Expert: Daniel Humphrey - 11/22/2008

Question
QUESTION: what other materials that can substitute to hollow-blocks?? and can you give substances that can be used to form a hollow-block?

ANSWER: Dear Sherly,

We had a concrete hollow block machine when I was growing up in New Mexico. The traditional aggregate is made of volcanic gravel from cinder cones, which we had nearby.

You can use regular gravel, but it makes a heavier, block with less insulation. The best aggregate to use is angular, like crushed rock.

A friend of mine makes paper blocks from paper pulp and cement powder, but they are not hollow.

Daniel

---------- FOLLOW-UP ----------

QUESTION: is it possible to add wood particles in forming hollow block? what is the difference of having wood particles in making hollow block to the original one?  

Answer
Dear Sherly,

Whenever wood particles make up part of the concrete, the compressive strength of the block would be less. Concrete also does not adhere to wood as well as to a rock; this reduces its strength, both compressive and tensile. The more wood in the block and the bigger the pieces, the less the block would hold together. When pressure is applied to wood, the cell walls collapse. This weakness is inherent in some kinds of volcanic cinder aggregate as well, but less so, since the air pockets are spherical and have stronger walls.

What this means is that you cannot expect a block with wood in it to perform the same way as a regular cinder block under a load, either directly vertically or horizontally in shear. Block walls typically are used for both of those qualities, and in some cases the cinder block (with its compressive strength of 1000 psi) is not good enough.  To compensate, you would need to fill the voids with reinforced concrete in strategic places such as corners and beneath posts, where the stresses are greatest. Concrete has a typical compressive strength of 4000 psi. I'm guessing you want to increase the insulating properties of the block, and having to fill more voids with concrete would somewhat decrease its R-value.

From my experience, forming the block itself would be tricky because of the tendency for the lighter particles to float up as the block is being vibrated on the block machine. The complications of adding aggregate other than rock most likely have posed too many challenges to make it commercially viable. If you do try adding different substances, you should consider adding fiber to increase tensile strength and bonding. Mineral fiber is available for adding to concrete in slabs and walls.

Another problem you would face with wood in a block is that the wood absorbs water and swells; or it dries out and shrinks. Either way, that would be very bad for a block. I mentioned making blocks from paper pulp, with is essentially wood, and those have a very low compressive strength and are not dimensionally stable in different environments.

Good luck on your project. You might find someone who has already experimented with these things and can pass some info on.

Daniel