Structural and base materials · Masonry & Blocks
Aerated Concrete Block (AAC Block)
Distinguishes lightweight autoclaved aerated blocks from dense concrete units, covering handling, cutting, fixings, finishes and the questions an owner should raise before they are specified.
Educational reference entry. Suitability depends on the complete assembly, substrate, exposure, installation method, manufacturer documentation, local requirements and qualified professional review.
Overview
What aerated concrete block is
Autoclaved aerated concrete blocks are made from a slurry of cement, lime, fine siliceous material and water, with a small amount of a gas-forming agent that causes the mix to rise before it is cut into units and cured under steam pressure in an autoclave. The result is a solid material full of tiny closed cells.
That cellular structure is what makes the material behave so differently from dense concrete block. Units are light enough to be handled comfortably, can be cut with a hand saw, and can be chased for services using simple tools. Some ranges are laid with a thin-bed adhesive mortar on ground flat faces rather than with a conventional bed.
Fixings are where the difference bites. A dense block will hold an ordinary plug and screw; an aerated block will crumble locally unless a fixing designed for the material is used, and heavier items such as wall-hung sanitaryware, cabinets and handrails usually need a solution worked out in advance. The material is also thirsty, drawing water quickly from mortar and from wet finishes.
Terminology
Common names and aliases
These names describe the same thing. The library keeps one entry per material so that an acronym, a trade name or a regional spelling never becomes a second, thinner page.
- AAC
- Aircrete block
- Autoclaved aerated concrete block
- Thermal block
Applications
Common applications
Where this material is typically encountered. A typical application is not a statement that it suits your project.
- Inner leaf of cavity walls in housing, later plastered or dry-lined as the internal finish.
- Internal partitions where a lightweight masonry wall is wanted in place of a framed and boarded one.
- Below-ground and foundation courses, where a variant intended for that position is specified.
- Infill panels within a structural frame, where the weight of the walling is a consideration.
- Walls built with thin-joint systems, where speed and dimensional accuracy are the drivers.
Consideration
Where it is commonly considered
- Where reducing the weight of masonry is helpful, particularly in alterations to existing structures.
- Where the thermal contribution of the wall material is part of the design thinking for the element.
- Where cutting, chasing and shaping on site need to be straightforward with ordinary tools.
- Where a thin-joint system is being used and the dimensional accuracy of the units matters.
- Where the wall will be plastered or lined rather than left exposed as a finished surface.
Professional review
Where additional professional review is especially important
These are the situations where the answer depends on the specific building, and where a qualified professional should be involved before anything is decided.
- Where heavy items must be fixed to the wall, since ordinary fixings do not hold well in aerated units.
- Where the wall is left exposed to weather without a protective finish, given how readily the material takes up water.
- In loaded or retaining positions, which are engineering decisions and depend on the particular unit chosen.
- In wet areas without a lining or tanking arrangement designed for this kind of absorbent substrate.
- Where impact resistance matters, such as busy circulation routes or garages, since the surface marks easily.
Properties
Key properties to discuss
Qualitative topics worth raising. Measured values, classes and grades come from the manufacturer's technical documentation for the specific product, not from a reference page.
- Moisture behaviour
- The cellular body takes up water quickly and gives it up slowly, so blocks are kept dry on site, protected in the wall and allowed to dry out before finishes are applied over them.
- Thermal behaviour
- Lower density is associated with less heat conduction than dense concrete block, which is why these units are used within thermal elements; the actual figures belong to the specific product's technical documentation.
- Substrate dependency
- As a background for finishes, aerated blockwork is absorbent and needs suitable preparation, and as a background for fixings it needs fixings made for the material rather than general-purpose ones.
- Acoustic behaviour
- Low mass changes how a wall behaves compared with dense blockwork, and the finished result depends on the whole build-up and its junctions rather than on the block on its own.
- Surface wear
- The surface is soft enough to be scuffed and gouged by ordinary knocks, which matters most where the wall is left with only a thin finish over it.
- Installation dependency
- Units can be cut with a hand saw, which makes accurate work easier, but the dust from cutting still needs controlling. Thin-joint work depends on flat, accurate courses from the very start.
- Documentation
- Products differ widely in density and in intended use, so the manufacturer's data for the exact block, including its fixing guidance, is the reference that matters.
Exposure
Exposure and environmental conditions
- Will the blockwork be left uncovered during construction, and how will it be protected from rain?
- Is any part of this wall below ground or in contact with soil, and is a suitable variant specified there?
- Does the wall bound a wet area, and what lining or protection is planned on that face?
- How much drying time is available before plastering or lining work begins?
- Is the wall exposed to knocks, trolleys or vehicles that a soft face would not tolerate well?
