## 1 Introduction  
Self-sensing is the ability of a structural material to perceive its own condition, which can include strain, stress, damage and temperature [1]. In this regard, the material does not need embedded sensors such as fibre-optic sensors or continuous carbon fibres to sense its own status because it becomes the sensor itself thanks to the inclusion of conductive additions, like carbon-based fillers and fibres [2]. In the last two decades, numerous studies have been carried out to investigate self-sensing cement-based materials containing carbon-based fillers or fibres to develop novel multifunctional structural materials [3]. These additions are able to improve the durability and mechanical, electrical and electromagnetic properties [3, 4, 5] because of their high mechanical strength, high aspect ratio (in the case of fibres), high specific surface area (in the case of fillers), lightness and high electrical conductivity [6, 7, 8]. With regard to carbon fibres, both long and short fibres can be used. However, the use of short fibres is preferable since they are cheaper and easier to disperse, even though they are less effective than long fibres as reinforcement in cement-based composites [9]. Being conductive, carbon fibres are able to improve cement-based composites' electrical properties [1] to create a conductive network that changes if the material is deformed or stressed [10]. [PERSON] et al. [11] found that 6 mm-long fibres are better than 3 mm-long fibres for decreasing the electrical resistivity of mortars. This was also confirmed by [PERSON] et al. [12], who found that the electrical resistivity of cement mortars is highly reduced when carbon fibres are added at 1.1 wt% of cement. Moreover, a 15% increase in flexural strength and 18% increase in compressive strength was found when 6 mm-long fibres are added at 0.8 and 2.0 wt% of cement, respectively. Furthermore, [PERSON] et al. [13] found that at the same length (6 mm) and volume dosage, carbon fibres are much more effective than steel fibres for decreasing the electrical resistivity of cement mortars. Among carbon-based fillers, graphene and its derivatives are some of the most innovative materials studied in construction applications. Graphene is the basic structural unit of graphitic materials and it is composed of a single-layer sheet of carbon atoms closely packed into a two-dimensional (2D) honeycomb framework [14]. Graphene, thanks to its high mechanical resistance and electrical conductivity [4] is an interesting material for decreasing the electrical resistivity of cement-based materials [15; 16; 17]. A good dispersion of carbon additions in aqueous media is essential for their effectiveness in cement-based composites. Indeed, carbon-based commercial fillers such as graphene nanoplatelets (GNP) or carbon nanotubes (CNT) are difficult to disperse in polar liquids like water because of their hydrophobic nature and their high specific surface area, which are responsible for the formation of bundles and agglomerates [18; 19]. In addition, the use of such materials at a large scale is unfeasible owing to their high costs. It is reported that some carbon-based by-products that are obtained by thermal treatment of biomasses [20] or by other industrial processes exhibit comparable properties to those of commercial ones, and being wastes, are also much cheaper. Moreover, thanks to a less specific surface area and/or a different chemical composition (that is, the presence of functional groups containing elements other than carbon), they are more compatible with water and then easily dispersible [20]. Two interesting materials are gasification char (GCH) and used foundry sand (UFS), which have already been tested by the present authors as possible additions to manufacture lime-based pastes with multifunctional properties [21]. GCH is an industrial by-product produced by the thermal treatment of biomasses.