Abstract: Gasification slag, as a solid waste product from the coal chemical industry, has seen a rise in production in tandem with the development of the coal industry, presenting a challenge in its disposal and utilization. This study explores the potential application of gasification slag in cement mortar materials for road engineering to achieve its green and efficient utilization. Coarse and fine slags were used to prepare cement mortar specimens, and the effects of their incorporation levels on the specimen strength were examined using analytical methods such as thermogravimetry (TG) and scanning electron microscopy (SEM). The results indicate significant differences between the coarse and fine slags: the cumulative yield of particles larger than 0.6mm for fine slag is only 8.87%, much lower than that of coarse slag, which is 62.61%, and the fine slag contains a higher residual carbon content and requires more water. The extensive incorporation of gasification slag reduces the strength of the mortar specimens, but the flexural and compressive strengths of specimens with coarse slag are superior to those with fine slag. The optimal specimen with 10% coarse slag at 28 days age achieved 81.0% of the flexural strength and 70.9% of the compressive strength of the control group. TG and SEM show that residual carbon significantly impacts the hydration process of the mortar specimens by hindering the interaction of hydration products, leading to a looser structure. Additionally, the study finds that the potential pozzolanic components in coarse slag can effectively promote the hydration process and simultaneously inhibit carbonation, providing a significant theoretical basis for the application of gasification slag in mortar materials. Fine gasification slag is not suitable as a raw material for cement mortar for road use before the impact of residual carbon is eliminated, whereas coarse slag is appropriate for replacing fine aggregate in mortar materials, with the best comprehensive strength and economic benefits when the coarse slag content is 10%.