[Greenbuilding] Crushing Clay

[natural building]

Rob, while you are correct that there is no chemical change occurring when using Clay as a binder, the issue of shrinkage and moisture control is HUGE. Every bit as important as with Concrete / Cement. Possibly more so.

I have been researching and testing unstabilised rammed earth samples for the last year. I've produced close to 90 samples, experimenting with different ratios of clay, aggregate and water and what I can say with total certainty is that the water content is extremely critical and optimum proctor occurs in a VERY narrow margin of moisture content. By that I mean quantifiable changes in strength can be seen (and documented) with changes of a tenth of one percent by weight of the sample.

Furthermore, I have experimented with mixing time that range from 10 minutes up to 40 minutes. Currently I have found that the optimum with the mixer I am using is about 20 - 25. That being said, I am using a conventional concrete drum mixer which I acknowledge is not the ideal.

You are right to suggest that a mixer which 'kneads' (such as the vertical or horizontal shaft stucco mixers) might produce better results but I can guarantee that they would not handle a ball of clay with only 30 - 35% clay much better!

Finally, your suggestion about hydrating and then leaving the clay to dry to the appropriate moisture ratio would not only return us to the same point of unworkable consistency, but be rather impractical if you are trying to make any progress on a construction site.

A herd of cows might work, but I don't have one...

Regards,
Steve Satow

www.naturalbuildingsite.net
naturalbuilding at shaw.ca

On 2011-08-03, at 3:53 PM, RT wrote:

> On Wed, 03 Aug 2011 16:55:44 -0400, natural building <naturalbuilding at shaw.ca> wrote:
> 
>> tried the "dissolve-all-of-the-clay-to-make-a-minimally-wet-clay-slip" approach at a fairly early stage in the process but it was totally unworkable.
> 
>> The ratio of water to clay necessary to achieve the optimum Proctor(compressive strength) is in the region of 30 - 35% by weight (water to clay)and resulted in a ridiculously sticky mixture that was impossible to thencombine evenly with the aggregate.It just turns into a large ball covered in gravel.
> 
> The "minimally-wet" stipulation mentioned earlier was intended as an attempt to address the issue of excessive shrinkage (and possibly shrinkage cracking) after the mixture has been placed in the forms.
> 
> However, I don't think that for clay mixes, there is any issue similar to the critical water:cement ratio that affects the strength of the final product as would be the case for concrete where the water is apportioned like a chemical formula for hydration of the cement.
> ie I don't think that there are any chemical changes going on in the clay. Molecular structure changes perhaps.
> 
> That is to say, I strongly suspect that one could make a soupier clay slip than would be implied by the 30-35% water: clay ratio you mention (for ultimate compressive strength).
> 
> ie Perhaps one makes the slip as soupy as is required to facilitate reasonable workability and then set the mud aside (perhaps covered if necessary to control moisture loss rate) to let it dry out to the desired moisture content to reach the 30-35% water: clay ratio  (maybe days, maybe weeks "curing" time) before placing in the forms for tamping ?
> 
> The term "unworkable" is pretty subjective too.
> 
> ie One of the most common mistakes that novices make when mixing cementitious mortars without the aid of a mechanical mixer is that they don't do enough mixing of the properly-proportioned mix and finding that it's too difficult to work with, add water to the mix to make mixing easier when in fact, what they should have done is mix longer with more elbow grease.
> 
> I know that a clay soil that seems fairly dry and hard and not amenable to being worked, will eventually turn into a pliable and plastic material when "kneaded" with sufficient (very high) pressure, not unlike what happens during the floating and trowelling processes in concrete finishing ... a process of compaction that amongst other things, brings water to the surface.
> 
> I'm wondering if the unworkable, large, gravel-covered ball you mention was worked only by hand or were mechanical means tried as well ? (Like maybe something similar to an institutional-sized bread dough mixer ... or a herd of cows  ... or [ ] ?)
> 
> 
> 
> 
> -- 
> === * ===
> Rob Tom
> Kanata, Ontario, Canada
> < A r c h i L o g i c  at  Y a h o o  dot  c a >
> manually winnow the chaff from my edress if you hit "reply"
> 
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