Monoliths that set rapidly and harden at ambient temperature have been prepared by exploiting sol-gel type condensation reactions between sodium silicate, formed in-situ by alkaline dissolution of silica fume, and a solution of sodium aluminate. Structural characterisation of the product was carried out by XRD, Al-27 and Si-29 MAS NMR spectroscopy) and the samples showed reproducibly good compressive strengths, indicating that the products display all the characteristics of typical aluminosilicate geopolymers.