What sodium level counts as 'low-soda' reactive alumina?

Industry convention: Na2O < 0.10 wt% (often labeled 'LS' or 'LR') qualifies as low-soda reactive. Standard reactive alumina is typically 0.20-0.40 wt% Na2O, with low-soda grades pushing 0.05-0.09 wt%. The drop matters for any castable that must survive 1200-1700 C service in petrochemical, cement, or steel preheaters because sodium is the dominant driver of low-temperature mullite formation — and unwanted mullite pulls oxide ions out of the binding glass, weakening the silicate network.

Why does Na2O matter in castables even when the service temperature is high?

Sodium lowers the eutectic point of the alumina-silica matrix by 50-150 C and seeds beta-alumina (Na2O·11Al2O3) platelets during the burn-in cycle. Beta-alumina hydrates violently if the casting is cooled below 900 C with moisture present — a re-commissioning risk in preheater linings that have been idled for shutdowns. Low-soda reactive alumina eliminates this hydration risk entirely and lets the casting densify by direct corundum-corundum bridging rather than through a transient sodium-aluminate intermediate.

What's the role of specific surface area (BET) in reactive alumina selection?

BET should land between 1.5 and 4.0 m²/g for a casting-grade reactive alumina. Below 1.5 m²/g the powder is too dead — it will not participate in sintering until 1500 C+, by which point the cement phase has already fired out. Above 4.0 m²/g you get runaway shrinkage (5-7% on first firing vs 2-3% for BET 2.5), poor packing density in the green body, and water demand spikes that drive down castable flow while increasing porosity. For 60-85% alumina castables, BET 2.0-2.8 m²/g is the operational sweet spot.

How does reactive alumina differ from tabular alumina in a castable recipe?

Tabular alumina (T-60/T-64, sintered to ~1950 C, BET <0.5 m²/g) is the inert aggregate — it brings refractoriness and thermal-shock mass. Reactive alumina (BET 2-4 m²/g) is the sintering-active binder. Typical recipes run 60-75% tabular, 8-15% reactive, 5-10% calcium aluminate cement, with the balance being microsilica and dispersing alumina. Reactive alumina's surface energy drives a 100-200 C drop in the matrix densification temperature compared to using dead sintered filler alone — often the difference between a castable that matures at 1100 C versus one that needs 1400 C.

What calcination temperature produces a true reactive alumina (vs just 'ground calcined')?

True reactive alumina is rotary-kiln calcined between 1100 and 1300 C — high enough to drive off 99% of the residual OH but low enough to retain gamma-alumina content above 60%. Above 1350 C the transition to alpha-alumina passes 90% and reactivity collapses; below 1050 C you still have active alumina that over-hydrates in the castable mix water. The label 'reactive Al2O3' on a COA implies this gamma-alpha window. If your supplier can give you only an alpha-phase percentage ('alpha 95%') and no gamma content, you are looking at a dead calcined alumina, not a reactive grade.

Does reactive alumina interact with burned magnesia (MgO) at high temperature?

Above 1300 C, sodium-free reactive alumina reacts with MgO to form MgAl2O4 (spinel) at the grain boundary — this is the desired bond in magnesia-rich tap-hole, ladle, and cement-preheater castables. The reaction is volume-expansive (~5-8%), which is how MgO-bonded castables self-heal microcracks through 1300-1500 C thermal cycling. The catch: residual Na2O in standard reactive alumina poisons the spinel — it sequesters alumina into beta-aluminate instead, suppressing spinel yield by 30-60% until sodium is pushed below 0.10 wt%. This is one of the strongest cases for specifying low-soda reactive alumina specifically in MgO-bonded systems.

What's the dosing range for low-soda reactive alumina in a 70% alumina castable?

Typical recipes: 8-12% reactive alumina in 70% alumina castables for general industrial service; 12-15% in low-cement castables (LCC) where calcium aluminate cement drops below 5%. Below 7% the matrix fails to densify and the casting becomes permeable above 1000 C; above 15% the mix water demand goes non-linear and the casting can shrink-crack during the first firing. For vibrating-cast installations requiring self-flow, 9-11% is the operationally verified range.

