Autoclaved aerated concrete (AAC, Aircrete)
Autoclaved aerated concrete is a versatile lightweight construction
material and usually used as blocks. Compared with normal (ie: “dense”
concrete) aircrete has a low density and excellent insulation properties.
The low density is achieved by the formation of air
voids to produce a cellular structure. These voids are typically 1mm -
5mm across and give the material its characteristic appearance. Blocks
typically have strengths ranging from 3-9 Nmm-2 (when tested
in accordance with BS EN 771-1:2000). Densities range from about 460 to
750 kg m-3; for comparison, medium density concrete blocks
have a typical density range of 1350-1500 kg m-3 and dense
concrete blocks a range of 2300-2500 kg m-3.
Autoclaved aerated concrete block with a sawn surface to show the
cellular pore structure
(Picture courtesy H+H UK Ltd.)
Detailed view of cellular pore structure in an aircrete block.
Autoclaved aerated concrete blocks are excellent thermal insulators and are typically used to form the inner leaf of a cavity wall. They are also used in the outer leaf, when they are usually rendered, and in foundations. It is possible to construct virtually an entire house from autoclaved aerated concrete, including walls, floors - using reinforced aircrete beams, ceilings and the roof. Autoclaved aerated concrete is easily cut to any required shape.
Aircrete also has good acoustic properties and it is durable, with good resistance to sulfate attack and to damage by fire and frost.
Autoclaved aerated concrete is cured in an autoclave - a large pressure
vessel. In aircrete production the autoclave is normally a steel tube
some 3 metres in diameter and 45 metres long. Steam is fed into the
autoclave at high pressure, typically reaching a pressure of 800 kPa and
a temperature of 180 °C.
Autoclaved aerated concrete can be
produced using a wide range of cementitous materials, commonly:
cement, lime and pulverised fuel ash (PFA)
- Portland cement, lime and
fine silica sand. The sand is usually milled to achieve adequate
A small amount of anhydrite or gypsum is also
Autoclaved aerated concrete is quite different from
dense concrete (ie: “normal concrete”) in both the way it is produced
and in the composition of the final product.
Dense concrete is
typically a mixture of cement and water, often with slag or PFA, and
fine and coarse aggregate. It gains strength as the cement hydrates,
reaching 50% of its final strength after perhaps about 2 days and most
of its final strength after a month.
In contrast, autoclaved
aerated concrete is of much lower density than dense concrete. The
chemical reactions forming the hydration products go virtually to
completion during autoclaving and so when removed from the autoclave and
cooled, the blocks are ready for use.
concrete does not contain any aggregate; all the main mix components are
reactive, even milled sand where it is used. The sand, inert when used
in dense concrete, behaves as a pozzolan in the autoclave due to the
high temperature and pressure.
The autoclaved aerated concrete
production process differs slightly between individual production plants
but the principles are similar. We will assume a mix that contains
cement, lime and sand; these are mixed to form a slurry. Also present in
the slurry is fine aluminium powder - this is added to produce the
cellular structure. The density of the final block can be varied by
changing the amount of aluminium powder in the mix.
The slurry is
poured into moulds that resemble small railway wagons with drop-down
sides. Over a period of several hours, two processes occur
The cement hydrates normally to produce
ettringite and calcium silicate hydrates and the mix gradually stiffens
to form what is termed a "green cake".
The green cake rises in
the mould due to the evolution of hydrogen gas formed from the reaction
between the fine aluminium particles and the alkaline liquid. These gas
bubbles give the material its cellular structure.
Slurry being poured into moulds (Picture courtesy H+H UK Ltd.)
Green cake rising in mould (Picture courtesy H+H UK Ltd.)
