AAC vs CLC Blocks: What Differs in Product and Plant

AAC is aerated by hydrogen from aluminum and cured in autoclaves under high-pressure steam. CLC (cellular lightweight concrete, or foam concrete) is aerated by blending preformed foam into a cement slurry and cured at ambient temperature. As a result, AAC reaches higher strength at lower density, typically with lower drying shrinkage and tighter dimensions. A CLC plant needs far less fixed equipment but much longer curing.

Both are sold as “lightweight blocks”, and many web pages treat them as near-equivalents. For a builder they are different products. For an investor they are different factories. This article compares them on process, curing, properties and plant scale, using published standards and declared product data. It gives no equipment prices; it explains what drives the difference in investment.

How AAC and CLC Are Made

The difference starts with how the air gets in.

AAC (autoclaved aerated concrete). ASTM C1693 defines AAC as a cementitious product based on calcium silicate hydrates, made porous by a void-forming agent and cured under high-pressure steam. On our reference line:

1. Sand or fly ash is wet-milled into slurry in a ball mill.
2. Slurry, lime, cement and gypsum are dosed; aluminum paste (about 0.45 kg per m³ of product) is added as a suspension 0.5–1 minute before pouring.
3. The mix is poured at 45–48 °C into 4.2 m steel molds. The aluminum reacts in the alkaline mix and releases hydrogen, which raises the slurry into a cellular cake.
4. The cake pre-cures at 40–50 °C for 2–4 hours until it reaches 0.15–0.18 MPa.
5. It is tilted 90° in the air and wire-cut on six sides.
6. It is cured in autoclaves at 1.3–1.4 MPa (about 190–200 °C), then separated and packed.

CLC (cellular lightweight concrete). India’s IS 2185 (Part 4):2008 describes these blocks as cellular concrete made at ambient conditions using preformed stable foam. The usual sequence is:

1. A foaming agent is diluted with water and turned into stable foam in a foam generator. In the US, foaming agents are specified by ASTM C869 and tested by ASTM C796, which measures how well they produce cellular concrete.
2. The foam is blended into a slurry of cement and water, usually with fine sand or fly ash.
3. The foamed mix is poured into molds, or pumped in place for screeds and fills.
4. The blocks are demolded after about 24 hours and cured as ordinary concrete. IS 2185 (Part 4) then requires 2–3 weeks of drying in shade to complete initial shrinkage before use.

AAC cells are made chemically inside a reacting slurry. CLC cells are made mechanically and must survive mixing and pumping. Foam stability therefore controls CLC quality, while gas release and cake rise control AAC quality.

Curing: Autoclave vs Ambient

Curing is the main difference, and it explains most of the others.

AAC figures from our reference line; CLC figures from IS 2185 (Part 4).

The autoclave reacts the silica with lime under heat and pressure, so the matrix is stronger and more stable than ambient-cured cement paste at the same density. The 2–3 week drying period for CLC is also a plant design issue: output is limited by how many molds and how much covered yard the plant has, not by the speed of any one machine. For autoclave sizing and cycles, see autoclave for an AAC plant.

Density and Compressive Strength

At the same strength, AAC is much lighter:

Test methods and specimen conditions differ between standards, so treat cross-standard comparisons as indicative.

The pattern is clear. CLC reaches 2.5 N/mm² at 800 kg/m³; AAC reaches it at under 400 kg/m³, and reaches 5 N/mm² at 675 kg/m³. IS 2185 (Part 4) classes CLC at 800 and 1,000 kg/m³ as non-load-bearing, with load-bearing grades starting at 1,200 kg/m³.

CLC’s advantage is range. The same standard covers CLC up to 1,800 kg/m³ and 25 N/mm², which AAC does not try to reach. For dense structural blocks, foam concrete competes with ordinary concrete blocks rather than with AAC.

Drying Shrinkage and Cracking

Shrinkage is the property buyers notice later, as cracks in plaster.

