My mail client has gone a little berserk lately. I thought I saw a
question from Sasha on sawdust-cement panels he was using for
wall-insulation, but the message either disappeared or was a figment of
my imagination. Anyway, here's an answer.
Advantages---very cheap, lightweight, good insulation factor,
nail-holding capacity, resistance to fire and insects.
Disadvantages---humidity or lack of it causes a lot of expansion and
contraction, causes panel movement and cracking. Increasing the
percentage of cement in the panels reduces expansion and contraction,
as does coating the panel surfaces with a sealant. Making the panels
smaller is another way to limit expansion and contraction---a flexible
grout between them allows movement without air-infiltration. Nail
vertically to the center and slanting nails horizontally toward the
center, allows the latter to slip without creating cracks. A cheap
panel-sealant would be Jack Bays asphalt-containing Rub-R-Slate---the
formulas available for $12.00 in a post I made a day or two back
In making the panels, presoaking the sawdust in water, the panels
requires less sawdust and less cement for optimum quality.
Size and condition of sawdust granules is important, Granules should be
firm, and screened through a 1/4 inch screen..
Conifer sawdust makes better quality panels than deciduous-tree sawdust
Good species are Norway Pine, Jack Pine, Aspen, . Poor softwood species
are Douglas Fir, Juniper aka Red Cedar, and Ponderosa Pine. Hardwood
sawdust in general is inferior for making sawdust-cement panels, i.e.,
sawdust from any wood that contains a large amount of sugars, starches
and tannins. High tannin content of Red Oak makes it one of the
lowest-quality panels.
You can offset poor quality sawdust to some extent by adding one-third
hy volume hydrated lime per one-volume of Portland Cement. Adding one
percent diatomaceous earth further improves the quality of panels from
poor-quality sawdust.
Another good mix is one part cement, one part sand;, three parts
sawdust, and one part clay or lime. Drying shrinkage of one part cement
to three parts sawdust is .55 percent. With sand added as above,
shrinkage is reduced to 14 percent. With a sand component as above, the
insulation factor of a two-inchy panel is equivalent to a one-half inch
fiber board.
A further improvement can be made by adding soapy water and waterglass
(sodium silicate) to the mix before agitating the mix---sort of a poor
man's foamed concrete withlut the use of a foaming machine. The water
glass coats individual sawdust granules and semi-seals them when the
water evaporates.