Abstract: A method is disclosed for improving the strength and impermeability of soils or engineering structures, which comprises the steps of:(a) applying to the site of the soil or of the engineering structure, an alkaline water glass solution comprising water glass in an amount of 15 to 40% by weight, calculated as dry substance;(b) allowing a time period to pass sufficient to permit the alkali present in the water glass solution to move by diffusion;(c) applying to the same site as in step (a), a silicic acid sol comprising SiO.sub.

Abstract: The invention relates to a process for improving the sedimentation and filtering properties of waste waters and/or slurries and muds containing clay minerals. The waste waters and/or slurries obtained in the various technologies and containing clay minerals and optionally also other solid components are treated with compounds containing ions having a hydrated ionic diameter of from 0.12 to 0.15 nm and a co-ordination number of 12.

Abstract: The invention relates to a new process for dewatering the sludges arising from the aerobic and/or anaerobic decomposition of e.g. household sewages, by treating with chemical agents, which comprises adjusting the pH of the sludge to a value between 9 and 12 by adding at least one of a compound selected from the group consisting of calcium and magnesium hydroxide and calcium and magnesium oxide, then mixing 50 to 5000 g./m.sup.3, of formaldehyde and/or dimethylolurea to the sludge suitably in the form of an aqueous solution, then optionally adding a dusty or granular substance capable of adsorbing oils and fats in an amount of 0.5 to 15% by weight as calculated for the dry substance content of the sludge, and adding prior to the adjustment of the pH or after the optional treatment with said substance capable of adsorption to the sludge 20 to 500 g./m.sup.3.