In this work, carbon of 2% and 4% by weight were added for the first time in a fly ash cement system to produce Activated carbon–fly ash composites in the form of pastes and mortars. Compressive strengths of the composites were then investigated. It was found that the use of Activated carbon resulted in higher strength of fly ash mortars. The highest strength obtained for 20% fly ash cement mortars was found at 4% Activated carbon where the compressive strength at 28 days was 22.39 MPa. This benefit can clearly be seen in fly ash cement with fly ash of 20% where the importance of the addition of Activated carbon means that the relative strength to that of cement became almost 100% at 28 days. In addition, scanning electron micrographs also showed that good interaction between Activated carbon and the fly ash cement matrix is seen with Activated carbon acting as a filler resulting in a denser microstructure and higher strength when compared to the reference fly ash mix without Activated carbon.