## 1 Introduction
One of these processes is gasification, which is carried out to produce energy at a temperature range of 500-1400 \({}^{\circ}\)C in the presence of oxygen (under stoichiometric conditions). As a result, it gives a gas (syngas), a liquid material (tar) and a solid material (GCH), which is highly porous and mainly composed of carbon [22]. To the best of the authors' knowledge, only two papers have already considered GCH in construction materials. In particular, [PERSON] et al. [23] found that when GCH is added to mortars at 1 wt% by cement, the compressive and flexural strengths remain comparable to those of control specimens, with a slight increase in fracture energy. [PERSON] et al. [21] found that GCH is able to decrease the electrical resistivity of pastes by 65%, regardless of the percentage added (0.25, 0.5, and 1.0 wt% on hydraulic lime). On the other hand, UFS is an industrial waste obtained by the ferrous and non-ferrous metal casting industries. Its great thermal conductivity makes it a suitable material for mould casting. In the casting process, moulding sands are recycled multiple times, but, after many cycles the UFS becomes unsuitable for the manufacturing process, so it is removed and becomes a waste. Many studies exist on the use of UFS in cement mortars; however, it is mainly added to construction materials as a replacement for natural sand [24; 25]. Even though UFS contains a high amount of silica, it also contains carbon-based additions and several metals [26], which makes it a good candidate for reducing the electrical resistivity of construction materials. In fact, as already found by the present authors, even a small quantity, equal to 0.25 wt% of hydraulic lime, provides to pastes 27% lower electrical resistivity to pastes, which decreases up to 65% in the case of 1.0 wt% UFS addition [21]. [PERSON] and [PERSON] [27] have reported that the combined use of conductive fillers and fibres could be beneficial to composites since they may decrease the electrical resistivity of the material providing a synergistic effect. They found that the electrical conductivity and the electromagnetic shielding effectiveness of cement pastes are maintained unaltered by partially replacing carbon fibres with carbon black at 50 wt%. The present authors have found that the combined use of GNP at 1.0 vol% and recycled carbon fibres (RCF) at 0.2 vol% entails very low electrical resistivity and clear piezoresistive properties to cement mortars [17]. However, the combined use of carbon-based by-products and carbon fibres in mortars (as well as in concretes) has never been tested. The aim of this work is to decrease the electrical resistivity of cement-based mortars manufactured with recycled carbon-based fillers and fibres as alternatives to more expensive commercial carbon fillers and virgin carbon fibres. In particular, GCH and UFS were used as fillers in place of GNP, whereas RCF was used as a substitute for virgin carbon fibres (VCF); also, the combined effect of fillers and fibres was investigated. Mortars were tested in terms of their flexural and compressive strength as well as electrical resistivity, which was measured by both direct current (DC) and alternating current (AC). Since water allows the ingress of Cl\({}^{-}\) and SO\({}_{4}\)\({}^{2-}\) ions into construction materials, thus impairing their durability [28; 29; 30; 31], the effect of carbon-based additions on the absorption of capillary water in mortars was also investigated.