mineral / Cementitious / Concrete/Masonry
Aerated Concrete Block (AAC) — CSR Hebel
CSR Hebel Thermoblok AAC masonry blocks (525–650 kg/m³ dry density) deliver one-fifth the weight of normal concrete, intrinsic R-value up to R1.9 per 190 mm leaf, FRL up to 240/240/240, and loadbearing capacity to three storeys. Laid with 2–3 mm thin-bed mortar per AS 3700 and AS/NZS 4455.1. Australia's dominant AAC manufacturer is CSR Hebel (Somersby, NSW), with the product in continuous production since 1990.
Atlas code
MIN-CEM-CMU-005
At-a-glance signals
CSR Hebel Thermoblok AAC masonry blocks (525–650 kg/m³ dry density) deliver one-fifth the weight of normal concrete, intrinsic R-value up to R1.9 per 190 mm leaf, FRL up to 240/240/240, and loadbearing capacity to three storeys. Laid with 2–3 mm thin-bed mortar per AS 3700 and AS/NZS 4455.1. Australia's dominant AAC manufacturer is CSR Hebel (Somersby, NSW), with the product in continuous production since 1990.
Overview
Executive summary
Autoclaved aerated concrete (AAC) blocks are lightweight masonry units manufactured by steam-curing a foamed cementitious slurry to produce a highly porous, closed-cell microstructure. The dominant Australian product is CSR Hebel's Thermoblok range, produced at Somersby, NSW since 1990. At a nominal dry density of 525–650 kg/m³, AAC blocks weigh roughly one-fifth of equivalent normal-weight concrete masonry units, dramatically reducing structural loads and manual handling risks on site. Despite their low density, Hebel blocks achieve a characteristic compressive strength f'm of 2.25 MPa and can be used in loadbearing masonry construction up to three storeys under AS 3700. The closed-cell air void structure delivers outstanding thermal insulation — a 190 mm Thermoblok wall achieves an intrinsic R-value of approximately R1.9 without additional insulation — and non-combustible fire performance up to FRL 240/240/240 in compliant systems. Blocks are precision-cut to ±1 mm tolerances and laid with a 2–3 mm proprietary thin-bed adhesive mortar rather than conventional 10 mm mortar joints, improving thermal bridging and construction speed. External faces require a vapour-permeable, water-resistant render or coating system because AAC is moisture-permeable. AAC is inorganic, contains no organic binders or volatile compounds, and poses negligible toxicity risk in use. CSR Hebel holds an Environmental Product Declaration and the embodied carbon of Thermoblok blocks is substantially lower than normal-weight concrete masonry on a per-m² wall basis.
Best when…
- One-fifth the weight of normal concrete masonry (~525 kg/m³ vs ~2200 kg/m³), reducing foundation loads, crane picks and manual handling risk.
- Superior intrinsic thermal insulation (k 0.10–0.16 W/mK) — 190 mm wall achieves R1.19–R1.90 without added insulation.
- Non-combustible (AS 1530.1) with FRL up to 240/240/240; satisfies all Bushfire Attack Level (BAL) categories including BAL-FZ.
- Loadbearing to 3 storeys under AS 3700 at 2.25 MPa characteristic compressive strength.
- Thin-bed mortar system (2–3 mm joint) reduces thermal bridging, mortar use and construction time compared to standard masonry.
- Easily worked with standard hand saws and power tools — can be cut, routed and drilled on site without specialist equipment.
- Excellent acoustic separation: 190 mm single leaf Rw ~45–48 dB, suitable for intertenancy walls.
- Termite and vermin resistant — inorganic material with no organic content.
- Low embodied carbon (~0.37 kg CO₂e/kg; ~85–100 kg CO₂e/m² for 190 mm wall) versus normal-weight concrete masonry.
- 50+ year service life with minimal maintenance when correctly rendered and painted.
- CSR Hebel EPD published under Australasian EPD Programme — verified lifecycle data available for Green Star and NCC J5 reporting.
