A comparative study of amorphous and crystalline forms of commercial aluminum hydroxides as inhibitors of alkalisilica reactions in Portland cement mortars has been performed. It was found that at dosages of 1% to 3%, amorphous aluminum hydroxide can efficiently inhibit alkali-silica expansion of Portland cement compositions. High inhibiting activity of amorphous Al(OH)₃ additives may be explained by their ability to actively bind Ca(OH)₂ formed by the hydration of silicate phases of cement, to form ettringite (with participation of gypsum). Crystalline Al(OH)₃ additives that do not possess the ability to interact with Ca(OH)₂ even after additional grinding, however, demonstrate week properties to inhibit alkali-silica expansion. This may indicate that the inhibitory effect of Al(OH)₃ at least—partly, may be given by its influence on the concentration of Al³⁺ ions in the pore solution. Some expansion of the samples with admixtures of Al(OH)₃ observed during the alkaline expansion accelerated test procedure is not associated with the formation of ettringite and is only due to alkali-silicate reactions.