Lightweight concrete, weighing from 35 to 115 pound per
cubic foot, has been used in the United States for more than 50 years.
The compressive strength is not as great as ordinary concrete, but it
weathers just as well. Among its advantages are less need for structural
steel reinforcement, smaller foundation requirements, better fire resistance
and most importantly, the fact that it can serve as an insulation material!
It can cost more that sand and gravel concrete, and it may shrink more
Lightweight concrete may be made by using lightweight
aggregates, or by the use of foaming agents, such as aluminum powder,
which generates gas while the concrete is still plastic. Natural lightweight
aggregates include pumice, scoria, volcanic cinders, tuff, and diatomite.
Lightweight aggregate can also be produced by heating clay, shale, slate,
diatomaceous shale, perlite, obsidian, and vermiculite. Industrial cinders
and blast-furnace slag that has been specially cooled can also be used.
Pumice and scoria are the most widely used of the natural
lightweight aggregates. They are porous, froth-like volcanic glass which
come in various colors and are found in the Western United States. Concrete
made with pumice and scoria aggregate weighs from 90 to 100 pounds per
The rock from which perlite is manufactured has a structure
resembling tiny pearls and when it is heated it expands and breaks into
small expanded particles the size of sand. Concrete made with expanded
perlite weighs between 50 to 80 pounds per cubic foot and is a very
good insulating material.
Vermiculite comes from biotite and other micas. It is
found in California, Colorado, Montana, and North and South Carolina.
When heated, vermiculite expands and becomes a fluffy mass, which may
be 30 times the size of the material before heating! It is a very good
insulating material and is used extensively for that purpose. Concrete
made with expanded vermiculite aggregate weighs from 35 to 75 pounds
per cubic foot.
Concrete made with expanded shale and clay is about as
strong as ordinary concrete, but its insulation value is about four
times better. Pumice, scoria, and some expanded slags produce a concrete
of intermediate strength, but with even more impressive value as insulation.
Perlite, vermiculite, and diatomite produce a concrete of very low strength,
but with superior insulation properties; however these are subject to
greater shrinkage. All of these kinds of lightweight concretes can be
sawn to some extent, and they will hold fasteners, especially screws.
Lightweight aggregate should be wetted 24 hours before
use. It is generally necessary to mix lightweight concrete for longer
periods than conventional concrete to assure proper mixing and it should
be cured by covering it with damp sand or by using a soaker hose.
The master sculptor/builder who created the images
in this section is Steve Kornher, who is now living in Mexico. His website, Flying
Concrete, describes more about these pictures, and has many
more of these amazingly beautiful designs to be seen. Steve can be reached
through his website for consultation. He used an unvitrified aggregate,
kind of like perlite, but not manufactured; perhaps called tuff. It
comes well graded, fine to 1 1/2", with a few rocks which are tossed
out. He screens it a bit when doing shells and adds the coarser stuff
when doing walls. Walls are mixed 8 espumilla/ one cement / 1/2 lime.
Structural roofs are 5/1/ 1/2 -- 2-3" of this, then 3" or more of 8/1.
Then 1/8" sand and cement on top, scratched, the same day so he can
easily bond the next coat--polish coat or add more lt. wt. roof fill
between vaults 10 / 1 / 1/2. Local blocks made out of the stuff are
10/1 vibrated. A dry, fluffy mix weighs about 75 pounds per cu. ft.
He figures that 4" = 2" styrofoam, but he isn't sure.
Pumicecrete has been used in constructing buildings for many years.
It is simply concrete that uses crushed volcanic rock as an aggregate
rather than conventional sand and gravel. Both pumice and scoria, when
used in this fashion, render a product that is much lighter than concrete.
It also transforms what is usually considered a thermal mass material
into something that is much more of an insulator (about R-1.5 per inch),
because of all the trapped air. This is very useful, because it makes
it possible to actually build a load-bearing structure with an insulating
material, as with earthbags filledwith the same crushed volcanic rock.
When mixing pumicecrete, the idea is to use just enough wet cement
to coat the aggregate so it will adhere to the surrounding particles.
Too much cement will defeat the purpose of maintaining all of that trapped
air; about three bags of portland cement per cubic yard of aggregate is
recommended. Once the material has set up a bit, the surface can be washed
to expose the natural color of the stone. The rough texture of pumicecrete
is ideal for adhering to further plasters that might be used.
Pumicecrete is best placed on an ordinary concrete foundation, and
most applications require a cement bond beam at the top of the wall, for
structural strength and to tie the roof structure to. Entire domes of
pumicecrete have been successfully constructed. A wall thickness of at
least 14 inches is advised, with thicker walls providing more stability
All of the photos shown in this section on pumicecrete are courtesy
of Scott MacHardy of Pumice-crete Building Systems of New Mexico. His
has many more pictures and details about this useful material. Scott offers
contracting services, training, consulting, etc.
Cellular Light-weight Concrete
Extensive research has been undertaken in the use of
industrial waste consisting of fly ash from power plants as a raw material
for manufacturing building materials. The large volume of waste has
became one of the most significant problems of environmental protection,
as its disposal is expensive and non-productive. Experiments show that
this waste material can be used for the production of high quality bricks,
blocks and other building elements which are less energy intensive than
their conventional counterparts. This research has yielded patented
technology for the production of concrete-like blocks based on oil-shale
and coal fly-ash, cured under normal atmospheric conditions.
A particularly interesting material that has been developed
is ash-based cellular concrete, which in addition to being based on
industrial waste, is also manufactured through a low-energy process.
The manufacture of conventional cellular concrete of comparable properties
requires very high energy input.
This material has been used in over 40 countries over
the past 25 years to build residential and commercial buildings. It
is an air-cured lightweight concrete that can be produced at the project
site, using standard concrete equipment and molds. A typical mix for
making blocks is:
Portland Cement..........190 kg
Fly Ash........................309 kg
plus a foaming agent
Here is a PDF file that explains more about this: CLC
For further information you might contact G. B. Singh
cellularconcretetechnologies.com explains how structural lightweight concrete is made and used.
Hempcrete is a mixture of chopped hemp, hydrated lime and a small amount of either Portland cement or quick-set gysum, and possibly includes sand or pozzolans. A reaction between the lime and the hemp results in a very lightweight material that still has reasonable compressive strength. The advantage of hempcrete over regular cement is that the hempcrete is both structural and insulative, so both ends are achieved in the same pour. It is also lower in embodied energy. The disadvantages are a longer set time (2-4 weeks) and lower strengths. It is easier to work than traditional lime mixes and acts as a moisture regulator. It lacks the strength and brittleness of cement and consequently does not need expansion joints . It is less dense than concrete and is marketed under names like Hemcrete, Canobiote, Canosmose, and Isochanvre. Where the high ultimate strength of concrete is not necessary, this option works well.
www.gizmag.com is an excellent article about building a hempcrete home in Asheville, NC, describing the attributes of this very sustainable material.
The Hempcrete Book Designing and Building with Hemp-Lime
by William Stanwix and Alex Sparrow
UIT Cambridge Ltd, 2014