This report deals with the influence of different sizing agents on carbon fiber (CF) heavy tows towards their impregnation with a geopolymer (GP) suspension and resulting properties of mineral-impregnated carbon-fiber composites (MCF) produced in an automated, continuous process. Three different commercial CFs were investigated after treatment with either thermoplastic, epoxy, or vinyl-ester sizing agents and then compared to unsized CFs. During impregnating, diverse yarn-spreading behavioral modes and degrees of wetness were observed, indicating different degrees of impregnation, both in quality and quantity. All sizing agents decreased the hydrophobic nature of unsized CFs significantly. Supported by microscopic investigation, water contact angle measurement, and thermal gravimetric and mercury intrusion porosity, the epoxy sizing showed the best fiber-matrix distribution over the cross-section and a dense microstructure. Moreover, in single-fiber pullout tests each sizing brought about a significant increase in maximum pullout force, indicating enhanced bond between fiber and matrix. This is attributed to more intense interaction in the interfacial region, fully corroborating the topological characteristics obtained in microscopic analysis. With higher interfacial bond strength, the thermoplastic sizing enabled the highest tensile properties of MCF despite slightly less impregnation quality in comparison with epoxy-sized samples.