There are some parallels between autoclaved aerated concrete
production and bread-making. In bread, the dough contains yeast and is
mixed, then left to rise as the yeast converts sugars to carbon dioxide.
dough must have the right consistency; too hard and the bubbles of
carbon dioxide cannot 'stretch' the dough to make it rise, but if the
dough is too sloppy, the carbon dioxide bubbles rise to the surface and
are lost and the dough collapses. With the right consistency, the dough
is sufficiently elastic to stretch and expand, but strong enough to
retain the gas so that the dough does not collapse. When risen, the
dough is placed in the oven.
Although a much more complex
process, Aircrete production conditions are precisely-controlled for, in
part, somewhat similar reasons. The mix proportions and the initial mix
temperature must be correct and the aluminium powder must be present in
the required amount and with the appropriate reactivity an an alkaline
environment. All of the materials be be of suitable fineness. A
complicating factor is that the temperature of the green cake increases
due to the exothermic reactions as the lime and the cement hydrate, so
the reactions proceed faster.
When the cake has risen to the
required height, the mould moves along a track to where the cake is cut
to the required block size. Depending on the actual production process,
the cake may be demoulded entirely onto a trolley before cutting, or it
may be cut in the mould after the sides are removed.
The cake is
cut by passing through a series of cutting wires.
Green cake being cut by wires (Picture courtesy H+H UK Ltd.)
At the cutting stage, the blocks are still green - only a few hours has passed since the mix was poured into the mould and they are soft and easily damaged. However, if they are too soft, the cut blocks may either fall apart or stick together; if they are too hard, the wires will not cut them - here too, the process has to be carefully controlled to achieve the necessary consistency.
The cut blocks are then loaded into the autoclave. It takes a couple of hours for the autoclave to reach maximum temperature and pressure, which is held for perhaps 8-10 hours, or longer for high density/high strength aircrete.
"Green" blocks being loaded into an autoclave (Picture courtesy H+H UK Ltd.)
When removed from the autoclave and cooled, the blocks have achieved their full strength and are packed ready for transport.
The essence of aircrete production is that lime from the cement and lime
in the mix reacts with silica to form 1.1 nm tobermorite.NB:
Cement chemistry notation is used below. If you are not familiar with
this, see our
During the green stage, the cement is hydrating at normal temperatures
and the hydration products are initially similar to those in dense
concrete - C-S-H, CH and ettringite and/or monosulfate. After
autoclaving, tobermorite is normally the principal final reaction
product due to the high temperature and pressure.
of other hydrated phases will also be present in the final product.
Additionally, hydrated phases form in the autoclave as intermediate
products, principally C-S-H(I). This is a more crystalline form of
calcium silicate hydrate than occurs in dense concrete; it can have a
ratio of calcium to silicon of (0.8<Ca/Si<1.5) but 0.8 to 1.0 is
desirable as this ratio facilitates the formation of 1.1 nm tobermorite.
compositions of the hydration products in aircrete are therefore quite
different from those in dense concrete cured at normal temperatures (ie:
calcium silicate hydrate (C-S-H), calcium hydroxide (CH), ettringite
and monosulfate. See the “hydration” page for more information).
at this in a little more detail from when the green blocks enter the
autoclave, the main reactions that occur are broadly as follows:
2 hours or so, as the pressure and temperature increase, the normal
cement hydration products that formed in the green state progressively
disappear and the sand becomes reactive.
- C-S-H(I) forms, partly
from silica derived from the sand.
- As more sand reacts, calcium
hydroxide from the lime and from cement hydration is gradually used up
by continued formation of C-S-H(I).
- With continued autoclaving,
1.1 nm tobermorite starts to crystallize from the C-S-H(I); the total
proportion of C-S-H(I) declines and that of 1.1 nm tobermorite gradually
increases. C-S-H(I) is therefore mainly an intermediate compound.
final hydration products are then principally:
- Possibly some residual C-S-H(I)
sand is likely to remain in the final product. There may also be some
residual calcium hydroxide if insufficient silica has reacted and some
residual anhydrite and/or hydroxyl-ellestadite if anhdrite was present
in the mix.
