JWR,
First of all thank you for your blog. I have been reading it every day for the last year. J.D. in Texas offers some good information regarding Concrete Masonry Units(CMUs), however I may be able to share some more details. I have also been in the concrete masonry business for about 22 years. The first thing to consider when using concrete masonry is to avoid breathing any dust from the units, such as when a unit is cut, split, or ground. At the very least use a N95 or N99 dust mask. If you are cutting a CMU then use a wet saw if you can. The concern of the previous article was with Fly Ash which is a product derived from the scrubbers from coal fired power plants. It can contain some potentially dangerous chemicals such as mercury, antimony, barium, and strontium to name a few. It is used as a partial replacement for regular cement to actually produce a better finished product. Fly Ash can increase the long term strength, durability, freeze-thaw resistance, permeability, and road salt resistance. Many State D.O.T.’s have requirements to use Fly Ash at certain concentrations to improve bridges and roadways.
An important concept to understand about concrete is that it gets stronger with age due to a reaction with water called hydration. Most concrete is considered cured at 28 days, Fly Ash concrete is generally considered cured at 56 days, although the curing process never truly ends. One hundred year old concrete has been tested and it was found to still be curing. The other important concept to know is that the ingredients of concrete are generally bound within the matrix (internal structure) of the concrete. There is likely only one pound or less of Fly Ash in a typical 8”x 8”x 16” CMU, which would only contain a very small percentage of potentially toxic materials that will not likely be released from the concrete.
Considering other building materials for a raised bed? Pressure treated lumber contains toxic materials, Railroad ties? – don’t even think about it [because they are permeated with toxic creosote, copper naphthenate, and other chemicals] Brick? – Clay brick can also contain fly ash. I would not hesitate to build a raised bed with concrete masonry units; in fact I have one in the works. If you are concerned I would just allow a little extra distance between your plantings and the sides of the CMUs. You could also paint the units with a low-volatile organic compounds (VOC) latex based paint to seal the units if you like. Also not all units will necessarily contain Fly Ash; if you have concerns you need to express them to your local concrete masonry producer. Some CMU manufactures use standard cement and or Slag Cement as a partial replacement for traditional cement and there are not any known contamination concerns with these products. Slag cement is derived from steel production and has some of the same benefits to concrete as Fly Ash without the negatives. – M.L. in Kentucky