Refractory materials are a class of inorganic non-metallic materials with a refractoriness of no less than 1580°C; refractoriness is defined as the temperature at which a conical specimen of the material, under no-load conditions, resists softening and collapsing when exposed to high temperatures. Widely used across various sectors of the national economy-including iron and steel, non-ferrous metals, glass, cement, ceramics, petrochemicals, machinery, boilers, light industry, electric power, and the defense industry-refractory materials are essential for the production operations and technological advancement of these industries, playing an irreplaceable role in the development of high-temperature industrial production.\
The main physical properties of refractory materials include the following:
(1) Structural properties of refractory materials (GB/T 2997-2015): porosity, bulk density, water absorption, gas permeability, pore size distribution, etc. Generally, as firing time increases, porosity (due to unbalanced diffusion and gas generation) and bulk density both increase.
(2) Thermal properties of refractory materials (GB/T 7320-2008): thermal conductivity, coefficient of thermal expansion, specific heat, heat capacity, thermal diffusivity, thermal emissivity, etc.
(3) Mechanical properties of refractory materials (GB/T 5072-1985, GB/T 3001-2000): compressive strength, tensile strength, flexural strength, torsional strength, shear strength, impact strength, abrasion resistance, creep resistance, bond strength, modulus of elasticity, etc.
(4) Service performance of refractory materials: refractoriness, refractoriness under load, permanent linear change upon reheating, thermal shock resistance, slag resistance, acid resistance, alkali resistance, hydration resistance, erosion resistance, electrical conductivity, oxidation resistance, etc.
(5) Workability of refractory materials: consistency, slump, flowability, plasticity, adhesiveness, rebound, setting characteristics, hardening characteristics, etc.
As the state places increasing emphasis on environmental protection and pollution control, environmental authorities are imposing stricter requirements on refractory material enterprises. During the production of refractory magnesia bricks, binders are added to ensure that the various raw materials bond tightly before firing, thereby enhancing the stability and deformation resistance of the green bricks. This is because the stability and structural integrity of the green brick are crucial prerequisites for the finished refractory material to meet performance standards.
Binders can be classified in various ways: by origin (natural vs. synthetic) or by chemical composition (organic vs. inorganic). Organic binders include synthetic resins, residual oil, tung oil, asphalt, rosin, flour, dextrin, syrup, pulp waste liquor, and furfuryl alcohol resin; inorganic binders include clay, gypsum, water glass (sodium silicate), cement, and phosphates.
Common binders currently used in refractory bricks include phenolic resins, pulp waste liquor, and organic powders. While these binders serve a purpose prior to firing, they become unstable impurities during the firing process and subsequent high-temperature use. When cement manufacturers use these refractory bricks, the binder impurities release harmful substances-such as organic compounds and sulfides-that not only compromise cement performance but also harm the environment. Refractory bricks containing organic binders are of particular concern, as complex chemical reactions during their production can result in the emission of significant quantities of harmful volatile organic compounds (VOCs).
VOC stands for Volatile Organic Compounds; the term commonly refers to the harmful chemical substances (excluding heavy metals) found in products like wall paint. Major components of VOCs include hydrocarbons, halogenated hydrocarbons, oxygenated hydrocarbons, and nitrogenated hydrocarbons. Specific examples include benzene series compounds, organic chlorides, Freons, ketones, amines, alcohols, ethers, esters, acids, and petroleum hydrocarbons-substances such as formaldehyde, ammonia, ethylene glycol, and various esters.
Commonly used organic binders include the following:
(1) Coal tar pitch: Often contains aromatic hydrocarbons.
(2) High-temperature resistant polymers: High-temperature resistant resins with excellent thermal stability, exemplified by polyimides-such as bismaleimide (BMI), polyarylacetylene (PAA), and COPNA. Among these, polyimides (PI) exist in both thermoplastic and thermosetting forms, feature aromatic heterocyclic structural units, and-due to high costs-are typically used in military applications. Heat-resistant matrix resins or precursor resins-represented by epoxy resins and phenolic resins-include furan resins (FA) and polycarbosilanes (PCS). Epoxy resin (EP) is a thermosetting polymer with a heat resistance of less than 200°C. Phenolic resin (PF) is generally produced through the polycondensation of phenol and formaldehyde; it offers good ablation resistance and a high char yield.
