A method of treating a treatment zone in a well to reduce the permeability of the treatment zone including the steps of: introducing into the treatment zone a water-based treatment fluid comprising: an aqueous silica sol; and a water-soluble chemical activator for gelling the silica sol, wherein the chemical activator is selected from the group consisting of: phytic acid, methylglycinediacetic acid, a water-soluble polyepoxysuccinic acid, salicylic acid, ascorbic acid, tannic acid, and an alkali metal salt or ammonium salt of any of the foregoing; and shutting in treatment zone for at least a sufficient time to allow the treatment fluid to in-situ form a solid gel at a design temperature for the method. Alternatively, a first treatment fluid including the aqueous silica sol and a second treatment fluid including the chemical activator can be introduced into the treatment zone separately, in any order.