Top advantages
- 01 One-fifth the weight of normal concrete masonry (~525 kg/m³ vs ~2200 kg/m³), reducing foundation loads, crane picks and manual handling risk.
- 02 Superior intrinsic thermal insulation (k 0.10–0.16 W/mK) — 190 mm wall achieves R1.19–R1.90 without added insulation.
- 03 Non-combustible (AS 1530.1) with FRL up to 240/240/240; satisfies all Bushfire Attack Level (BAL) categories including BAL-FZ.
- 04 Loadbearing to 3 storeys under AS 3700 at 2.25 MPa characteristic compressive strength.
- 05 Thin-bed mortar system (2–3 mm joint) reduces thermal bridging, mortar use and construction time compared to standard masonry.
Top limitations
- 01 Brittle and easily chipped at arrises — requires careful storage, handling and site protection.
- 02 High water absorption (20–35%) means external faces must be protected with a vapour-permeable, water-resistant render or coating.
- 03 Lower compressive strength (f'm 2.25 MPa) than clay brick or normal concrete masonry; loadbearing use limited to 3 storeys.
- 04 Low abrasion resistance — not suitable as an exposed, uncoated floor or pavement surface.
- 05 Poor impact resistance — vulnerable to hard impacts from tools, forklifts and construction traffic.
Technical
Physical ·9
- Density
- 525-650 kg/m3 CSR Hebel block product nominal dry density range (Thermoblok 525; other block products up to 650). Working density at 30% moisture content ~682 kg/m3 for Thermoblok.
- Specific gravity
- 0.53-0.65 Calculated from nominal dry density range 525–650 kg/m3 relative to water (1000 kg/m3). Thermoblok SG ~0.53.
- Porosity
- 70-80 % AAC's closed-cell void structure constitutes approximately 70–80% of total volume by design. This high porosity is the source of its low density and thermal insulating properties.
- Water absorption
- 20-35 % AAC blocks have high water absorption due to open capillary structure. AS/NZS 4455.1 limits initial rate of absorption; typical values 20–35% by mass for AAC masonry units. This is why external render or coating is mandatory.
- Hardness
- 2-3 Mohs AAC is approximately equivalent to hardened gypsum plaster. No published Mohs value; estimated 2–3 based on comparable cementitious products. CSR notes AAC is 'not less hard than plaster'.
- UV resistance
- Good with coating AAC itself is not UV-degradable (inorganic mineral). Uncoated external surfaces are susceptible to carbonation and moisture cycling but not UV breakdown per se. Protective coating system required for UV and moisture durability.
- Chemical resistance
- Moderate — protect from sulphates, chlorides and strong acids AAC is alkaline (pH 9–10.5) and resistant to mild alkalis and weak acids. Susceptible to carbonation in high-CO2 environments and attack by sulphates, chlorides and strong acids. External render provides primary chemical protection. Not suitable in permanently immersed or highly aggressive chemical environments without additional protection.
- pH tolerance
- 9.0-10.5 pH CSR Hebel Technical Manual notes AAC is alkaline (pH 9.0–10.5) due to residual calcium hydroxide and tobermorite phases.
- Surface roughness
- Wire-cut smooth, Ra ~1.5-3mm Block faces are wire-cut during manufacturing, producing a smooth but slightly textured surface. Top and bottom bed faces may be ground or profiled for thin-bed mortar application. Typical surface Ra ~1.5–3 mm macro-texture.
Mechanical ·7
- Tensile strength
- 0.25-0.44 MPa Direct tensile strength of AAC is low, approximately 10–15% of compressive strength. Estimated 0.25–0.40 MPa for Thermoblok grade. Modulus of rupture (flexural tensile strength) f'ut = 0.44 MPa per CSR Hebel Technical Manual.
- Compressive strength
- 2.25 MPa CSR Hebel Technical Manual: characteristic compressive strength of AAC masonry f'm = 2.25 MPa; mean unit strength fuc = 2.50 MPa (AS 3700). Sufficient for loadbearing walls to 3 storeys.
