Straw Crete?

Hello everyone, I’m looking for some advice.
I’m a former contractor with experience in traditional building, and have had a strong interest in alternative construction. It’s finally time to build my own home, and have been considering Hempcrete, unfortunately hemp hurd has skyrocketed in price, and I’m considering cob as the alternative. I have a farm and can grow my own straw.
I can’t find anything on Strawcrete, has this been looked at as an alternative to hemp? Is there any knowledge on its properties, if anyone has any info regarding using Strawcrete. I appreciate and advice and direction. Thank you

Kelly,

I experimented with strawcrete that I'm going to use as the infill between the curvature of the brick vault and the cornice of the barn/coop that I built. I have made two samples:

Sample 1:
compacted straw   4 parts
coarse sand   2 parts
portland cement   0.5 part
lime   0.5 part
water   2 parts

Sample 2:
compacted straw   4 parts
coarse sand   1 parts
portland cement   1 part
lime   0.2 part
water   2 parts

I used rice straw. I did not chop it. I compacted it well before measurement and mixed with all ingredients. Then I loaded the mix into a wooden gothic brick sized form (30x14x9 cm). I removed it from the form the next day. It took 3 weeks to fully cure and dry in current wet season.
The weights turned out to be similar. I did not put them on the scale yet. They are quite strong but also a little too soft. I would not use them as load bearing, but as the infill.
The wall could be also built by directly loading the mix on the wall.
If I ever built the wall I would make another sample using lime only instead of cement and lime/pozzolan instead of cement.

Look at Mudbrick construction in Australia, straw and earth is used a lot to create dried bricks, sometimes called adobe or mudbrick.
It works well.

This is great info, thank you for replying.
I’ll make a test batch with your ratios and see how the hold up. I appreciate this!

New here, and first post, I was looking at https://www.themudhome.com/ and yea the hempcrete seemed like the best with the insulation value but I guess it's become expensive. Ya I wonder what the insulation of strawcrete. Obviously straw is super available but there must be a reason they use hemp and I think its just the inner part of hemp? straw cob pretty much has no insulation, I live in cold climate area so it is pretty critical to consider insulation values.
Hope to find out more on it if you do any tests with insulation please share.

Dan,

I just measured the weight of the brick I made - the first recipe. The second was partially eaten by my sheep.
The weight of 20x14.5x7.5 cm brick is around 4 kg which translates to specific gravity of 1.23 kg/l.
For comparison a handmade fired clay brick is 1.75 and compressed earth block is 1.87 (cob could be slightly less). So it would not offer any substantial insulation value, but still better than solid masonry materials. Now I'm thinking they could be used as load bearing.
For example a hemp block from HempBlock in Australia has gravity of 0.37. It means they used much less aggregate (if any). It would be possible if the straw was finely chopped, then adding binder could be sufficient.

Now, after weighing the brick I'm thinking that I may use perlite concrete as the infill, instead of straw concrete, because I'm not sure how to chop large amount of straw. For plaster I use scissors. I could try my chipper/shredder or rock crusher.
Very light perlite concrete that I made has specific gravity of 0.2 and excellent thermal conductivity of 0,06 W/mK. I used 10 parts of perlite (1-5 mm) and 1 part of portland cement and enough water to make it slushy.
It's the weakest perlite concrete, but also having highest insulating value. Cement to perlite ratio can be increased from 1:10 up to 3:10.

I'm going to make a test with shorter straw fibers and cement only.

What you want for insulating solid infill is "light straw clay" or some variant on that name. It is essentially straw tumbled with clay slip so there is just enough clay to serve as binder, then packed into forms and dried in place. It has around R-1 per inch more or less depending on how much clay is in the mix. It is not structural, but few decent insulators will be structural.