Al2O3 is a major constituent of municipal solid waste (MSW) bottom ash that may cause swelling and weakening failure. This paper explores the influence of Al2O3 on the long-term mechanical properties of bottom ash. Experimental results indicate that increased Al2O3 corresponds to decreases in both the uniaxial compressive and tensile strengths of cemented bottom ash. More specifically, the strength and Young's modulus of cemented bottom ash with high Al2O3 content (9.2%) decreased by 22.1% and 40.1%, respectively, after 365 days. Therefore, the long-term mechanical stability of bottom ash with a high Al2O3 content is unsuitable for engineering applications.