Aluminium (Aluminum) Powder in Autoclaved Aerated Concrete (AAC): Gas Evolution Mechanism & Grade Selection

Your AAC has sink marks on top. The block weighs more than the spec sheet allows. Or the rise simply stopped before the mould was full.

Nine times out of ten the culprit is the gasifier, not the mix design — an aluminium powder whose activity, fineness, or gas curve no longer matches the casting window you are running.

Why aluminium is the AAC gasifier

Autoclaved aerated concrete gets its pores from a chemical reaction, not from mechanical foaming. In the alkaline slurry — lime (Ca(OH)₂) plus the alkalis released as cement hydrates — metallic aluminium is attacked and evolves hydrogen:

2Al + 3Ca(OH)₂ + 6H₂O → 3CaO·Al₂O₃·6H₂O + 3H₂↑

The hydrogen forms bubbles. A foam stabilizer — usually protein- or saponin-based — keeps them from coalescing and escaping, and the setting cement freezes the pore structure in place. By stoichiometry, roughly 1 gram of active aluminium releases about 1.24 L of H₂ at STP: 2 mol Al (54 g) yields 3 mol H₂ (67.2 L). That is why a very small dose creates a large, uniform void volume.

Typical AAC dosing sits in the range of 0.4–0.8 kg per cubic metre of concrete — a fraction of a percent by mass, enough to cut the block density roughly in half.

Flake or atomized for AAC

Flake (ball-milled) aluminium is the mainstream choice for AAC. Flat platelets disperse evenly through the slurry and gas off on a steady, controllable curve — exactly what you want when the bubble structure has to stay uniform across a full mould. Made on our ball-milling line.

Spherical (atomized) powder is the exception rather than the rule here: its high surface area drives a fast, early gas peak, and the round particles can agglomerate in a dense slurry. It earns its place in flash and report compositions, not in AAC. We cover the shape split in our fine vs spherical selection guide.

What “active aluminium” buys you

“Active aluminium” is the fraction of the powder that actually reacts. The remainder is the oxide skin (Al₂O₃) that forms the moment fresh flake meets air — and that same skin is what makes the powder safe to handle and stable in storage.

Low active aluminium means low gas yield: the block comes out heavy and under-aerated, no matter how carefully you dose. Commercial AAC grades normally specify an active-aluminium floor of 85% and up. The exact figure for your product is on the grade page — match the spec sheet, not the label.

The grade dimensions that matter

Four numbers decide whether a grade works in your plant. Everything else is secondary.

Moisture and oil content sit behind all four: they change flow and reactivity, and a batch that cakes in the silo will dose unevenly even if the certificate says otherwise.

Matching the gas curve to your casting window

The most common AAC defect is a gas curve that does not fit the slurry. If gassing peaks before the mix thickens, bubbles break through the surface and the top sinks. If it peaks after the mix has set, the block never reaches full height and you get a dense bottom and a short rise.

You tune the curve with three levers: fineness, surface treatment (a mineral-oil coating or dispersant on the flake), and slurry temperature. Our FLPA particle-size grade guide explains how D50 maps to real behaviour, and our air-atomized line is where spherical grades come from when a different application calls for them.

The safety line

Finely divided aluminium that is damp is a Division 4.3 dangerous good: it releases flammable hydrogen on contact with water and can form explosive dust clouds in air. In finished AAC the metal has been fully consumed into the cement matrix, so the cured block is inert. The hazard lives upstream, in storage and handling:

• Store dry and sealed, away from acids and oxidizers.
• Never let fine powder meet standing water or high humidity during handling.
• Follow local dangerous-goods transport rules for shipping (Class 4.3; UN 1396 or 1309 by form).

If you are still screening suppliers, our supplier selection guide covers MOQ, samples, and the questions that separate a real manufacturer from a reseller.

Before you send that RFQ

Save a round of emails. Tell us:

• Application — AAC block or panel, and target density
• Shape — flake
• Active-aluminium floor you need
• Desired gas curve — onset t₁ and peak t₂, or your current cycle time
• Monthly volume, and whether you need dangerous-goods documentation

Send it through our contact form and an engineer replies within a day.

FAQ

Q: Why is aluminium powder used in autoclaved aerated concrete?

A: It reacts with the alkaline slurry (lime, Ca(OH)2) to release hydrogen, creating the uniform pores that make AAC lightweight. Roughly 1 g of active aluminium yields about 1.24 L of H2 at STP.

Q: How much aluminium powder is added to AAC?

A: Typically around 0.4-0.8 kg per cubic metre of concrete – a fraction of a percent by mass, enough to cut the block density roughly in half.

Q: Flake or atomized aluminium for AAC?

A: Flake (ball-milled) is the mainstream choice: it disperses evenly and gases off on a steady, controllable curve. Spherical powder gases too fast and can agglomerate, so it is the exception in AAC.

Q: What active aluminium content do I need?

A: It depends on your target density, but commercial AAC grades normally specify an active-aluminium floor of 85% and above. Confirm the exact figure on the grade page for your product.

Q: Is aluminium powder in concrete dangerous?

A: As a raw powder it is a Class 4.3 dangerous good: it releases flammable hydrogen on contact with water and carries a dust-explosion risk. In finished AAC the aluminium is consumed into the cement matrix, so the cured block is inert.