to reaction. The use of cupola slag as a replacement for cement provides a sustainable application--in which there are hardly any studies--in the context that cement production accounts for 7% of global CO\({}_{2}\) emissions [3]. From an environmental point of view, the use of this waste in the manufacturing of concrete structures could mean, on the one hand, avoiding a generation of waste that would be deposited in a landfill and, on the other, the reduction of the carbon footprint derived from reducing the demand for cement. The immense volumeof concrete that is produced every day could absorb all of the production of cupola slag, even in the case of not improving the physical-mechanical properties of the concrete, so that the residue is only encapsulated. Cupola slag, like other wastes generated in the processes of the steel industry, must be reused or recovered following the principles of sustainability that European policies mark in their sustainable development plans. When discussing waste from the iron and steel industry (mainly slags) for the production of cement, it is essential to speak of ground-granulated blast furnace slag (GGBS). GGBS have been used worldwide for many years, due to their pozzolanic/cementitious properties and their ability to improve many of the performance characteristics of the concrete, such as strength, workability, permeability, durability, and corrosion resistance [4]. Additionally, the use of GGBS as an ordinary Portland cement (OPC) additive also has the advantage of its potential use in waste stream structures as a reducing agent and in immobilizing medium level radioactive waste [5], which confers its countless potential applications. The slag generated by cupola furnaces is usually composed of 30% refractory material, 10% scrap sand, 40% CaO, 10% coke ash, and 10% burned material [6]. Although studies are rare, the first in 1982 [7] studied the pozzolanic properties of cupola slag, finding little pozzolanic activity, and establishing the limit of ordinary Portland cement (OPC) replacement at 20% (13.5% reduction in strength). The use of cupola slag would significantly reduce the cost of concrete and waste would be disposed of without further applications. The study of this material--with application in cements and concrete mixes--was not reconsidered again until the 2000s, with a few studies on its use, both as an aggregate and as a replacement for OPC. In the literature studies, it is common to find information on the production of ecological concrete (recycled concrete) through the use of siderurgical aggregates or the use of construction and demolition wastes (C and DWs) of different types [8]. Most of these C and DWs are from building demolition waste; although there are already innovative proposals, such as for use in railway superstructures [9], siderurgical aggregate concrete [10], or the application of multi-recycling techniques [11].