How is low-soda reactive alumina used in shotcrete (gunning) mixes?

Shotcrete mixes need fast early-strength set (return-to-service in 8-24 h) plus low rebound loss. Low-soda reactive alumina at 6-9% by weight, paired with calcium aluminate cement at 12-15%, gives the right balance. Lower soda means the hydration heat does not spike prematurely — crews get 60-90 min working time without flash set, and rebound drops from typical 15% down to 6-9%. This is one of the few applications where reactive alumina is not just a sintering aid but a set modifier.

What's the difference between 'fine' and 'reactive' grades — are they the same?

No. Fine-grade calcined alumina (D50 = 5-15 micron, BET <0.5 m²/g) is ground calcined alumina — alpha-phase dominant, low surface area, used to plug pore structure. Reactive grade (BET 2-4 m²/g, gamma-content 60-90%, D50 1-5 micron) is calcined under controlled conditions to preserve metastable phases that drive sintering. A 5-micron fine grade may have one-tenth the surface area of a 5-micron reactive grade — the only correct way to tell them apart is BET and X-ray diffraction peak ratios, not particle size alone.

What storage / shelf-life considerations apply to reactive alumina?

Reactive alumina is hygroscopic — pickup of 1.5-3.0 wt% moisture over 90 days is normal under uncontrolled warehouse humidity (60-80% RH). Once moisture exceeds 3%, the powder begins to form bayerite (trihydrate) that disrupts castable set kinetics and produces steam-induced porosity during first firing. Best practice: store in sealed drums or foil-lined supersacks, keep warehouse RH below 50%, and rotate stock FIFO with 90-day maximum hold. For high-humidity tropics, climate-controlled storage is worth the cost if batch quality must be reproducible year-round.

ATH Flame Retardant Mechanism: Why Aluminum Hydroxide Cools and Chars

Q: Why does Na2O matter in castables even when the service temperature is high?

Sodium lowers the eutectic point of the alumina-silica matrix by 50-150 C and seeds beta-alumina (Na2O·11Al2O3) platelets during the burn-in cycle. Beta-alumina hydrates violently if the casting is cooled below 900 C with moisture present — a re-commissioning risk in preheater linings that have been idled for shutdowns. Low-soda reactive alumina eliminates this hydration risk entirely and lets the casting densify by direct corundum-corundum bridging rather than through a transient sodium-aluminate intermediate.

Q: What's the role of specific surface area (BET) in reactive alumina selection?

BET should land between 1.5 and 4.0 m²/g for a casting-grade reactive alumina. Below 1.5 m²/g the powder is too dead — it will not participate in sintering until 1500 C+, by which point the cement phase has already fired out. Above 4.0 m²/g you get runaway shrinkage (5-7% on first firing vs 2-3% for BET 2.5), poor packing density in the green body, and water demand spikes that drive down castable flow while increasing porosity. For 60-85% alumina castables, BET 2.0-2.8 m²/g is the operational sweet spot.

Q: How does reactive alumina differ from tabular alumina in a castable recipe?

Tabular alumina (T-60/T-64, sintered to ~1950 C, BET <0.5 m²/g) is the inert aggregate — it brings refractoriness and thermal-shock mass. Reactive alumina (BET 2-4 m²/g) is the sintering-active binder. Typical recipes run 60-75% tabular, 8-15% reactive, 5-10% calcium aluminate cement, with the balance being microsilica and dispersing alumina. Reactive alumina's surface energy drives a 100-200 C drop in the matrix densification temperature compared to using dead sintered filler alone — often the difference between a castable that matures at 1100 C versus one that needs 1400 C.

Q: What calcination temperature produces a true reactive alumina (vs just 'ground calcined')?