Assembly
Substrate and system dependencies
What this material sits on, is fixed to or depends on. This is usually where performance is won or lost.
- The finish over aerated blockwork has to suit an absorbent, relatively soft background, and preparation is part of that.
- Fixings for kitchens, sanitaryware, handrails and screens need planning before the wall is finished, often with built-in backings.
- Where the blocks form part of a cavity wall, the ties and the cavity arrangement belong to the wall design.
- Movement joints and their positions are a design decision, since the material shrinks as it dries after construction.
- Thin-joint systems depend on the adhesive mortar intended for those units, not on ordinary bedding mortar.
Appearance
Appearance and finish considerations
- The natural face is pale, uniform and slightly open in texture, and it is rarely left as a finished surface.
- Suction is high, so the preparation regime affects how the finished plaster or render surface looks.
- Because the units are dimensionally accurate, thin-joint walls can often be finished with a thinner coat than traditional blockwork.
- Damage from handling shows readily on arrises and corners well before any finish goes on.
Durability
Durability questions
Questions to raise rather than a service life to expect. No material has a universal lifespan — it depends on the assembly, the exposure and the upkeep.
- Is the wall protected from persistent wetting, which this material tolerates poorly when left bare?
- How will fixings applied later be handled, given the risk of local crushing around them?
- Which block variant is appropriate below ground or in ground contact, and where does the specification change?
- Are the arrises and corners of the wall protected in areas likely to take knocks?
- How is shrinkage as the wall dries being accommodated in the finishes?
Upkeep
Maintenance and repair considerations
- How would a hole left by a failed fixing be made good, and what fixing would replace it?
- If a finish is removed later, will the block face survive that without extensive repair?
- Are records kept of where heavy-duty fixing backgrounds were built into the wall?
- What happens where a wall has been chased repeatedly for services over the years?
Installation
Installation dependencies
What the installation depends on — not how to do it. Method belongs to the manufacturer's instructions and the trades carrying out the work.
- Blocks are kept dry and covered before laying, because saturated units behave differently once they are in the wall.
- Cutting with hand tools is straightforward, but dust control is still required on site.
- Thin-joint work needs a very accurate levelling course, since the system leaves little room for later correction.
- The mortar or adhesive must be the one intended for the units, and mixing systems together is unwise.
- Newly built walls are protected from rain until they are covered by the outer construction.
Documentation
Documentation to request
Ask for these in writing, for the specific product being proposed, and keep them with the project record.
- The manufacturer's technical data for the specific block, including its intended positions of use.
- The manufacturer's fixing guidance, which is the reference for anything to be hung on these walls.
- Details of the thin-joint mortar system where one is used, from the same source as the blocks.
- The specification for finishes over the blockwork and any preparation those finishes require.
Conversations
Questions for qualified professionals
Bring these to the relevant qualified professional, manufacturer or authority. Requirements vary by project and location.
- Why has a lightweight unit been chosen here rather than a dense block?
- What will be fixed to these walls, and how will those fixings be made?
- Is any part of this blockwork loaded or retaining, and who has checked it?
- Which block variant is being used below the damp-proof course?
- What finish is going on this wall, and what preparation does the substrate need?
- Are we using a thin-joint system, and does the trade have experience of it?
- Where are the movement joints, and how do they relate to the finishes?
Blind spots
Commonly overlooked points
- Ordinary wall plugs pull out of aerated blocks, and the discovery usually comes when a heavy item is being hung.
- The blocks are thirsty enough to change how plaster and mortar behave against them.
- Corners and arrises damage easily during construction, well before any finish is applied.
- Not every aerated block is intended for every position, and the variants within one range differ from each other.
- Thin-joint construction is a different way of working and is not simply faster conventional blockwork.
What this page does not do
- Thermal behaviour is one reason these units are considered, but the figures belong to the specific product's documentation and to the assembly as a whole.
- Structural use, including loaded and retaining walls, is a matter for a qualified professional and for the particular unit.
- Fixing capacity depends on the fixing, the block and the load, and is not something a reference page can state.
- Whether these units may be used in a given position depends on local requirements that must be confirmed.
Related entries
Related materials
Materials from the same family, an adjacent role in the assembly, or a shared application.
Inspiration
Related Ideas Library pages
Design directions where this material commonly appears.
Preparation
Related planning checklists
Owner-side preparation before the conversation where this material comes up.
Go deeper
Related Build Design Hub guides
Planning guidance and neutral comparisons behind the decisions on this page.
Masonry & Block Materials
Reference entries for unit masonry — clay and concrete bricks, blockwork, mortars, and the ties, lintels and accessories that hold masonry construction together.
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