Adobe Test Blocks Again

26 Jan

“A cement mixer is your friend.” We’ve heard this many times from people making their own adobe blocks and concrete for various projects. After mixing components in a wheelbarrow three times in a day, I was lusting after a mixer.

Ed adding components to the mixer.

We finally bought a new 4 cubic foot mixer that was priced as a special from Northern Tool Company during a trip for a family event in Fort Worth. Like most any consumable product these days, it came in a box. We wondered how a good sized cement mixer could fit in a box, but it did. (I questioned if products are now standardized to fit in boxes.)

As Ed expected, the instructions for putting the mixer together were more than inadequate (pidgin Chinese?); some segments were wrong. Because of Ed’s mechanical background and experience with poorly written manuals, he had little problem putting it together. From my experience editing science and technical manuscripts written by non-English authors, I could only imagine. Regardless, it was soon put together all burgundy and shiny, waiting with gaping mouth and hungry for hard stuff; “Feed me!”

One sunny and warm day this week, we decided to try our hand at making adobe blocks with our newest member: Audrey.  (Classic musical cue here) Ed had several five-gallon buckets of adobe mix components waiting next to the front of the mixer:

  • Clay: sourced from a hillside near Cedar Springs area and which has proven to be good clay for adobe blocks (see former post),
  • Sand: crushed and screened recycled adobe blocks that were rejects because they were too high in sand (they broke apart VERY easily)
  • Cement: regular Portland cement
  • Water from our rainwater catchment tanks
  • We have at this time forms for only four blocks.

We began by screening the clay on a 3/8-inch screen over the wheelbarrow. Very little aggregate was discarded after using gloved hands to break up clumps on the screen.  About ¾ of a large bucket of raw clay (before screening) was enough for each batch.

We made two test batches. The amount for each test batch was as follows:

  • Clay: 5 shovelfuls
  • Sand mix: 15 shovelfuls
  • Cement: 2 shovelfuls
  • Water: 5.5 buckets (bucket holds 80 oz.., ergo 3.4 gallons)

Mixing time was a new variable that we also wanted to test based on comments in Gernot Minke’s book (Building with Earth: Design and Technology of a Sustainable Architecture, 2nd edition, pub. 2006). Regarding clay, mixing time was an important influence on mixtures for plaster and blocks/bricks that contained clay. According to tests at the university lab in Germany, extending the mixing time from one to ten minutes enhanced bonding capacity of the clay and water adsorbance in the mixture (1). On the other hand, extending duration to 20 minutes reduced the bonding capacity. We tried two durations: Batch 1, 7-10 min. Batch 2, ~15 min.

Batch 1

Number of blocks: 3 whole blocks + ~1/2 of a block left in the wheelbarrow.


  1. Ed started mixing the second batch while I filled the block forms with mix from the first batch.
  2. This mix had a nice slump, probably between the consistency of grout and mortar (2). The consistency was similar to the mixes for blocks made at New Mexico Earth near Albuquerque; loose, but not watery. It is fluid enough to easily fill the form corners without a lot of hand-forming, but not too wet that the blocks crack easily.
  3. As with both batches, Ed added all the dry ingredients first, then added water. With the first batch, he added the water a bucket at a time with ~30 sec of mixing in between, then adding another ½ bucket after mixing for a minute or two.
  4. Because there was not enough mixture for another whole block, I left the remainder in the wheelbarrow to mix with the second batch. It is important to shade the leftovers from the sun to prevent drying too quickly.
  5. Forms were pulled while second batch was mixing. (~12-15 min).

Batch 2

Number of blocks: 4 whole and 2/3 of a block


  1. This mix was noticeably drier and grainier than Batch 1. Although the water content added was the same, the difference may have been larger shovelfuls of the sand or clay. Ed discovered that constancy between shovelfuls was difficult to maintain because of trying to scoop components from a bucket
  2. This batch, Ed added all the water in quick succession and then let mix for extended time (~15 min).
  3. Related to #1, the slump of this batch was stiffer and required a lot more time from both of us to push mud into the corners, lightly compress with our hand and also screed the top with our hand. This is a good reason for a looser slump of mud!
  4. More blocks were made from this batch because of the residue left in the mixer from Batch 1 and the unformed mix in the wheelbarrow, also from Batch 1.
  5. Because of the drier consistency (slump), Ed pulled off a form from one of the blocks to fill in with the left-over mix to make the 2/3 block.

Overall, from beginning the first batch to clean-up (tools, mixer, wheelbarrow, hands) was ~ 2 hours.

‘Green’ adobe blocks. Top, Batch 1. Bottom four+, Batch 2.


  1. Set-up and clean-up times are time intensive. Ideally, we would like to have a larger volume of materials in big piles on tarps. We ran out of our sand component (the crushed recycled blocks).
  2. This would also facilitate longer production runs (4-6 batches) and consistency in shovelfuls as measurement of mix components.
  3. For clean up, a large shallow pan or stock tank ½ full of water would facilitate efficiency of water and cleaning tools. It would also facilitate wetting the inside surface of the block forms to avoid the mud sticking to the forms.
  4. More materials. Can’t be said enough.
  5. If we can make 4-5 blocks/batch, than we will probably make ladder forms for that many blocks. I also want to get some landscape cloth to put on the ground underneath the forms.
  6. We still need to experiment with adding the water. The second batch had some dry clumps in the bottom of the mixer. Add water a bit at a time, like we did in Batch 1? Or all at once, as in Batch 2? I think this still needs experimentation.

A note on the recycled blocks. These were reject blocks from a project north of us. They break and crumble easily and spall (particles fall off the block when lightly rubbed with the fingers, or as they erode from weather). Consequently, they have very poor tensile and compression strength. We also discovered that plasters do not adhere to these blocks because the granules are so loose (spalling).

Another contribution to their loss of integrity is that they were stored outside, stacked flat on pallets, and with pallets on top of pallets. Most of these blocks that we picked up are being recycled. Those that are stronger are being used in the west wall of the ramada. However, new blocks that we are making and that pass our strength and compression tests will be used for the arched window in that wall.


    1. Clays are like ‘chemical sponges.’ Clay minerals are colloids with large flat surface areas and carry negative or positive charges on their external and internal surfaces. Thus they have the ability to attract other charged particles, especially many water molecules. Because this attraction is a surface phenomenon, it is called adsorption (which is different from absorption because the ions and water are not attracted deep inside the clay grains).
    2. Slump is commonly the consistency of masonry materials in a specific batch. The fluidity and plasticity of material varies for different applications, such as concrete, mortar and grout. For discussions on slump, visit these two websites: What Is Grout and Slump Testing.

The adobe west wall of the ramada. It’s also our plaster test wall.



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