A lost-circulation material including a mixture of an aqueous colloidal dispersion and fatty acid. The aqueous colloidal dispersion includes silica nanoparticles and has a pH of at least 8. Combining the colloidal dispersion and the fatty acid initiates gelation of the lost-circulation material when the pH of the lost-circulation material is less than 8 and a temperature of the lost-circulation material is in a range of 5° C. to 300° C. Sealing an opening in a portion of a wellbore or a portion of a subterranean formation in which the wellbore is formed may include providing the aqueous colloidal dispersion and the fatty acid to the wellbore, mixing the colloidal dispersion and the fatty acid to yield the lost-circulation material, initiating gelation of the lost-circulation material, and solidifying the lost-circulation material in the wellbore to yield a set gel.