[GSBN] Plaster thoughts

[huffnpuff at q.com]

Mark and All, 

  

I am sure you will be aware of some of what I suggest, as it's common knowledge in the earth plaster world. Too much clay, too little sand/aggregate, the size of the sand/aggregate (too fine sand) can all cause excessive cracking. Removing clay, adding more sand/aggregate or upping the size of the sand/aggregate could help. I've found that the more/longer straw added can eliminate higher clay content cracking. I've used a 1:1:1 ratio of clay, long straw, masons sand for a very sticky, super strong, low/no cracking application to stick as a base over foam, lumber and plywood (placing some of this mix on the surface, tack mesh to subsurface, place more mix, for an added mechanical bond). 

  

When you start adding lime and cement to a mix sounds like what we do in the US. If it doesn't move, get a bigger hammer. It's not better living through 'bigger hammer' chemistry. Often less is more. I think you'll find some of your answers to cement earth plaster ratios in a study by the U. of Penn as they attempted to do some repairs to the mortar on ancient ruins in the Mesa Verde and Chaco Canyon adobe structures. http ://repository. upenn . edu / cgi / viewcontent . cgi ?article=1044&context=hp_theses You'll find test results on "tensile strength, modulus of rupture, water vapor permeability, and water porosity (respectively) (and freeze/thaw) vs . volume-percent of Portland cement for amended mortars" .  vs . volume-percent of Portland cement for amended mortars" .  

  

Also, you might check out Stabilized Compressed Earth Block sites (adobe block on steroids...it's what happens when engineer types make adobe block). My limited experience with SCEB's is they only use about 6-12% cement...lime may be substituted, but don't quote me. Check out the Auroville Earth Institute   www.earth-auroville.com for mixes using cement. Maybe others can chime in here with knowledge on substituting lime for cement. Of coarse, different areas have different clays that react in varying ways so experimenting with different mixes/ratios would be prudent.  The one advantage of using cement amended earth mixes is that it hardens much faster so the results of your experimenting is swifter. 

  

For a shameless plug for the Conference in Sept. 2012 http ://www. strawbaleconference .com/index. html  the Auroville Institute will be represented at the conference by a presentation by Richard Presley.  

  

If the locals insist on using cement additives to the earthen plaster mix, visiting the information on these two sites should help with your educated guess as to ratio's. I did some extensive stone work this last summer and wanted to use an earthen mortar. I used a 9:1 mix of my earth plaster mix/cement. My mason buddy thought I was nuts. Eight months later, 4 feet of snow and lots of freeze/thaw in our Colorado winter climate and that mix seems to be holding up amazingly. There's little to no degradation of the mortar. 

  

For those interested in pre-fab straw bale walls and those using cement stucco's these amended earthen mixes may be worth noting.   

  

Hope this helps Mark... Regards , 
Mark (see you in September) Schueneman 
Colorado Straw Bale Association 
303-444-6027 hm. 
303-591-9841 cell 


  

  




----- Original Message -----




From: "Mark Jensen" < markj @ redfeather .org> 
To: GSBN@ sustainablesources .com 
Sent: Saturday, March 3, 2012 3:44:05 AM 
Subject: [GSBN] Plaster thoughts 

Hello et al , 


I am in Kenya right now working on two straw bale buildings, stacking for both Monday and Tuesday. I am struggling with plaster thoughts. I guess the last plaster they did was clay, sand, cow dung, chopped straw, lime, cement and water. Nobody knows the proportions but they had severe cracking, like a dried up lake bed. 


Now they want to do 1 1/2 bags of lime, 1/2 bag cement, 4 wheelbarrows of sand, chopped straw and water. This seems like a recipe for disaster to me. They are on a very short budget. I suggested harvesting clay and just doing a earthen plaster, it's free other than some labor to harvest.  


Any thoughts on the recipe above? 


Your input would be appreciated. 


Asante Sana, Thank you very much, 


Mark 
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