Abstract: A grouting composition, a method of grouting and a formation so grouted are disclosed. The composition comprises a very small particle size slag, an equal or greater weight of water and the optional components cement, alkali silicate, anionic dispersant, a source of orthophosphate ions, sodium hydroxide and sodium carbonate. The grout is particularly adapted to the treatment of "tight" or low permeability formations being low in viscosity and having controllable set time and hardening time as well as high strength upon hardening. Being based on slag, a byproduct, the grout is economical.