Silicomanganese slag is a solid waste in metallurgical industry and can be transformed into an alkali-activated silicomanganese slag-based cementitious-material (ASSC) for the first time. The ASSC shows quite low electro-conductivity and can be raised dramatically by incorporated carbon black (CB) in the matrix of ASSC to create an electro-conductive alkali-activated silicomanganese slag-based cementitious-composite (EASSC), served as a low cost and environmentally-friendly photocatalyst for the removal of dye pollutant in the paper. The interrelationships of mechanical, optical, electroconductive, microstructural, and photocatalytic properties are evaluated. The network of CB plays a critical role in the electron transfers. The electrical conductivity of EASSC doped 4.5% CB drastically increases by 594.2 times compared to that of ASSC. The FESEM, XRD, and XPS results indicated that the EASSC with mean grain size about 50 nm is composed of amorphous calcium silicate hydrate (CSH), alabandite (α-MnS) and CB. The UV–vis DRS and PL exhibit that the absorption edges of electro-conductive alkali-activated silicomanganese slag-based cementitious-composite EASSC samples are gradually blue-shifted and the photoluminescence intensities progressively decrease with increasing CB content. The activities of photocatalytic degradation of basic violet 5BN dye are positive correlated to the electro-conductivities. The separation efficiency of photo-generated electron-hole pairs is enhanced due to the electron transfers from α-MnS to the network of CB. The photocatalytic degradation of dye pollutant belongs to the second order kinetics via a reaction mechanism of superoxide radical (•O2−) intermediate.