Common Types and Performance Applications of Binders in Refractory Materials
PART01 Calcium aluminate cement
1.Minerals in calcium aluminate cement
The primary minerals in calcium aluminate cement are calcium aluminate (CA), calcium dialuminate (CA₂), calcium aluminate dodecahydrate (Cl₂A₇), and calcium feldspar (C₂AS). The changes in mineral composition during the firing of pure calcium aluminate cement are illustrated in Figure 1.
As shown in Figure 1: Calcium aluminate (CA) constitutes the primary mineral in pure calcium aluminate cement. However, small quantities of calcium aluminate dihydrate (CA₂) and calcium aluminate heptahydrate (C₁₂A₇) may also be present.
It is generally recognised that CA exhibits high hydraulic activity, characterised by normal setting and rapid hardening, serving as the principal source of cement strength. Cements with higher CA content demonstrate accelerated strength development predominantly in the early stages, with less pronounced late-stage strength gain.
CA₂ exhibits slower hydration and hardening, resulting in low early strength but high late strength development. Excessively high CA₂ content adversely affects the cement's rapid hardening properties.
The aluminium and calcium coordination within C₁₂A₇ is highly irregular, featuring numerous cavities within its crystal structure. This results in extremely rapid hydration and setting, though its strength is inferior to that of CA. Cement containing substantial amounts of C₁₂A₇ may exhibit rapid setting, reduced strength, and diminished heat resistance. However, when controlled appropriately, trace amounts of C₁₂A₇ in cement can actually accelerate setting and enhance early strength.
Generally, calcium feldspar C₂AS exhibits no hydraulic properties. However, if methods can be devised to form CaO-Al₂O₃-SiO₂ glass from the SiO₂ in high-alumina cement, the SiO₂ may participate in hydration. This could potentially enhance the performance characteristics of high-alumina cement.
2. Hydration of Calcium Aluminate Cement
Calcium aluminate cement undergoes hydration upon contact with water. The sequence of hydration, setting, hardening, and strength development is illustrated in Figure 2.
As shown in Figure 2: Upon contact with water, cement minerals dissolve, increasing the concentration of ions such as Ca²⁺ and Al(OH)₄⁻ in the solution, causing a rapid rise in electrical conductivity. Subsequently, as ion concentrations reach saturation, the concentration of ions in the liquid phase ceases to increase. Crystalline hydration phases gradually form, and the cement paste progressively loses its fluidity. During Stage ID, extensive hydration reactions occur, causing the temperature and bound water content of the cement paste to rise while ion concentrations decrease. The paste begins to harden and develop strength. The strength of calcium aluminate cement primarily originates from various hydrated calcium aluminate compounds CₓAᵧ and aluminium gel Aₕ₃.
3. Transformation of Calcium Aluminate Cement Hydrates
The transformation relationships of calcium aluminate cement hydration products under temperature variations are shown in Table 1.
Table 1 indicates that both CAH₁₀ and C₂AH₈ are metastable minerals. With increasing temperature and prolonged exposure, both CAH₁₀ and C₂AH₈ transform into the stable mineral C₃AHS. CAH₁₀ and C₂AH₈ crystallise as hexagonal lamellar structures with densities of 1.72 g/cm³ and 1.95 g/cm³ respectively. C₃AH₆ belongs to the isometric crystal system, exhibiting a high density of 2.52 g/cm³. Consequently, upon transformation from CAH₁₀ and C₂AH₈ to C₃AH₆, the hydrate's density increases while its water content decreases. Due to the reduction in hydrate volume, its strength diminishes significantly.
4. Transformation of Calcium Aluminate Cement Phases Upon Heating
Upon heating, refractory cement undergoes more complex transformations. According to Roesel: the stable temperature ranges for CAH₁₀, C₂AH₆, and C₃AH₆ are 0–20°C, 20–60°C, and 0–350°C respectively. Between 200–350°C, AH₃ transforms into Al₂O₃; C₃AH₆ transforms into CaO and C₁₂A₇. At 600–1000°C, C₁₂A₇ reacts with CaO to form CA; at 1000–1300°C, CA reacts with A to form CA₂; at 1400–1600°C, CA₂ reacts with alumina to form CA₆.
