The basic composition of thick-coating fire-retardant coatings for steel structures includes: binder (silicate cement, magnesium chloride oxide, or inorganic high-temperature binder, etc.), aggregates (expanded vermiculite, expanded perlite, silicon-alumina fiber, mineral wool, rock wool, etc.), chemical additives (modifiers, hardeners, waterproofing agents, etc.), and water. The base materials for steel structure fire-retardant coatings include silicate cement, magnesium chloride oxide cement, and inorganic binders, with commonly used inorganic binders including alkali metal silicate types and phosphate substances, etc.
Free alkali metal ions are often present in alkali metal silicate compounds, which react chemically with acidic gases in the air and CO2. When used alone as the base material for coatings, alkali metal silicate compounds result in coatings that are not water or moisture-resistant, have poor weather resistance, and are prone to cracking and flaking. Therefore, when employing alkali metal silicate compounds as the base material for fire-resistant coatings on steel structures, the primary key issue to address is their modification, specifically the suppression of the free alkali metal ions.
Phosphate-based binders are also commonly used inorganic binders. When used as the base for fire-retardant coatings, they prevent issues with alkaline oxides reacting with acidic gases in the air, thereby enhancing the coating's weather resistance and water resistance, among other physical and chemical properties. However, the molar ratio of M/P (M stands for metal, P for phosphorus) in phosphate-based binders directly affects various properties such as storage stability, water resistance, and adhesion to steel substrates. Therefore, controlling the base molar ratio is crucial in the development of fire-retardant coatings for steel structures with phosphate as the base.
Due to the thick coating and high volume of fire-resistant coatings for heavy-duty steel structures, the weight of the building is increased. Therefore, it is important to add some lightweight materials and high-efficiency thermal insulation aggregates to reduce the density of the coating. The commonly used lightweight thermal insulation aggregates are expanded vermiculite and expanded perlite.
Vermiculite is an iron and magnesium-containing aluminum silicate mineral with a layered structure and interlayer crystalline water. When heated, it expands and wiggles like a leech, hence its name. After drying, crushing, and screening, vermiculite is calcined at temperatures between 850 and 1000 degrees Celsius, where its particles expand over 20 times to form expanded vermiculite. It has a density of 80 to 200 kg/m³ and a thermal conductivity of 0.17 to 0.25 W/(m·K), boasts strong refractory properties, excellent sound absorption and insulation capabilities, and is non-toxic and odorless, making it an important additive for fire-resistant coatings.
Expanded Perlite is a glassy rock that, after being crushed, sifted, preheated, and instantaneously fired, can expand to about 20 times its original size, forming an expanded perlite with a honeycomb-like structure. The expanded perlite is lightweight, with a density of 80-250 kg/m3, and a thermal conductivity of 0.042~0.076 W/(m·K). It boasts insulation, heat resistance, non-flammability, non-toxicity, and good chemical stability, making it an important filler in fire-retardant coatings.
