Inorganic coating technology: curing technology and curing mechanism of water glass

The main characteristics of water glass are:
Good bonding performance;
The silica gel precipitated during hardening has the effect of blocking capillary pores and preventing water penetration; it is non-flammable and heat-resistant. At high temperatures, the silica gel dries more strongly without reducing its strength and even increases its strength. It also has high acid resistance.
The curing technology and curing mechanism of water glass: Liquid water glass absorbs carbon dioxide in the air to form amorphous silica and gradually dries and solidifies. The reaction formula of its curing film-forming mechanism is as follows:

In the above reaction formula, I is an alkali metal ion, such as Na, K, etc.; n is the modulus of water glass; x is a natural number greater than or equal to 1. The process is very slow. To accelerate the curing and improve the water resistance of the coating, a curing agent is often added to the water glass coating. The curing agent can react with the alkali metal ions to form water-insoluble substances, and can also promote the further condensation of silica colloids into a water-resistant coating. This method is called the curing technology of water glass, also known as the alkali metal ion fixation technology of water glass. In architectural coatings, it is typical to use condensed phosphates (Al salts, Mg salts, Ca salts, etc.) or β-dicalcium silicate as curing agents; or in addition to using condensed phosphate materials as the main curing agent, one or more auxiliary curing agents (mainly ZnO, Al2O3, borates, etc.) are also used to achieve better curing effects. When condensed phosphates are used as curing agents, condensed phosphates slowly hydrolyze in the coating to release H+, which reacts with water glass to precipitate colloidal silica, which condenses itself to form an inorganic network film with -Si-O-Si- as the main chain, and phosphate reacts with metal ions Al3+, Na+, K+, etc. to form water-insoluble complex salts. The preparation method of condensed phosphates is to react metal oxides or hydroxides such as aluminum, zinc, magnesium, and calcium with a certain amount of phosphoric acid aqueous solution to obtain a suspension, then evaporate to remove water, and then calcine in steps at a certain temperature. For example, 320 g of alumina hydrate and 250 mL of water are used to prepare a suspension, which is slowly added to 1300 g of phosphoric acid (containing 55% P2O5) at 60 ° C in a reactor with constant stirring, and the temperature is raised to 90-95 ° C to allow for a full reaction. The reaction product is dehydrated, dried to constant weight, calcined at 350-400 ° C for 2.5 h, and then calcined at 700-800 ° C for 2.5 h to polymerize and crushed.