One of the most serious causes of degradation of concrete is the alkali–silica reaction. Amorphous silicates present in certain aggregates react with the alkaline pore solution of the concrete to form a gel, which absorbs water and swells, leading to the expansion of the concrete. It is known that the deleterious effects of ASR can be suppressed by the incorporation of supplementary cementitious materials in the concrete, but the mechanisms operating are not clear. In this article, we study a model system of amorphous silica plates in simulated pore solutions by X-ray photoelectron spectroscopy (XPS), to demonstrate that aluminum in solution can directly suppress the dissolution of amorphous silica under the highly alkaline conditions found in concrete. XPS shows that aluminum species are incorporated into the framework of the silica structure. Scanning electron microscope (SEM) and atomic force microscope (AFM) images confirmed the reduction in dissolution of silica in solutions containing aluminum.