Microhouse 2 - Soil Testing

This was the guide we used for the soil test - http://www.ext.colostate.edu/mg/gardennotes/214.html

There are two piles of soil to use, one is yellow and very clayey, the other is dark and more sandy and silty. the objective is to produce a stabilised earth brick

I'm quite sure that yellow pile is almost all clay and according to gernot minke's book, more than 30% clay for CEB's is no good. using cement for stabilising soil at the higher end of this clayey spectrum is not ideal either because it is incompatible with the clay - to much fine particle matter/glue with nothing to glue onto (but the different types of clay affect how true this statement is). you need to balance it out with more grainy sand to get a better matrix happenning. clay and cement are like the glue, the brick shouldn't be all glue but use the glue to bond ideally an event distribution of different sized particles. the higher clay content CEB's need other crazy ingredients instead of cement for stabilising like horse pee and ox blood, or lime.

If the intent is to get the most ideal concrete stabilised block at the maximum end of concrete in the mix @ 10%, then i would recomend from memory (will dive in deeper) that there be around 15% clay in the mix, using very sandy soil or sand to balance out the rest. Using less than 4% concrete in any ceb from stress tests appears to be a no no, so getting it right is pretty important here.

The dark soil pile being tested could be ideal. Although i don't know its history (how deep it was dug from the original top layer where it had its biology in there), if it was taken from underneath all the biological layer (usually about a foot deep and under is what you want). So far it looks like its going to come out at least 30% sandy 30% silt, perhaps more, but a good healthy distribution of the larger grains. depending on how much clay settles we can mix a small amount of that yellow pile in there to get a likely very good mix in terms of granularity and then add our cement. only thing about the dark soil is it may have a bit of biological content, I have no idea. it has no smell to me which is good. if there is biological content then it can create mold later, but i think for now this is going to be our best bet unless theres something I'm missing.

Thinking about this mixing of proportions now, we need to mark a meter out on the bucket when in its tilted position to gauge our volumes. We can fill it with water and add our markings every 50-100 liters of water added or so to do this, Marcin should I add this to the tasks? We need the tractor to do it and to weld the tabs on the mixer too.

I'm reading more about what our ideal proportions can be tonight. I shared the gernot mink book with everyone, the thing is deep but a must read anyone that wants to build with soil. the information in it can also be applied to rammed earth.

Updating on building with Earth - Gernot Minke

I will add this to the wiki but heres an update-

From the book - getting more coarse grain sand can also achieve a better weather resistance in higher clay content soils. more coarse grain sand improves compressive strength too.

It says cement blocks must cure for at least 7 days. if not protected against direct sun and wind they must be sprayed with water while curing. sodium hydroxide can be added to the water to hasten and enhance curing. Maybe we should get mixing on monday and put them in the hab lab?

If opting for lime as a stabiliser instead of cement in the high clay case, the bricks must be tested to find the optimum amount (this makes it a bit more inconvenient in my opinion)

We also should do clay ball tests with the soil mixer to determine optimum mixing time to get maximum strength

An interesting statement is that often adobes and kneaded loam can result in higher strength than compression see attached p44. but the difference is less with higher sand or clay content loams. So it make sense that optimum soil for the CEB is to have more sand in it and enough clay such that it doesn't impact the 10% cement - cement negatively affects clay's bonding ability to a degree.

Also interesting is it says compressing under static force does not achieve as higher strength as beating while vibrating "Here it can be seen that the compressive strength of a sandy loam under constant pressure for ten seconds and vibrating at 3,000 cycles per minute is enhanced by 14%. For each technique of preparation, there is an optimum water content that can be determined only by testing "

Maybe something to consider for the next version of the CEB press.

Also intersting is on p45, optimum water content at which maximum density is achieved ad its relationship to compressive strength. "if there is lesser compaction and higher water, then the same compressive strength may be achieved by using higher compaction and less water" p45 So there can be higher tolerance for having less water under compression. How much this transfers to a cement stabilised block though im not sure, probably to some degree.

Perhaps at the point at which we get the highest density brick adding water and making bricks with different amounts of water, we could take a step a little back on the water from that maximum density we got - since we are compressing. how much though I'm not sure. also to factor in is the cement and the fact that we may not be curing them? with more cement, then optimum water content is probably a logical thing to aim for, but no curing again could be a deal breaker. I really hope we try to aim for as much curing time as we can.

"When adding cement to loam, the mixture should be used immediately, since the setting of cement starts at once. If the mix is allowed to stand for several hours before being pressed into soil blocks, the compressive strength of these blocks may be reduced by as much as 50%. However, if lime is added, this time lag has no negative influence on the final strength. If less than 5% cement is added, the drying process affects the compressive strength"

-so we add cement just before we press, stands to reason.

I think it would be a worthy experiment for the future to make 5 bricks for each test - with enough sand content, 10% cement, and do variations on clay from 5% - 50%.

Test 3 brick on compressive strength, the rest on erosion (but I believe they all will pass with 10% cement on the erosion test)

Kaolinite clay can improve compressive strength by 50%, Montmorilinite clay by 100% in some tests. how to know which one we have though? in another test with 17% clay, bentonite showed 2.4x improvment over kaolinite. all are good, but kaolinite is not as good as the others in this regard to compressive strength. we know that a 30% clay brick needs much more cement to get goot bonding, so I think if we go back to something like 15% clay that would be sound value - No empirical data yet on the optimum brick yet with 10% cement as an axiom from my reading.

"The addition of lime and cement, usually intended to increase the weather resistance of loam, also generally increases compressive strength. As described here, however, compressive strength may also be decreased by these additives, especially in amounts lower than 5%. This is because lime and cement interfere with the binding force of clay minerals. The greater the clay content, the higher must be the amount of lime or cement added." p45

So it stands to reason that high clay content soil means you will need much more cement to get it to work optimally, and less than 5% cement or lime is generally a bad idea. "Tests have shown that as a rule, lime offers better stabilisation with rich clayey loams, while cement gives better results with leaner loams. Furthermore, cement is more effective with Kaolinite and lime with Mont- morillonite. In practice, it is always recommended that relevant tests be conducted. When doing so, the following points are to be kept in mind: 1. When loam is stabilised with cement or lime, some pores should remain. Only the points of contact of the larger particles should be cemented together, but fewer pores should be filled than with concrete. 2. When the cement hydrates, free lime is formed. This reacts with the silicate acids of the clay minerals so that in addition to the early stabilisation caused by cement, a longer lasting hardening also occurs. Unlike cement concrete, therefore, the strength of cement-stabilised loam increases a little even after 28 days. 3. When adding hydraulic lime, an ion exchange between the clay minerals and the added calcium ions takes place, lasting between four and eight hours. The addi- tional hardening process caused by the reaction of the hydrated lime with the car- bon dioxide from the air occurs very slowly."

So without any test data to say, this is the way to do it, based on what I'm reading I'm recommending mixing to achieve these proportions Current optimum mix -

10% cement (we can lock this one down, because while adding more cement can improve weathering performance, most graphs show a great benefit already at 8%, and if need too much cement it kind of defeats the purpose of making CEB's in the first place) 15% clay, (potentially less but unknown how much)- generally, if theres more clay you need more lime or cement to stabilise so we want to avoid this for reasons above. Clay bonding forces are destroyed by clay and lime. generally stabilising high clayey soil is better with lime instead, but i doubt lime is as great with water erosion than cement is. 40% sand 35% silt These last two can be split, but balance on the side of more sand if possible since it plays nice with clay and improves stabilisation/erosion resistance even without cement. it also reduces shrinkage.