cob....sand vs perlite or pumice

Anyone have any thoughts on using perlite or pumice in place of sand in adobe or cob. I see it is used in rocket stoves for insulation on the riser pipe. How about in between cordwood in cord wood construction in place of the sawdust. I believe the sawdust is mostly to keep the mortar from drying too slow. I think wet perlite or pumice would have the same effect.

I have access to pumice and perlite but no clean sand for 120 miles away.

Hello Billy,

I am not sure if what I am about to share is of much use, but take it for what value it has.

Sand comes in many forms, and "sharp" sand is much better than smooth or rounded sands for cobb making. That said, one can always find exceptions in traditional work of cobb in all the different places it is employed.

I would recommend experimenting with it and making test bricks. This will provided you the empirical understanding that you need, plus give you experience at the same time in working with cobb. Sawdust used in kubbhus architecture (now called cordwood) does slow the curing process of the mortar, yet really is not a necessity, nor how it was traditionally performed. Lime mortars and even just plan cobb mortars work much better in my opinion with this style of architecture, and is how it has been done for a very long time.

So, in short, experiment, and see what you get...

Regards,

j

Mr white cloud,
Thanks, probably what I am going to do is some experimenting. I was just trying to not re invent the wheel. I can imagine the the mohs hardness of the sand vs. the pumice or perlite is a determining factor on
durability and how it holds up under the weight of compression.

I work with sands and clay for baseball diamonds, the mixtures we use are right in line with cob making. 70 percent sand and 30 percent silt and clay with the silt to clay ration of less silt than clay. Too bad its not in NM.

Thanks again

I can imagine the mohs hardness of the sand vs. the pumice or perlite is a determining factor on durability and how it holds up under the weight of compression.

Compression strength is a big factor in “structural walls,” yet has little to no bearing on infill systems.

70 percent sand and 30 percent silt and clay

Silt clay is not acceptable for cobb and other clay building practices. Silt in general is to be avoided.