True reactive alumina is rotary-kiln calcined between 1100 and 1300 C — high enough to drive off 99% of the residual OH but low enough to retain gamma-alumina content above 60%. Above 1350 C the transition to alpha-alumina passes 90% and reactivity collapses; below 1050 C you still have active alumina that over-hydrates in the castable mix water. The label 'reactive Al2O3' on a COA implies this gamma-alpha window. If your supplier can give you only an alpha-phase percentage ('alpha 95%') and no gamma content, you are looking at a dead calcined alumina, not a reactive grade.

Q: Does reactive alumina interact with burned magnesia (MgO) at high temperature?

Above 1300 C, sodium-free reactive alumina reacts with MgO to form MgAl2O4 (spinel) at the grain boundary — this is the desired bond in magnesia-rich tap-hole, ladle, and cement-preheater castables. The reaction is volume-expansive (~5-8%), which is how MgO-bonded castables self-heal microcracks through 1300-1500 C thermal cycling. The catch: residual Na2O in standard reactive alumina poisons the spinel — it sequesters alumina into beta-aluminate instead, suppressing spinel yield by 30-60% until sodium is pushed below 0.10 wt%. This is one of the strongest cases for specifying low-soda reactive alumina specifically in MgO-bonded systems.

Q: What's the dosing range for low-soda reactive alumina in a 70% alumina castable?

Typical recipes: 8-12% reactive alumina in 70% alumina castables for general industrial service; 12-15% in low-cement castables (LCC) where calcium aluminate cement drops below 5%. Below 7% the matrix fails to densify and the casting becomes permeable above 1000 C; above 15% the mix water demand goes non-linear and the casting can shrink-crack during the first firing. For vibrating-cast installations requiring self-flow, 9-11% is the operationally verified range.

Q: How is low-soda reactive alumina used in shotcrete (gunning) mixes?

Shotcrete mixes need fast early-strength set (return-to-service in 8-24 h) plus low rebound loss. Low-soda reactive alumina at 6-9% by weight, paired with calcium aluminate cement at 12-15%, gives the right balance. Lower soda means the hydration heat does not spike prematurely — crews get 60-90 min working time without flash set, and rebound drops from typical 15% down to 6-9%. This is one of the few applications where reactive alumina is not just a sintering aid but a set modifier.

Q: What's the difference between 'fine' and 'reactive' grades — are they the same?

No. Fine-grade calcined alumina (D50 = 5-15 micron, BET <0.5 m²/g) is ground calcined alumina — alpha-phase dominant, low surface area, used to plug pore structure. Reactive grade (BET 2-4 m²/g, gamma-content 60-90%, D50 1-5 micron) is calcined under controlled conditions to preserve metastable phases that drive sintering. A 5-micron fine grade may have one-tenth the surface area of a 5-micron reactive grade — the only correct way to tell them apart is BET and X-ray diffraction peak ratios, not particle size alone.

Q: What storage / shelf-life considerations apply to reactive alumina?

Reactive alumina is hygroscopic — pickup of 1.5-3.0 wt% moisture over 90 days is normal under uncontrolled warehouse humidity (60-80% RH). Once moisture exceeds 3%, the powder begins to form bayerite (trihydrate) that disrupts castable set kinetics and produces steam-induced porosity during first firing. Best practice: store in sealed drums or foil-lined supersacks, keep warehouse RH below 50%, and rotate stock FIFO with 90-day maximum hold. For high-humidity tropics, climate-controlled storage is worth the cost if batch quality must be reproducible year-round.

What It Actually Is (And What It Is Not)

ATH flame retardant mechanism - technical from Aluminaworld. ISO 9001 certified manufacturer with 15+ years export experience. Free sample available.

Aluminum hydroxide (ATH) is the world's largest-volume flame retardant by volume, with global consumption exceeding 1.5 million MT/year. Its flame retardant mechanism is fundamentally different from halogen-based systems: instead of interfering with the gas-phase chemistry of combustion, ATH works in the condensed phase by absorbing heat, releasing water, and promoting char formation.