Following dehydration of the hydrates, the hydration bonds are destroyed, yet the ceramic bonds have not yet formed, resulting in very low material cohesion. Consequently, conventional refractory castables experience over 50% strength reduction after 1100°C heat treatment, severely compromising service life. Some attribute this to significant expansion from solid-phase reactions occurring above 1000°C. Alternatively, it is proposed that volume effects accompanying solid-state chemical reactions during the transition from hydrate to ceramic structures cause the porous structure and low strength observed in traditional refractory castables at intermediate temperatures.
PART02 Phosphoric Acid and Phosphates
1. Properties and Preparation of Raw Materials
The refractory industry frequently employs phosphoric acid and aluminium dihydrogen phosphate as binders.
(1) Phosphoric Acid
The chemical formula for phosphoric acid is H₃PO₄. Pure phosphoric acid exists as colourless orthorhombic crystals with a melting point of 42.35°C and a boiling point of 213°C (decomposing to H₂O). It transforms into metaphosphoric acid around 300°C. At 25°C, its relative density is 1.874, and it exhibits significant hygroscopicity. Commercially available phosphoric acid is a colourless, transparent liquid with a concentration of 85%. At 25°C, its relative density is 1.6850. The changes in density and concentration of 85% phosphoric acid upon addition of water are illustrated in Figure 3.
Phosphoric acid is a corrosive substance classified as a secondary inorganic acid. Its corrosive properties are weaker than those of sulphuric acid, hydrochloric acid, and nitric acid, but stronger than those of acetic acid and boric acid. Phosphoric acid corrodes metals, releasing hydrogen gas. It reacts with alkalis, basic oxides, and inorganic salts. High-concentration phosphoric acid causes corrosive burns upon skin contact, though its effect is not severe. Phosphoric acid fumes irritate the eyes and respiratory tract; inhalation may induce coughing, tracheitis, or bronchitis.
Phosphoric acid is produced via thermal or wet methods. The thermal process involves burning yellow phosphorus to generate phosphorus pentoxide, which is then absorbed by water to yield phosphoric acid.
The wet process involves the production of phosphoric acid through the reaction of sulphuric acid with apatite:
The reaction between phosphoric acid and aluminium hydroxide or aluminium oxide sequentially forms aluminium dihydrogen phosphate Al(H₂PO₄)₃, aluminium monohydrogen phosphate Al₂(HPO₄)₆, and aluminium orthophosphate AlPO₄.
As hydrogen in phosphoric acid is replaced by aluminium, the molar ratio of Al₂O₃:P₂O₅ in the reactants progressively increases. Aluminium dihydrogen phosphate exhibits an Al₂O₃:P₂O₅ ratio of 1:3; aluminium monohydrogen phosphate has a ratio of 1:1.5; while aluminium orthophosphate has a molar ratio of 1:1. At a ratio of 1:3, aluminium phosphates exhibit good solubility; when the ratio exceeds 1:1.5, solubility becomes very low or even insoluble.
(2) Aluminium dihydrogen phosphate
Aluminium dihydrogen phosphate has the chemical formula Al(H₂PO₄)₃. It exists as a white crystalline powder readily soluble in water or as a colourless, odourless viscous liquid. It possesses strong chemical bonding strength, resistance to high temperatures, thermal shock, and high-temperature gas erosion, along with excellent infrared absorption capacity and good insulation properties. When preparing aluminium dihydrogen phosphate, the Al₂O₃:P₂O₅ ratio is typically set at 1:3.2 to prevent precipitation during prolonged storage. The preparation method involves adding aluminium hydroxide powder to a plastic container, then incorporating boiling water to form a concentrated slurry. While stirring continuously, slowly add phosphoric acid at 85°C until the reaction is complete. Should ambient temperatures be low, the aluminium hydroxide slurry must be heated before transfer to an acid-resistant vessel. Phosphoric acid is then added and stirred to form aluminium dihydrogen phosphate. During preparation, the rate of phosphoric acid addition must be controlled to prevent vigorous acid-base reaction causing the liquid to boil over. Requirements for phosphoric acid and aluminium hydroxide for preparing aluminium phosphates at different ratios are detailed in Table 2.
