This study aims to achieve the fast-setting and early strength gain of cementitious mineral-impregnated carbon-fibre (MCF) reinforcements, which is designed as a fire-resistant, cement-compatible alternative to traditional steel reinforcement, by utilizing electric Joule heating in combination with intermittent and controlled water spray treatment. The MCF is to replace traditional steel reinforcement in concrete to provide fire resistance and better consistency to the cement matrix. This study begins by examining the thermoelectrical behavior and early flexural strength development of MCF composites under a range of applied voltages, identifying 15 V as the optimal condition. The influence of cement substitution with metakaolin (MK), various electric curing durations (0.5–8 h), and the effect of intermittent water spray treatment were further explored to optimise early-age performance. Compared to air-cured MCF composites, which failed to harden within 24 h, early strength values exceeded 40 MPa and 100 MPa after 30 min and 4 h of thermal curing, respectively, attributed to the accelerated hydration driven by internal Joule heating. The incorporation of MK enhanced the early strength at initial stages, but resulted in reduced strength after 8 h of Joule heating. Microstructural analyses using scanning electron microscopy, porosity measurements and μCT indicated that MCF composites treated with water spray exhibited denser and more homogenous microstructures than those without treatment. These outcomes highlight the potential of electric Joule heating combined with water spray post-treatment in enhancing the rapid strength gain of MCF composites, paving the way for their rapid and flexible industrial prefabrication.