- Flexural strength
- 0.44 MPa CSR Hebel Technical Manual: modulus of rupture f'ut = 0.44 MPa for Thermoblok block product. Used in design of spanning AAC members.
- Shear strength
- 0.3 MPa CSR Hebel Technical Manual: characteristic shear strength of AAC masonry f'ms = 0.30 MPa per AS 3700.
- Poisson's ratio
- 0.20-0.25 Typical value for AAC masonry. Lower than normal concrete due to the cellular microstructure. Used in FEA modelling of AAC masonry systems.
- Impact resistance
- Limited — protect arrises from mechanical damage AAC has low impact resistance due to its cellular microstructure — corners and arrises chip readily under hard impact. Not suitable in areas subject to heavy mechanical impact without protection. Qualitative rating: Limited/Low compared to clay brick or dense concrete masonry.
- Creep resistance
- Good AAC exhibits low long-term creep compared to normal concrete due to the stable tobermorite microstructure. The long-term modulus of elasticity (562 MPa) accounts for creep deflection in AS 3700 design. Generally considered good creep resistance for masonry applications.
Acoustic ·1
- Sound absorption
- 0.04-0.08 NRC Low sound absorption — AAC wall surfaces reflect rather than absorb sound. NRC 0.04–0.08 for smooth rendered finish. Not used as an acoustic absorber.
Sustainability & Health
Embodied carbon & energy ·7
- Embodied carbon
- 0.37 kg CO2-eq/kg CSR Hebel EPD (Australasian EPD Programme, 2024) reports embodied carbon for AAC products at approximately 0.37 kg CO₂e/kg (A1–A3 cradle-to-gate). Lower than normal-weight concrete (~0.10–0.15 kg CO₂e/kg) on a per-kg basis but comparable on a per-m² wall basis due to lower density. Significantly lower per-unit thermal resistance than heavyweight alternatives.
- Carbon footprint
- 14-20 kg CO2-eq/m2 wall (190mm) For a 190 mm single-leaf Thermoblok wall: approximately 13 blocks/m2 × ~3 kg/block (dry) × 0.37 kg CO₂e/kg ≈ 14–20 kg CO₂e/m² (A1–A3, excluding mortar, render and finishes). Lower than equivalent normal-weight concrete masonry wall (~40–60 kg CO₂e/m²).
- Embodied energy
- 3.5-5.5 MJ/kg Autoclaving is energy-intensive (steam at 180°C, 10–12 bar). Embodied energy approximately 3.5–5.5 MJ/kg (A1–A3). Higher than air-cured masonry products but offset by reduced structural requirements due to low weight.
- Water footprint
- 2-5 L/kg Manufacturing uses significant water in slurry preparation and steam autoclaving. Estimated water footprint 2–5 L/kg (A1–A3). CSR Somersby facility recycles process water. Refer CSR Hebel EPD for site-specific values.
- Recycled content
- <5 % CSR Hebel EPD confirms minimal recycled content (<1%) in standard Thermoblok blocks. Primary inputs are Portland cement, quicklime, fine siliceous sand (or fly ash in some formulations), aluminium powder and water. Some fly ash inclusion is possible but not standard in Australian product.
- Renewable content
- 0 % AAC is a mineral-based product. Renewable content is effectively 0%. All inputs are mined or manufactured inorganic materials.
- Circular score
- 5 /10 AAC blocks can be crushed and used as lightweight aggregate or fill. Not readily reused as intact blocks after demolition. Recyclability as aggregate is limited by contamination with render/mortar coatings. Rated Moderate on circular economy principles.
Compliance & Fire
Fire performance ·6
- Combustibility class
- Non-combustible — AS 1530.1 / NCC Group 1 CSR Hebel AAC blocks are classified Non-Combustible per AS 1530.1. Group 1 material under NCC Volume One Spec C1.10.