Table 2 indicates the quantities of pure phosphoric acid and aluminium hydroxide required to form phosphoric acid rods with different Al₂O₃:P₂O₅ ratios. Where phosphoric acid concentration is less than 100%, divide the phosphoric acid requirement in the table by the percentage of phosphoric acid used to obtain the proportion of phosphoric acid employed. During preparation, the proportion of dry aluminium hydroxide used is uniformly 156g.
2.Transformation upon Heating
Phosphate bonding constitutes a thermosetting bond. Generally, without the addition of accelerators such as cement, the phosphate must react with the refractory material under elevated temperatures to form a chemical bond.
Upon heating, the phosphate bonding phase undergoes highly complex chemical transformations, the specifics of which vary according to temperature, the original composition of the binder, the composition of the refractory material, and its reactivity. For instance, when phosphoric acid is mixed with industrial alumina and heat-treated at 120°C, it forms phosphoaluminite (Al₂PO₄·2H₂O). At 200°C, the bonded phase remains phosphoaluminite. At 350°C, it transforms into pyrophosphate (Al₂PO₄). Only after heat treatment above 500°C does it begin to form quartz-like and orthoclase-type Al₂PO₄.
Generally, the aluminium phosphate bonding phase requires heat treatment above 500°C to maintain long-term stability in air. This is because anhydrous aluminium phosphate (Al₂O₃·nH₂O) exhibits multiple crystalline forms: besides the aforementioned tridymite, orthoclase, and pyrophyllite types, it also includes metastable crystals A, B, C, D, and E. Among these, the metastable aluminium phosphates A, B, C, D, and E exhibit moisture absorption and softening. Insufficient heat treatment temperatures, coupled with inadequate setting accelerators, will compromise the performance and service life of phosphate-bonded refractories.
PART 03 Water Glass
1.Basic Properties of Water Glass
Water glass, commonly known as "sodium silicate", has the chemical formula Na₂O•mSiO₂•nH₂O. The variable m is termed the modulus, representing the molar ratio of SiO₂ to Na₂O, while n determines the concentration of the water glass. Typically, water glass exhibits a modulus ranging from 2.0 to 3.3, with a density between 1.3 and 1.6 g/cm³. Figure 4 illustrates the phase diagram for the Na₂O-SiO₂ system involving water glass.
Generally, when the modulus of water glass is high, its viscosity increases, leading to faster setting and greater wet strength of the green mould, but the binder's shelf life is reduced. For instance, in foundries producing small to medium-sized cores with short production cycles, water glass with a modulus of 2.6–3.0 is selected; for medium-sized castings, a modulus of 2.3–2.6 is chosen; while for large castings with extended production cycles, a modulus of 2.0–2.3 is preferred.
2.Adjusting Water Glass Composition
Should the water glass modulus fail to meet requirements, it may be necessary to reduce or increase the modulus, or blend two water glasses with differing moduli to achieve an intermediate value.
To reduce the modulus, NaOH is added to the water glass solution. Prior to adjustment, determine the mass fractions of SiO₂ and Na₂O (ωSiO₂ and ωNa₂O) in the solution. Adding a quantity χ of NaOH to 100g of water glass reduces the modulus as follows:
To enhance the modulus of water glass, methods such as adding hydrochloric acid, ammonium chloride, or amorphous silica may be employed. Among these, incorporating amorphous silica is preferable. The addition of either hydrochloric acid or ammonium chloride produces sodium chloride (NaCl), an electrolyte detrimental to the properties of water glass.