The Hidden Mechanism Behind It

At Aluminaworld, we manufacture ATH specifically for flame retardant applications, with annual capacity of 80,000 MT across multiple surface-treated and untreated grades. The data below reflects our standard grades and the performance our customers achieve in cable, compound, and engineered stone applications.

This article explains the chemistry of ATH flame retardancy, the practical loading limits in common polymers, and how to formulate for optimal performance.

Measured Numbers Across Grades

Property	This Material	Common Alternative	Test Method
Surface area	High	Low	BET N2
Bulk density	Grade specific	Different	ASTM C527
Pore volume	Uniform	Narrow	Hg intrusion
Mechanical strength	Attrition resistant	Friable	ASTM D5757
Service life	Longer cycle	Shorter	Field data

Industry-typical values are a starting point, not a guarantee. Your duty cycle, feed gas and service interval decide the final grade — always run a 50 kg trial before committing to a 20 t shipment.

When ATH is heated to 200 to 300°C, it decomposes endothermically:

2 Al(OH)3 = Al2O3 + 3 H2O (vapor), delta-H = +1,300 kJ/kg

Where It Earns Its Premium

Al2O3 · nH2O → Al2O3·nH2O (surface) + Al(OH)3 (bulk)

This is the working principle behind every number in the table above. Get the mechanism right and grade selection becomes a lookup; get it wrong and no supplier will save the run.

This single reaction accomplishes four things simultaneously:

The decomposition temperature window (200 to 300°C) is critical: it must be below the polymer's processing temperature but above its normal use temperature. For most thermoplastics processed at 180 to 220°C, ATH decomposes during processing. For polyolefins processed at 200 to 240°C, special surface treatment is needed to prevent premature decomposition.

The Product Across Variants

ATH loading: 50 to 65 wt% (the maximum that allows reasonable melt flow). Below 40%, flame retardancy is marginal. Above 65%, mechanical properties (especially elongation at break) suffer significantly. Common compromise: 55 to 60% ATH with a coupling agent to maintain mechanical performance.

ATH loading: 60 to 75 wt% achievable because the elastomeric matrix tolerates higher filler content. EVA + 65% ATH is the industry-standard LSZH cable compound.

Variant	Form	Grade	Key Spec	Typical Service
Standard	Spherical	Type 1	Bulk density	General drying
High strength	Spherical	Type 2	Attrition index	High-cycling beds
Bead	Extrudate	Type 3	Crush strength	Pressure swing
Fine	Powder	Type 4	Particle size	Coating and binder
Catalyst support	Spherical	Type 5	Pore volume	Catalyst carriers
Specialty	Custom	Type 6	Tailored spec	OEM applications

Working Time, Turnaround and Operating Discipline

ATH loading: 40 to 60 wt% depending on the application. Lower loading (40 to 50%) is used in cast polymer countertops where surface aesthetics matter. Higher loading (55 to 60%) is used in electrical laminates where maximum flame retardancy is needed.

ATH loading: 20 to 40 wt% as a smoke suppressant and acid scavenger. Used together with antimony trioxide synergist in flexible PVC. The combination cuts smoke emission by 50 to 70% in cone calorimeter tests.

Molecular Sieve
Activated Alumina
Alumina Powder
Aluminum Hydroxide

Heat absorption (1,300 kJ per kg ATH). The polymer is cooled below its ignition temperature, slowing or stopping the combustion cycle.
Water vapor release (35 wt% of the original mass). The vapor dilutes flammable gases and oxygen in the flame zone.
Alumina residue formation. The Al2O3 char acts as a barrier, insulating the underlying polymer from heat and oxygen.
Smoke suppression. Unlike halogen systems, ATH produces minimal smoke and no corrosive or toxic gases.

Procurement Specification

Untreated ATH has polar surface that does not bond well with polyolefins. The result is poor mechanical properties (especially impact strength and elongation) and high viscosity that makes processing difficult. Surface-treated ATH (silane, stearate, or titanate coating) costs 20 to 30% more but improves compound properties dramatically. Use it for any loading above 40 wt%.

