Primary silica donor
Class F fly ash
Fly ash is the preferred silica source across many Indian AAC plants because amorphous silica reacts efficiently in the alkaline slurry. Consistency matters as much as headline chemistry.
- Combined SiO₂ + Al₂O₃ + Fe₂O₃
- Minimum 70%
- Reactive silica
- Approximately 45–50%
- Calcium oxide
- Below 10% for Class F
- Loss on ignition
- Below 5%; preferably below 3%
- Fineness
- <15% residue on 45 μm sieve; Blaine 2,800–3,800 cm²/g
- Reference standard
- IS 3812 (Part 1), verify current applicable edition
High unburnt carbon increases water demand and can disturb aluminium-driven pore formation, cake stability and final strength.
Alternative silica donor
Quartz silica sand
Sand is suitable where consistent fly ash is unavailable, but crystalline quartz requires controlled wet grinding and sufficient hydrothermal energy.
- Silica, SiO₂
- Typically above 75–85%
- Iron oxide, Fe₂O₃
- Below 3%
- Alumina, Al₂O₃
- Below 10%
- Clay, silt and organics
- Below 3% by volume
- Milled fineness
- <10% residue on 90 μm; Blaine 3,000–4,000 cm²/g
Sand and fly ash are not one-to-one substitutes. Grinding, water demand, binder balance and autoclave settings must be revalidated.
Alkaline activator · Calcium donor
Quicklime
Quicklime supplies calcium, creates the high-pH environment and releases heat during slaking. Its reaction curve directly affects cake rise, temperature and cutting time.
- Active CaO
- Above 85%; preferred 88–94%
- Magnesium oxide
- Below approximately 2.0–2.5%
- Silica impurity
- Low; commonly controlled around 1.5–5%
- Slaking reactivity, t₆₀
- Target 4–14 minutes
- Particle size
- Approximately 45–75 μm / 100–300 BSS mesh
Very fast lime can cause thermal spikes and cake fallback. Slow lime can delay setting and damage the green cake during cutting.
Hydraulic binder
OPC cement
OPC provides early green strength for mould handling and wire cutting, while contributing calcium and maintaining alkalinity for gas generation.
- Typical selection
- OPC 53 Grade
- Reference
- IS 269; verify current applicable edition
- Blaine fineness
- Approximately 3,000–4,200 cm²/g
- Preferred behaviour
- High early strength, controlled aluminate response
- Total alkalis
- Controlled; report recommends below 1.0%
More cement is not automatically better. Excess consumption increases cost and may alter cake temperature, setting and cutting performance.
Sulfate regulator
Gypsum or anhydrite
Gypsum regulates early hydration, moderates setting and supports formation of the desired mineral structure during autoclaving.
- Calcium sulfate purity
- Approximately 70–90% or better, source dependent
- Chloride
- Low and controlled
- Loss on ignition
- Low and consistent
- Milled fineness
- <10% residue on 90 μm
Gypsum dosage must be matched with lime reactivity and cement behaviour; it is a process-control input, not merely a filler.
Gas expansion agent
Metallic aluminium
Aluminium reacts in the alkaline slurry to release hydrogen. The temporary bubbles form the cellular pore structure that gives AAC its low density.
- Active metallic aluminium
- Approximately 85–95%
- Particle morphology
- Leafing, flake-like
- Median particle size
- Approximately 25–45 μm
- Screen passage
- More than 98% through 75 μm / 200 mesh
- Typical dosage
- 0.38–0.40 kg per finished m³ for B06 benchmark mixes
Small dosing errors have a large effect on density, cake height, pore structure and rejection. Automated precision dosing is strongly recommended.
Process modifier · Release aid
Soluble soap oil
A water-dispersible emulsion can moderate early reaction, stabilise pore walls and act as a mould-release boundary lubricant.
- Typical slurry dosage
- Approximately 0.3–0.7 L per finished m³, mix dependent
- Mould-release dilution
- Commonly 1:5 to 1:20 with water
- Primary functions
- Bubble stability, water retention, clean demoulding
- Control requirement
- Compatibility trials with lime and aluminium system
Only use plant-approved formulations. A change in surfactant chemistry can shift gas-release timing and cake stability.