In most concrete, fly ash replaces part of the cement. In autoclaved aerated concrete it is the dominant
raw material, at 60 to 70 percent of the dry mix. When ash quality moves in an AAC plant, the product moves
with it - immediately and visibly.
The three properties that matter
1. Fineness
Silica has to dissolve fast enough to react with lime during autoclaving and form tobermorite, which is what
actually carries the strength. Coarse ash reacts too slowly, leaves unreacted silica behind and produces soft,
crumbling blocks. Target 3500 cm2/g Blaine or better, with 45 micron residue under 20 percent.
2. Loss on ignition
Unburnt carbon interferes with the aluminium powder reaction that makes the cake rise. High LOI gives an
uneven pore structure - dense patches, oversized voids, inconsistent density across the cake. Keep it under
3 percent; under 2 percent is better.
3. Free lime and sulphate
The mix design assumes a known lime contribution from the ash. If free lime drifts upward the slurry sets
before it finishes rising and you get short cakes; if sulphate runs high you risk delayed expansion after
autoclaving. Both need to be reported lot by lot, not assumed.
What good looks like
| Property | Target for AAC |
|---|
| Fineness (Blaine) | 3500 cm2/g minimum |
| 45 micron residue | 20 percent maximum |
| Loss on ignition | 3 percent maximum |
| SiO2 | 55 percent minimum |
| Free lime | 1.5 percent maximum |
| SO3 | 2.5 percent maximum |
| Moisture | 1 percent maximum |
The practical consequence
An AAC plant cannot re-tune its mix every time a truck arrives. Consistency is worth more than any single
headline number, which is why we supply AAC plants from a fixed set of units and hold the specification band
tight rather than chasing the cheapest lot available that week.
Running an AAC line and fighting density variation? Send us your last three ash reports and we will tell you
honestly whether the ash is the problem.