ATH particle size affects both flame retardancy and processing. Coarse ATH (10 to 20 μm) gives better flame retardancy (more char, slower heat release) but worse mechanical properties. Fine ATH (1 to 3 μm) gives better surface finish and mechanical strength but slightly worse flame retardancy. Most LSZH cable compounds use a bimodal blend: 70% coarse + 30% fine.

COA Parameter	Report Method	Typical Limit	Why It Matters	Acceptance
Surface area	BET	As specified	Capacity driver	>= 95% of nominal
Bulk density	Tamped	As specified	Filling and mass transfer	Within tolerance
Particle size	Sieve / laser	Grade window	Pressure drop	No fines excess
Crush strength	ASTM D5757	Grade minimum	Attrition loss	>= 95% of spec
Moisture	Loss on ignition	Low	Prevents caking	Below limit
Appearance	Visual	Free flowing	Handling	No lumps or fines

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Storage, Shelf Life and Inventory Hygiene

A coupling agent (typically 0.5 to 1.5% of ATH weight) bridges the polar ATH surface to the non-polar polymer matrix. Without coupling, the compound has 30 to 50% lower elongation at break. Common coupling agents: silanes (best for polyolefins), titanates (best for filled composites), and maleated polyolefins (cost-effective all-rounder).

For new ATH formulations or troubleshooting existing ones, our technical team can review your recipe and recommend specific grades, treatments, and coupling agents. We supply free 1 kg samples of multiple grades for lab evaluation.

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✉️ Email: barry@okkinc.com
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Store in the original sealed container, off the floor.
Target a cool, dry, ventilated store away from direct sunlight.
Use first-in first-out and log every lot you open.

Total Cost of Ownership

Is ATH safer than halogen flame retardants?

Yes. ATH releases only water when it decomposes. Halogen systems release HCl or HBr, which are corrosive and toxic. ATH is also non-bioaccumulative, unlike some brominated flame retardants.

Cost Component	Standard Grade	Engineered Grade	What Changes
Purchase price per tonne	Lower	Higher	Upfront premium
Service life per charge	Shorter	Longer	Fewer changeouts
Product loss to carry-over	Higher	Lower	Less product waste
Unplanned shutdown frequency	More frequent	Less frequent	Less downtime
Lab and QA cost per batch	Higher unit cost	Lower unit cost	Fewer deviations
Total cost per tonne of product	Higher	Lower	Payback on volume

The payback shows up in the second and third changeout, not the purchase order. Model your own numbers on the duty cycle you actually run, then compare the landed cost per tonne of finished product rather than the price per tonne of adsorbent.

What is the maximum temperature ATH can withstand?

ATH starts losing water at 200°C and is fully converted to alumina by 300°C. For continuous high-temperature applications above 200°C, consider aluminum hydroxide carbonate (AHC) or magnesium hydroxide, which decompose at 300 to 400°C.

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Next Steps — Get a Sample or Talk to Our Engineers

Does ATH affect polymer color?

Pure ATH is white (whiteness at least 96%). It is used as a white pigment extender in many formulations. For colored compounds, use our low-iron grades (Fe2O3 below 50 ppm) to avoid yellow tint.

What is the price for ATH flame retardant?

$400 to $800 per MT depending on grade, particle size, and surface treatment. Surface-treated grades are 20 to 40% higher than untreated. Fine-particle grades are 30 to 50% higher than coarse.

Is ATH suitable for food contact applications?

Yes, our FDA-grade ATH meets 21 CFR 186.1307 for direct and indirect food contact. Common in food packaging films and disposable containers.

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Tell us your feed conditions and target outlet spec. Our engineers reply within 24 hours with a recommendation and a 1 kg free sample.

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We hold stock on hand and ship within 15-25 days of order confirmation. If you need a custom specification, a blended grade or a trial lot, tell us the duty cycle and we will run the sample from our own production line.

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