SEM image of polished section showing a detail - a cell wall - of a block made with cement, lime and sand mix. Some residual unreacted sand particles remain (examples arrowed), often with rims of hydration product showing the size of the original particle. Most of the matrix is composed of tobermorite. Black areas at top left and bottom right are epoxy resin used in preparing the polished section filling air voids (air cells).
The objective is to react sufficient silica from the sand to form
tobermorite from the available lime supplied by the lime and cement.
This will depend on a range of factors, including the inherent
reactivities of the materials, their fineness (especially the
sand), and the temperature and pressure. If the autoclaving time
is too short, the tobermorite content will not be maximised and some
unreacted calcium hydroxide will remain and block strengths will be then
less than optimum. If the autoclaving time is too long, other hydration
products may form which may also be detrimental to strength and an
unnecessary energy cost will be incurred.
There are different forms of
tobermorite: 1.1 nm tobermorite and 1.4 nm tobermorite. Also, there are
different types of 1.1 nm tobermorite and these behave differently when
heated. Their crystal structure is that of layered sheets, with water
molecules between the layers - on heating, the inter-layer water is
lost; as a result, some 1.1 nm tobermorites shrink (a process known as
lattice shrinkage) but some don’t.
1.4 nm tobermorite (C5S6H9)
- forms at room temperature and is found as a natural mineral. It
decomposes at 55 °C to 1.1 nm tobermorite, and so is not found in AAC.
silicate hydrate compositions in AAC
- 1.1 nm tobermorite
(C5S6H5) is usually the main hydration
product in AAC where cement, lime and sand are used
- C-S-H(I) -
more crystalline than C-S-H in dense concrete, typically
- Xonotlite (C6S6H) -
forms with longer autoclaving times, or higher temperatures
'Normal'tobermorite shows lattice shrinkage, while non-shrinking tobermorite is
called 'anomalous' tobermorite. Tobermorite in AAC made with cement,
lime and sand is usually normal tobermorite. Tobermorite in autoclaved
aerated concrete made with cement, lime and PFA is usually anomalous
tobermorite. Aluminium and alkali together in solution (such as will be
present in mixes of cement, lime and PFA) tend to produce anomalous
tobermorite, with some aluminium and alkali taken up into the
tobermorite crystal structure. The differences between the different
forms of autoclaved calcium silicate hydrates are not well-defined; in
an AAC block, intimately-mixed hydrates of different compositions and
crystallinity are likely to occur.Other hydrothermally-formed minerals
- Gyrolite (C2S3H2)
- not normally found in AAC
- Jennite (C9S6H11)
occurs as a natural mineral; not found in AAC
- C-S-H(II) - Ca/Si≈ 2.0. Does not occur in AAC
- C2SH (α-C2S hydrate) can occur in autoclaved products but is undesirable
- Hydroxyl-ellestadite (C10S3.3SO3.H2O) - may be
found in AAC; also occurs at the cooler end of cement kilns
Environmental benefits of Autoclaved Aerated Concrete
The use of autoclaved aerated concrete has a range of environmental
- Insulation: most obviously, the insulation
properties of aircrete will reduce the heating costs of buildings
constructed with autoclaved aerated concrete, with consequent fuel
savings over the lifetime of the building.
- Materials: lime is
one of the principal mix components and requires less energy to produce
than Portland cement, which is fired at higher temperatures. Sand
requires only milling before use, not heating, and PFA is a by-product
from electricity generation. NB: lime may require less energy to
manufacture compared with Portland cement but more CO2 is produced per tonne (cement
approx. 800-900 kg CO2/tonne
compared to lime at 1000 kg CO2
- Carbonation: less obviously, the cellular structure
of aircrete gives it a very high surface area. Over time, much of the
material is likely to carbonate, largely offsetting the carbon dioxide
produced in the manufacture of the lime and cement due to the calcining
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