- Fire resistance level
- Up to 240/240/240 (system-dependent); all BAL categories minutes CSR Hebel PowerBlock guide documents FRL up to 240/240/240 (structural adequacy / integrity / insulation) for compliant wall systems. FRL depends on wall thickness, reinforcement and system detailing. A 190 mm single-leaf unreinforced Hebel block wall can achieve FRL -/240/240. All BAL categories including BAL-FZ are satisfied.
- Ignition temp
- Does not ignite C AAC does not ignite. No ignition temperature is applicable to an inorganic mineral material. Structural integrity begins to reduce above ~300°C as tobermorite progressively dehydrates, but this is a structural (not combustion) phenomenon.
- Flame spread index
- 0 AAC does not spread flame. Flame spread index = 0 per AS 1530.3 early fire hazard indices testing.
- Smoke dev. index
- 0 Smoke development index = 0. AAC generates no smoke as it does not combust. AS 1530.3 tested.
- Heat release rate
- 0 kW/m2 AAC is inorganic and non-combustible. Heat release rate = 0 kW/m² (no organic content to combust). Tested to AS 1530.1.
Cost & Lifecycle
Capex & lead time ·6
- Material cost (range)
- 80-160 AUD/m2 wall face Approximate 2025 Australian pricing for Hebel Thermoblok 390x190x190mm blocks only (excluding mortar, accessories, render). Block cost ~$6–12 per block at ~13 blocks/m2 = approximately $80–160/m2 wall face. Prices vary by distributor, volume and location. Supply from a single manufacturer limits price competition.
- Material cost (per unit)
- 6-12 AUD/block CSR Hebel Thermoblok 390x190x190mm. Approximate 2025 trade pricing. Retail pricing higher. Volume discounts available for large projects.
- Lead time
- 2-6 weeks CSR Hebel Thermoblok blocks are manufactured at Somersby, NSW and distributed nationally. Standard stock items: 2–4 weeks typical from order to delivery in major metropolitan areas. Regional or non-stock products: 4–8 weeks. Supply from a single manufacturer introduces lead time risk for large projects.
- Lifecycle cost
- 180-350 AUD/m2 Estimated lifecycle cost over 50 years including material, installation, coating maintenance (every 10 years at ~$15–25/m2) and minimal structural maintenance. Lower lifecycle cost than many alternatives due to long service life and low maintenance requirements.
- Annual maintenance
- 1.50-2.50 (external walls) AUD/m2/year Annual maintenance cost is minimal when external coating system is maintained. Coating refresh every 10 years at $15–25/m2 per cycle = $1.50–2.50/m2/year amortised. Internal walls require no maintenance beyond standard painting.
- Market availability
- Good — national distribution, single manufacturer Available nationally through CSR Hebel distributor network. Single Australian manufacturer (CSR Hebel, Somersby NSW) — strong supply network but concentration risk. Stocked by major building product distributors in all capitals and most regional centres.
Service life & durability ·3
- Expected lifespan
- 50-100+ years AAC masonry structures have demonstrated service lives of 50+ years in Australia and 60–80+ years in European buildings where AAC has been in use since the 1930s. Service life is contingent on maintaining the protective render/coating system on external faces.
- Maintenance interval
- 5-10 (coating refresh) years External render and paint/coating system should be inspected every 5–10 years and refreshed as required. Internal AAC requires minimal maintenance beyond standard wall finishes.
- Warranty period
- 10 years CSR Hebel provides a 10-year product warranty on Thermoblok blocks when installed in accordance with the CSR Hebel Technical Manual and with an approved coating system on external faces.
Layer D
Where it's used
Loadbearing external walls (residential and low-rise commercial to 3 storeys)
Non-loadbearing external cladding
Internal partition walls
Fire-rated separation walls (FRL up to 240/240/240)
Intertenancy acoustic walls — Class 2 apartments
Bushfire-resistant construction (all BAL categories including BAL-FZ)
Thermally efficient residential construction
Community and institutional buildings
Infill panels in steel or concrete framed buildings
Acoustic screen and boundary walls