This paper focuses on sustainable cementitious composites in terms of their conductivity, hydration and self-sensing properties, which are key features for smart city infrastructures. Smart cities have massive infrastructures that are interconnected, transmitting data and information for health-monitoring and performance optimization. In this regard, having them made of sustainable building materials (concrete) that are also sufficiently conductive, will be a suitable solution for structures’ performance. The studied sustainable cementitious mixtures are made by sea components (sea water and sea sand), which are abundant resources. The primary goal of this study is to improve the mixes’ electrical conductivity and sensitivity. To achieve this goal, milled carbon fibres (MCFs) and chopped carbon fibres (CCFs) in 6 different proportions were added to the cementitious mixes. The experimental study is divided into impedance spectroscopy to study the conductivity and hydration development, and self-sensing properties, conducted on various mix designs. The results show that incorporating sea components improve the electrical conductivity of the mixes by 40-50%. Further improvements were achieved by adding MCF as it shows a remarkable reduction by 60% compared to the plain ss-sw (sea sand and sea water) samples. Adding CCF improved the conductivity even further and resulted in sample’s resistivity as low as 53 Ωcm after 1 year of curing time.