• AAC. The FIDE AAC specification, which follows ASTM C1693 strength classes, sets a maximum average drying shrinkage of 0.02%. H+H and Termalica declare moisture movement of ≤0.2 mm/m (0.02%) under EN 771-4.
• CLC. IS 2185 (Part 4) allows up to 0.05% for load-bearing blocks and 0.08% for non-load-bearing blocks, and requires 2–3 weeks of drying before the blocks are used.

Test methods and reference conditions differ between these documents, and AAC values also vary by producer: the Tarmac aircrete declaration, for example, gives a nominal 0.70 mm/m. Compare products tested to the same method.

On site, higher shrinkage means more care with joint reinforcement, movement joints and plastering times, and more cracks when blocks are laid too soon. For the plant and site causes of cracking in AAC, see why AAC blocks crack.

Thermal Insulation and Dimensional Accuracy

Thermal conductivity. For AAC, the limits cited from GB/T 11968 run from about 0.10 W/(m·K) for class B03 to 0.18 for B07 (as cited in the T/CASME 2022 draft; check the current edition). IS 2185 (Part 4) sets maximum values for CLC of 0.32 kcal/(m·h·°C) at 800 kg/m³ and 0.36 at 1,000 kg/m³, about 0.37 and 0.42 W/(m·K) (1 kcal/(m·h·°C) = 1.163 W/(m·K)). These are upper limits, so actual CLC blocks may test lower, but on these published figures AAC insulates considerably better: even the limit for the lightest CLC grade is more than twice the top of the AAC range.

Dimensions. CLC blocks are cast in individual molds; IS 2185 (Part 4) allows ±5 mm on length and ±3 mm on height and width. AAC blocks are wire-cut from a large cake. On our reference design the cutter holds ±3 mm on length and ±1 mm on width and height, and European thin-layer AAC (TLMB) is declared at ±1.5 / ±1.5 / ±1.0 mm. That accuracy is what allows thin-bed mortar joints. CLC is normally laid on conventional mortar.

Plant Scale and Investment: What Differs

No prices here, because they depend on capacity, scope and site. What differs is the type and amount of equipment, and that sets the investment scale.

AAC figures from our reference line; CLC curing figures from IS 2185 (Part 4).

What this means for an investor:

1. AAC is capital-intensive with economies of scale. The ball mill, boiler, autoclaves and cutter are fixed costs that do not shrink much with lower output. The plant makes sense at volume, which is why most export AAC lines are 50,000, 100,000 or 200,000 m³ a year.
2. CLC is modular with a low entry point. A small plant can start with a foam generator, mixer and molds, then add molds and yard as sales grow. But it ties up more labor, more molds and more space per cubic meter of output, plus 2–3 weeks of stock in the yard.
3. Consistency costs differ. In AAC, process control is concentrated in dosing, pouring temperature and autoclave cycles. In CLC, it depends on foam stability, mixing and yard curing, which vary more with weather and crew.
4. Utilities differ. AAC needs steam: a boiler burning coal, biomass, oil or gas. CLC needs water and power, and shade from the sun in hot climates.

Which One Should You Produce?

• Choose AAC if you are targeting volume markets of framed buildings, contractors who want thin-bed systems, export to markets that specify ASTM C1691 or EN 771-4 (both written for autoclaved aerated concrete; C1691 requires units made entirely of AAC), or the best insulation per kilogram. You will also need a steady supply of lime and cement and either silica sand or fly ash.
• Choose CLC if you are testing a small local market, want to pour in place (roof insulation, screeds, void fill) as well as cast blocks, or need denser load-bearing blocks of 1,200 kg/m³ and above.
• Plan both only if you understand that they share little equipment; the foam line is not a step towards an AAC plant.

For how AAC compares with the traditional alternative, read AAC block vs red brick.

FAQ

Is CLC the same as AAC?

No. Both are cellular concretes, but AAC is aerated by hydrogen from aluminum and cured in an autoclave under high-pressure steam, while CLC is aerated with preformed foam and cured at ambient temperature. ASTM C1693 defines AAC by its steam curing and calcium silicate hydrate matrix, so a foam concrete block is not AAC under that standard.