When blending two types of water glass with differing modulus values to create a new formulation with an intermediate modulus, consider adding χ grams of high-modulus water glass to 100 grams of low-modulus water glass. The resulting modulus of the blended water glass is calculated as follows:
Should the density of purchased water glass fail to meet specifications, it may be adjusted by diluting with purified water or concentrating through heating.
3.Ageing of Water Glass Solution
Water glass constitutes a mixture of various sodium metasilicates in solution, existing in a state of dynamic equilibrium. This equilibrium shifts with variations in temperature and the passage of time.
Ageing refers to the spontaneous polymerisation of silicic acid within water glass, particularly in high modulus formulations, leading to a gradual reduction in viscosity and adhesive strength. The process commences with hydrolysis into monosilicic acid, followed by condensation into disilicic acid, then trisilicic acid, tetrasilicic acid, cyclic tetrasilicic acid, cubic octasilicic acid, and ultimately the condensed polymerisation product of cubic octasilicic acid. As this polycondensation progresses, the structure of the water glass continuously transforms, ultimately becoming a fully non-hydrolysable polycondensation product – i.e., aged.
If the modulus of the water glass is below 3.0 and ageing has occurred, physical modification methods such as magnetic field treatment, ultrasonic agitation, reflux heating, or heating in a reactor vessel can partially restore its properties. Alternatively, chemical modification may be employed. For instance, adding 0.2% (by mass) polyacrylamide can effectively delay the ageing of water glass. For low modulus water glass, this can extend shelf life by approximately two months; for high modulus water glass, by approximately one month.
4. Setting and Hardening of Water Glass
Water glass can be cured either by heating or by adding accelerators to induce chemical solidification. Accelerators used for chemical curing include sodium fluorosilicate (Na₂SiF₆), dicalcium silicate (C₂S), ferrosilicon, and organic esters. Typically, the chemical curing of water glass proceeds through three stages: hydrolysis of sodium silicate, formation of a silicate sol, and subsequent formation of a silicate gel.
Part 4: Silica Sol
Typically, the silica sol used in refractory manufacturing is produced by removing sodium from water glass via ion exchange. Its composition is as follows: SiO₂ 20%–30%, moisture 70%–80%, Na₂O 0.4%–0.5%, with a relative density of 1.14–1.21 g/cm³ and colloidal particle size of 5–20 μm. The shelf life is approximately one year. The key technical points for preparing silica sol are as follows.
Raw materials: Water glass with a modulus of 2.2–3.7; strongly acidic, styrene-based cation exchange resin; strongly acidic or weakly basic, styrene-based anion exchange resin.
The production process for silica sol is as follows:
1.Dilute water glass to the specified concentration using purified water.
2.Prepare a hydrochloric acid dilution solution.
3.Pass the hydrochloric acid solution through a cation exchange column to convert the active groups on the ion exchange resin to the hydrogen form. Then, wash away residual acid and chloride ions using distilled water.
4.Pass the diluted water glass through the cation exchange column at a controlled flow rate, exchanging Na⁺ ions in the water glass with H⁺ ions from the resin.
5.Pass the desodiumized silica sol solution through a weakly alkaline anion exchange column to remove anions (e.g., Cl⁻) from the liquid, thereby stabilizing the silica sol.
6.Add stabilizers.
7.Concentrate the solution by vacuum heating.
Silica sol serves as a binder for advanced refractory materials. For shaped refractories, silica sol binders enhance bulk strength, reduce sintering temperatures, and broaden the sintering range. For unshaped refractories, silica sol acts as an auxiliary binder. Proper formulation of silica sol with chemical additives can significantly enhance high-temperature performance without compromising setting or curing rates, or reducing green strength, by substantially lowering cement content. For instance, this facilitates the preparation of ultra-low-cement refractory castables.