(3) Pulp waste liquor: Sulfite pulp waste liquor binders, primarily consisting of mixtures of various lignosulfonates, thiolignins with sulfite structures, and their derivatives. Lignosulfonates used as binders for refractory materials mainly include calcium lignosulfonate, sodium lignosulfonate, and mixed salts containing both calcium and sodium. These materials are typically added at levels of 1% to 3.5% and decompose into trace amounts of CaO and Na₂O.
(4) Polymer types: Carboxymethyl cellulose, polyvinyl alcohol, and hydroxypropyl methyl cellulose.
Since organic substances generally tend to emit VOCs at high temperatures, environmentally friendly binder systems must be inorganic. Inorganic binders offer high bonding strength, structural stability at high temperatures, low raw material costs, and zero volatility.
The most common types of inorganic binders are silicate, phosphate, and borate binders; additionally, sulfate binders are used in certain specific environments.
(1) Clay. Clay is a mixture of hydrated aluminosilicates-primarily composed of SiO₂, Al₂O₃, and water of crystallization-that serves as a binder for green sand molds. When wetted, clay exhibits plasticity and binding properties; upon drying, it hardens to provide dry strength. Crucially, its plasticity and binding capabilities can be restored by adding water to the hardened clay, making it highly reusable. However, the production cycle for clay-bonded sand is long, requiring large-scale drying equipment, and the resulting castings often suffer from severe sand adhesion on their surfaces.
(2) Silicate binders (e.g., water glass). Silicate binders are soluble inorganic binders-also known as sodium silicate or "water glass"-produced from alkali metal oxides and silicon dioxide. They offer advantages such as simple preparation, low cost, and excellent high-temperature stability, making them popular choices for refractory coatings and fire-resistant insulation materials. However, their performance and utility are limited by drawbacks such as poor water resistance and high brittleness. Sodium silicate, the primary component, is a water-soluble silicate. As one of the most successful inorganic binders currently used in foundry production, it is highly favored because it does not emit toxic gases or fumes during the casting process. Nevertheless, water glass-bonded sand has a short shelf life and is prone to "aging"-a problem for which no satisfactory solution has yet been found. Additionally, poor collapsibility and recyclability remain major obstacles to its widespread application.
(3) Phosphate inorganic binders. Phosphate inorganic binders are inorganic chemical binders primarily composed of the reaction products of phosphoric acid and aluminum hydroxide. Initially used in dentistry for filling cavities, they have since been applied to bonding metal tools and in the fields of refractory materials and ceramics. When used as an inorganic binder in casting, this material shares the characteristics of water glass-being colorless, odorless, and non-toxic to humans-while offering superior collapsibility. However, as phosphate-based inorganic binders were adopted for casting applications relatively late, there remain significant challenges regarding their implementation in actual production.
Common inorganic binders include bentonite (primarily montmorillonite), water glass (sodium silicate), dolomite (including clay and claystone varieties), and kaolin. Phosphate binders are among the most widely used inorganic binders; they typically consist of a phosphate matrix, a curing agent, and fillers, with special additives-such as diluents or foaming agents-sometimes incorporated to meet specific application requirements. Depending on the reactivity of the added oxides, phosphate binders generally cure via either low-temperature or high-temperature processes. When substances with lower reactivity, such as aluminum oxide or copper oxide, are added, the reaction rate between the phosphate matrix and the metal oxides is slow, requiring heat to facilitate curing and film formation. Conversely, when highly reactive substances like zinc oxide or magnesium oxide are added, the binder can cure and form a film at room temperature. Calcining the oxides to reduce their reactivity can enhance the bonding performance.
This paper presents a novel inorganic binder system that is free of organic components, thereby avoiding the presence of complex or harmful organic substances. Additionally, the thermodynamic and degradation behaviors of the binder were analyzed using techniques such as infrared spectroscopy and thermogravimetric analysis, and the evolution of the chemical composition of the self-developed binder during the heating process involved in refractory brick fabrication was investigated.