## 4 Conclusions  
In this paper, we studied the use of a used foundry sand (UFS) and a gasification char (GCH) as possible low-cost green and alternative fillers to commercial graphene nanoplatelets (GNP) for manufacturing electrically conductive cement mortars. Since GCH provides mortars with the best performance in terms of both mechanical and electrical properties, it was also compared to virgin carbon fibres (VCF) and recycled carbon fibres (RCF)  
Figure 12: Water absorbed per unit area (Q\({}_{i}\)) by mortars after 28 days of curing.  
used alone and coupled with GCH itself. Fillers were added at 1.0 vol%, whereas fibres were added at 0.05 and 0.2 vol%. The obtained results show that:  
* GCH is a valid alternative to GNP because it decreases the electrical resistivity of mortars by maintaining their mechanical properties whereas GNP decrease their compressive strength.
* VCF and RCF have a similar effect on the compressive strength of mortars but RCF are better to improve their flexural behaviour, especially when the content of the fibres is low.
* GCH can be used alone to decrease the electrical resistivity of mortars; to further decrease the resistivity, VCF or RCF can also be added. The best performance can be obtained by adding RCF at 0.2 vol%.
* Fillers or fibres used alone reduce the capillary water absorption of mortars and the lowest absorption is provided by the addition of GCH.
* To measure the resistivity of composite systems, alternating current (AC) should be preferred to direct current (DC) in order to avoid polarisation of the electrodes or variation in properties over time due to ion migration (attributable to material polarisation).  
Therefore, it can be concluded that it is possible to substitute commercial and expensive carbon-based fillers like GNP with more sustainable and low-cost industrial by-products, such as the GCH obtained by the gasification of wood chips to manufacture durable and electrically conductive cement mortars. Moreover, more sustainable and less expensive RCF can be used instead of VCF. Finally, the combined use of GCH filler and fibre by-products can have a synergistic effect on mortars in terms of decreased electrical resistivity, even though there is a slight reduction in compressive strength and water absorption.  
In the future, it is planned to study how water penetration and other contaminants such as chlorides and CO\({}_{2}\) influence the electrical resistivity of mortars/concretes.  
Conceptualization, A.M., F.T. and G.M.R.; methodology and software validation, A.M., F.T. and T.B.; formal analysis, A.M. and C.G.; investigation, A.M. and C.G.; resources, F.T. and G.M.R.; data curation, A.M. and C.G.; writing--original draft preparation, A.M.; writing--review and editing, C.G., T.B., G.M.R. and F.T.; supervision, F.T.; funding acquisition, G.M.R. All authors have read and agreed to the published version of the manuscript.  
This research was carried out within the EnDurCrete (New Environmental friendly and Durable conCrete, integrating industrial by-products and hybrid systems, for civil, industrial and offshore applications) project, funded by the European Union's Horizon 2020 research and innovation programme under grant agreement n\({}^{\circ}\) 760639. The APC was waived.  
Not applicable.  
Data Availability Statement: Data is contained within the article.  
The authors wish to thank LA.BO S.r.l. for the used foundry sand, Procotex Belgium SA for the recycled carbon fibres and Dolder Massara S.r.l. for the virgin carbon fibres kindly offered for this work.  
The authors declare no conflict of interest.