The addition of carbon fibres is used in cement composite materials with a twofold objective: enhancement of their mechanical performance, and functionalisation of the composite, e.g. as strain sensor. Alkali-activated blast furnace slag pastes were reinforced with carbon fibres, which increased bending and compressive strengths, controlled drying shrinkage, and their resistivity could be related to strains during cyclic compression tests. However, in order to optimise this strain self-sensing property, changes in water saturation of samples are necessary, because fully saturated samples did not work properly as a strain sensor.