9.3. Soluble and liquid glass, Magnesia cements

Soluble glass is a glassy material, consisting of alkaline silicates. Its general formula is R₂O·nSiO_2, where, R₂O - alkaline oxides (Na₂O, K₂O),

n – silica modulus.

The silica modulus value can vary from 1 to 6.5. The sodium soluble glass with silica modulus 1.5-3 is usually used in construction, potassium glass is used basically to produce silicate paints. The soluble glass is produced in the form of the solid monolith, which is crushed into pieces or into fine product. The raw meal that includes silica and alkaline components, such as sand and soda, or sodium sulfate, is used for its production. The batch is fused in the glass-making furnaces at 1100-1400 ° C.

Silicate- boulder is formed with slow cooling of the glass bath in the air, and silicate finely crushed soluble glass by cooling in running water. Dissolution of the silicate- boulder in water is carried out in autoclave conditions under a pressure of 0.3-0.8 MPa. The finely crushed soluble glass can dissolve under atmospheric pressure and temperature of 90 - 100 ° C.

Water solution of soluble glass is called also a liquid glass. It can be produced from silicate- boulder or silicate finely crushed soluble glass by dissolution, or directly from the autoclave treatment of amorphous silicate products in solutions of caustic alkalis. Liquid glass is a colloidal solution with density of 1.4-1.5 g/cm^3. In closed vessels it can be stored for a long time but in the air conditions it slowly hardens. The essence of the hardening process of the liquid glass is the evaporation of the liquid phase, increasing the concentration of free colloidal silica, and the following coagulation and sealing. Air carbon dioxide neutralizes the alkalis, which are contained in water solution and promotes the silica coagulation. The fluorosilicate sodium additive and some other matters significantly speed up the liquid glass solidification. The liquid glass is characterized by high adhesive ability, which is 3-5 times higher than that of cement and other mineral binders. It is widely used for gluing of cardboard, paper, wood and silicate products, etc.

Silication – involves pressurization of liquid glass in soil together with solutions of additives through a system of the perforated pipes. It is used for soil fixing under foundations and as protection from ground-waters at passing of mines and tunnels.

Liquid glass serves as a sealer compound for quartz fluorosilicate in acid-proof cements(- products of the combined grinding or mixing of quartz sand and fluorosilicate sodium). There are two types of such cements: I - for acid-proof putties, II - for concretes and mortars. They differ with respect to the content of fluorosilicate sodium and time of setting. Acid-proof cements are used for agglutination of chemical-resistant materials, casing of different apparatus and protective coverings of building constructions. Acid-proof cements cannot be used in conditions of alkali action, fluorohydrogen and fluorosilicic acids, boiling water and stream.

The high heat resistance is also an excellent feature of mortars and concretes on the basis of liquid glass next to an acid resistance.

Magnesia cement is a product of the moderate burning of magnesite (caustic magnesite) or dolomite (caustic dolomite) tempered with solutions of electrolytes: magnesium chloride and sulfate, iron sulfate and others. Caustic magnesia is produced at 800-850 ° C, and caustic dolomite - at 650-750 °C.

Temperature rise, above the optimum, leads to compaction of MgO crystals, and during the burning of dolomite – to CaCO₃ dissociation, which impairs the quality of magnesia binders.

The highest strength of magnesia cement is obtained with use of magnesium chloride as sealer compound. So, caustic magnesite, tempered with the aqueous solution of magnesium chloride with a density of 1.2 g/cm³, shows a compressive strength of 50-60 MPa and higher at 28th days of hardening. Magnesia binders are the best binders for materials using wood sawdust (xylolite) and chips (fibrolite). That differs with respect to high impact strength, heat - and sound insulation properties. The strength of products based on magnesia binders become reduced in water and in humid atmosphere. Phosphate additives can be used to increase their water resistance. Using of the salts solutions at tempering of the magnesia binders is sharply limits their application in construction.