This from Andy in Indonesia:
Right now we are testing different soils to see what the best mix is. It
appears to me that our best bet is to use mostly sand with just enough
soil to hold it together to get it off the press. We have done some
samples of this with about 14% cement. They test our around 25Kg/Cm2.
We have some soil sand blocks that have been as good as far as compression
goes, but the soil is not readily available, and the corners still break
too easily. The people here want them perfect.
Mixes, compressive strength, corner breakage
We are testing our blocks after 7 days, including two days soaking in water.
As far as why we are going to more sand in the block...
First of all, I would say that is what seems to be most likely to be accepted here. Most people who have seen the blocks prefer them to be more like cement than earth. All production sites that I know of here have moved toward sand/cement, except Sigli which has produced plenty of earth blocks that no one is willing to use.
Generally speaking the earth blocks pass the compression test for two story buildings, but the quality is not acceptable to the public because of the perception of a poor product. This is because it can easily be scratched, and chipped. Also, during the construction process many blocks are not used because of corners being broken off. Some say as much as 20% or more were thrown away when our office was built. This percentage of waste was not calculated into the overall cost of the building, so we do not have an accurate comparison of construction costs. Also, with time, blocks that are exposed to the weather began to disintegrate. The outside exposed to the elements cracks and peels off. Unfortunately many of the applications of ICEB here so far have been for outside walls around graves, or other common areas. These applications are exposed to the weather, and many people see when the product begins to degrade with time.
Finally, though sand/cement is more expensive to produce, when we figure in the amount of waste with the earth blocks we may come out better as long as the majority of the blocks produced can actually be used during construction. Those who are interested in the blocks seem to be willing to pay a higher price, as long as they see the blocks as very strong, more like cement.
I understand your concerns about sand, but so far we have not been able to find a soil which is readily available, and produces durable blocks. I wish we could!
Andy
Most commonly civil engineers specify concrete mixes in a weight ratio.
So, your 14% cement could be expressed as 1:6 by weight. This is a high cement content for an earth block. Here we use 1:11 and our blocks pass 2MPa (20 ksc) in 1 week. At what age are you testing your blocks? Civil engineers actually use 28 days as a standard. A sand block will need more cement than an earth block.
I recommend using as little sand as you can. Sand is not compressible, is more expensive, uses more cement and will wear the walls of your press box down quicker than soil. What is your sand to soil ratio by volume?
If corner breakage is a concern then I suggest you study the corners. Which corners break and which ones don't? Top, bottom, Point "A," "C," "D," or "F"? How much force is required to break a corner? Quantifying characteristics helps us begin to control them. We get a little breakage here; it seems to be somewhat normal. The penetrometer you have can be used on 6 points on a block to ensure even compression of the correct ammount. We normally check the first block of every mix batch. When they put soil in the press box, if they press the soil down to far, it will get less compression against the lid; have them press the soil down just enought to close the lid.