The invention provides compositions and methods for sealing subterranean spaces such as natural or induced fractures, vugs or annular spaces. The composition is composed of a base fluid consisting of a soluble alkali silicate, a gas generating additive, solids and a setting agent. The gas generating additive may be coated or uncoated. The gas generating additive may also be in the form of a slurry. In the case of coated additives, the coating may act as a retarder or an accelerator to the expansion and setting agent of the soluble alkali silica. Similarly, the choice of carrier fluid in a slurry may retard or accelerate the expansion and setting of the alkali silicate-based plug.