## 1 Introduction  
Nowadays, recycling and reusing waste is an important challenge for achieving a more sustainable environment. Between the different wastes produced by industrial processes, it is important to highlight red mud, which is a residue generated by the digestion of bauxite minerals with caustic soda during the production of alumina through the Bayer process. For every tonne of alumina extracted, approximately 1.6 tons of red mud are produced and it is estimated that more than 66 million tons of this waste are generated annually worldwide [1, 2]. This high amount constitutes a serious environmental problem, especially in areas where this industry is implanted. In addition to this, it also has a relatively high economic cost (between 1% and 2% of the price of the alumina), because red mud residues must be stored in rafts which require a large amount of space (it is estimated that there is a surface of 1 km\({}^{2}\) of rafts every five years for a plant which produces one million tons of alumina per year) [3]. Furthermore, the removal of red mud by direct discharge into the sea is also highly polluting, since the toxic metals present in this waste can be dispersed over long distances. Red mud is generally presented as sludge with a concentration of solidsbetween 10% and 30%, and with a highly alkaline pH. In the last few decades, several investigations have been performed about the possible reuse of red mud in various fields. Among the different studies, it is interesting to highlight those focused on its use in the treatment of water and soil [4, 5], its use as a catalyst [6] and its application in construction and structural materials [7, 8, 9, 10]. The construction sector is a major contaminant industrial sector [11], so the lessening of its CO\({}_{2}\) emissions is now a relevant issue of study. One way to reduce those emissions is by substituting clinker with additions [12, 13, 14] and the majority of them are pollutant residues originated from other industrial processes, such as fly ash, ground granulated blast-furnace slag and silica fume [15, 16, 17, 18, 19, 20]. In relation to the use of red mud in cement-based materials, considering its composition, it is possible that it has pozzolanic activity and some research has pointed out this possibility [21, 22]. Along this line, there are several dispersed works related to the use of this addition in those materials and some of them have shown that red mud seems to improve protection against corrosion of reinforcements embedded in concrete [23, 24, 25, 26], as well as the transport properties of cement-based materials [27]. In addition to this, there are also some recent studies in which the mechanical strengths of cementitious materials with red mud have been analysed. Some examples are self-compacting concretes [28, 29], geopolymers [30] and lightweight concretes [31]. Regarding the presence of hazardous elements in red mud, there are several studies in which it has been shown that the presence of red mud is not dangerous for the users of structures or works made with construction materials with this addition. One of these studies was developed by [PERSON] et al. [10]. In that work, the healthy safe application of red mud in different mixtures was tested, studying the radiological properties of their constituents and their resulting construction materials. These authors [10] concluded that from a radiological point of view, all materials with red mud studied in their research were safe, and they are able to be used. In addition to this, they pointed out that even a red mud incorporation of 90% by mass would be safe for the particular application of road construction [10]. In another study, developed by [PERSON] et al. [22], an environmental assessment of cement-based materials with red mud was also performed.