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Proper mix for making good CEB

Thank you, Geoffrey for answering my many questions. I appreciate you sharing your knowledge and experience.
I've read that the ideal soil mix for solid CEBs is:
15% gravel (2-20 mm)
50% sand (0.06-2 mm)
15% silt
20% clay
What are your thoughts on this ideal mix?
Our soil here is very sand with measurable amounts of silt and clay  in various places (up to 20% clay and silt combined).
The remander of our soil is fine sand (80%) with essentially no gravel. Is some gravel essential to a strong CEB block? What minimum percentage would you suggest? Gravel will have to be purchased whereas the sand and clay can be found in vllagers' fields.
Also, I am finding it a challenge to distinguish clay from silt. Do you have any suggestions for doing so?
Thank you!

 

 

CEB mixes

When I train people to develop an ICEB mix I start with the materials that are available locally.  In your case I'd make several different mixes with different ratios (by volume using a standard measuring bucket) of soils and/or sand.  After I found a mix that had the characteristics I liked (color, texture, mixability, compressability, ejectability and liftability), I'd add different ratios of cement and make at least 20 blocks from the one I felt was most promising.  After curing (7 to 28 days) I'd check the expansion by lining up 20 blocks (soaked for 48 hours) on a flat surface and measuring the theoretical versus the actual length.
 
The maximum particle size I like in a mix is about 9 mm; I usually use a maximum of about 4 mm.  A 20 mm in a corner would be problematic.  You can try this manual for soil surveying:  http://www.cvbt-web.org/uploads/Pubs/Suitability%20of%20Local%20Soil%20for%20Cost%20Saving%20Construction%20Techniques.pdf  It is new and perhaps in need of a little more editing but I think it is worthy of trying.
This one is good: http://www.cvbt-web.org/uploads/Pubs/Compressed%20earth%20blocks_Vol1.pdf
When learning something new, expect to learn a lot.  Respectfully yours, Geoffrey