Which is stronger, CLC or AAC block?

At the same density, AAC. A Termalica AAC block declares 5.0 N/mm² at 675 kg/m³, while IS 2185 (Part 4) sets 2.5 N/mm² average for CLC at 800 kg/m³. CLC reaches higher absolute strengths only at much higher densities, for example 12 N/mm² at 1,400 kg/m³, where it competes with ordinary concrete blocks.

Why do CLC blocks shrink more than AAC?

CLC is cured at ambient temperature, so its binder is ordinary hydrated cement paste, which keeps shrinking as it dries. AAC is cured in an autoclave, where silica and lime form calcium silicate hydrates that are far more stable. IS 2185 (Part 4) allows up to 0.05–0.08% for CLC, while FIDE 052 and the H+H and Termalica declarations give about 0.02% for AAC. Test methods differ, so compare like with like.

Is a CLC plant cheaper to set up than an AAC plant?

For the same small output, usually yes, because a CLC plant has no ball mill, boiler, autoclaves or wire cutter. But the comparison changes with scale. CLC output grows only by adding molds, yard space and labor, while an AAC line spreads its fixed equipment over large volumes. The right comparison is the investment per cubic meter at your target output.

Can CLC blocks be laid with thin-bed mortar?

Usually not. Thin-bed mortar needs tight height and width tolerances and flat bed faces. Molded CLC blocks under IS 2185 (Part 4) may vary by ±5 mm in length and ±3 mm in height and width. On our reference design, wire-cut AAC holds ±1 mm in height and width, which allows thin joints.

Planning an AAC Block Production Line?

See how 20,000–300,000 m³ a year lines are configured on our AAC block production line page, and read the AAC block plant setup guide for the steps from raw material tests to commissioning.

Sources

• ASTM International, ASTM C1693-11(2025) Standard Specification for Autoclaved Aerated Concrete (AAC): https://store.astm.org/c1693-11r25.html
• ASTM International, ASTM C1691-21 Standard Specification for Unreinforced AAC Masonry Units: https://store.astm.org/c1691-21.html
• ASTM International, ASTM C796/C796M-19 Standard Test Method for Foaming Agents for Use in Producing Cellular Concrete Using Preformed Foam: https://store.astm.org/c0796_c0796m-19.html
• Bureau of Indian Standards, IS 2185 (Part 4):2008 Concrete Masonry Units, Part 4: Preformed Foam Cellular Concrete Blocks: https://law.resource.org/pub/in/bis/S03/is.2185.4.2008.pdf
• GB/T 11968-2020, Autoclaved aerated concrete blocks (China), SAMR catalog entry: https://std.samr.gov.cn/gb/search/gbDetailed?id=B13990C15C1D5DDAE05397BE0A0A0D35
• T/CASME draft group standard (2022), as cited for GB/T 11968 thermal conductivity limits: https://www.ttbz.org.cn/upload/file/20221115/6380410546558867949161119.pdf
• FIDE, Especificación Sello FIDE No. 052, Concreto Celular Autoclaveado (AAC), Rev. 3: https://www.fide.org.mx/wp-content/uploads/sello-fide/especificaciones/ESP052_03.pdf
• H+H Sverige, Declarations of performance, Celblocket 375: https://edge.sitecorecloud.io/hnhinternational-2bb7370b/media/sites/hplushse/documents/dop_celblocket_375.pdf
• Termalica, Declaration of performance No. 63/BKTA/2019, block 5.0/700: https://termalica.pl/wp-content/uploads/2019/dwu-eng/dwu-nr-63-bkta-2019-bloczek-termalica-50-700_en.pdf
• Tarmac Building Products, Declaration of performance No. 008, aircrete blocks: https://www.barbourproductsearch.info/008%20CE%20Mark%20Aircrete%20Blocks%20Feb%202020-file111963.pdf