Use of a foamed geopolymer as a fire-resistant sealant material, a method of sealing an aperture or cavity for housing services in a building comprising (i) applying a curable foamed geopolymer composition to the aperture or cavity; and (ii) curing the foamed geopolymer composition, thereby creating a seal in the aperture or cavity; and wherein the cured foamed geopolymer has fire-resistant properties, and a kit of parts for preparing a foamed geopolymer for use as a fire-resistant sealant material, comprising (i) a container holding a dry mixture of components suitable for preparing a foamed geopolymer including a blowing agent and (ii) a container holding an aqueous alkaline liquid mixture of components suitable for preparing a geopolymer.