| Summary: | This study evaluated the effect of carbon nanofibers (CNFs) coated by aluminum oxide Al<sub>2</sub>O<sub>3</sub> as a reinforcement on compressive strength, frost resistance, and drying shrinkage of cement mortars. Three weight ratios of 0.125%, 0.25%, and 0.5% of Al<sub>2</sub>O<sub>3</sub>/CNFs and bare CNF cement mortars were compared with reference cement mortar samples. The reactive porous and high surface area layer of alumina induced the hydration reaction and promoted the production of well-distributed hydration gel. Derivative thermal analysis–differential thermogravimetric (TGA-DTG) and X-ray powder diffraction (XRD) characterization showed that Al<sub>2</sub>O<sub>3</sub>/CNFs reinforcement led to greater hydration gel production than bare CNFs. Transmission electron microscopy (TEM) and scanning electron microscopy (SEM) were performed to study the coating and microstructure of the cement mortars evaluated in this paper. The results show that the optimum enhancement of the cement mortar properties was obtained at ratios of 0.125% for Al<sub>2</sub>O<sub>3</sub>/CNFs and 0.25% for CNFs. This enhancement was greater with Al<sub>2</sub>O<sub>3</sub>/CNFs-reinforced specimens in terms of high compressive strength, less compressive strength degradation after 150 cycles, and less drying shrinkage. The low use of the CNFs in Al<sub>2</sub>O<sub>3</sub>/CNFs samples indicates the coating is an economical and promising approach for improving the performance of cement mortars.
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