Gardening Note: Toxins in Concrete Blocks and Concrete


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Gardening Note: Toxins in Concrete Blocks and Concrete

By George Giltner, Adv. Master Gardener, MS Biology (and Pesticide Applicator Certified)

Gardeners love raised beds.  And, raised beds are so easily built with concrete blocks which are thought of as the most inert and safest product on the market for gardening.  Gee, I hate spoil your perceptions.  Like the commercial food we consume, some is good and some is bad, just like concrete products used in gardening.

Masonry block and concrete producers use coal combustion byproducts in two ways – fly ash as a cement replacement and bottom ash as a partial replacement for the sand and/or coarse aggregate. The fly ash composition varies from 5 to 60+ percent depending on the product.  Adding fly ash prevents hydration of lime which increases its strength and makes concrete less porous, makes finishes smooth, and eases concrete pours.  Seems like the ideal way to recycle hazardous waste that has historically created problems in water supplies.

Hazardous waste.  Yes, there are small amounts of heavy metals in concrete products.  Typically the main composition of Class C fly ash contains 3.5 to 40% calcium oxide, 0.5 to 40% aluminum oxide, and 2.5 to 25% Magnesium oxide.  However smaller percentages of strontium, chromium, nickel, lead, arsenic, cadmium, and other heavy metals are also present which cause it to be classified as hazardous waste by the EPA.

The spherical particle size of coal fly ash varies from 5-120 microns which is similar to that of silty sand to silty clay.  The good news is that this material is bonded, insoluble, and immobile in concrete.  However if the concrete or blocks are pulverized in destruction or become soluble with acids, the surface area exposed is greatly increased and the heavy metals may become mobile.  Example: You would not want to use pulverized concrete to “lime” a garden where it could be acted upon by microbes and organic acids.

Concrete blocks are porous unless they are sealed for outside and inside use.  Sealing with paint and other products prevents mineral (like hydrochloric acid, a concrete cleaner) and organic acids (from microbes and other life forms) from decomposing the locked in heavy metal chemical structure in concrete materials.

Variation in solubility and composition of concrete heavy metals is all over the map, depending upon the type of coal burned to produce the fly ash, percent of fly ash used, sealants incorporated in the blocks, acids in the environment and other factors.

If you are concerned about heavy metals in concrete materials, there is a reliable way to test for their presence.  Grow hyper accumulator plants in the concrete structures of concern, and then have chemical analysis performed on specific plant parts.  These plants can be found in Wikipedia and on other web sites.  Example: Sunflower parts can be used for soil and water hyacinths can be used in aquatic environments.

Another option is to separate the soil from the concrete structure.  Line the container with plastic or polymer paint.  Therefore decomposition products are not available for plant absorption.  This is probably your best and cheapest option.

Decomposition of concrete is usually slow; therefore your risk level is probably small in comparison with other heavy metal routes of entry into your body.  Rice, chicken, and other foods have been in the news with carcinogenic arsenic concerns recently.  Lead always comes up with old paints in buildings, and possibly lead shot cooked in wild game acid stews and gumbos.  Lead as an example can cause DNA genetic damage, nerve damage, and child learning disorders.

The science behind heavy metal poisoning is very well documented and proven.  Awareness of toxins in our environment, not phobias, leads to healthy gardening and living.

Author’s note: I recently had a conversation with fellow Master Gardener, Emily Shirley and after giving her the above information about concrete blocks, her question to me was “Bottom line – would you use concrete cinder blocks to build raised beds that you would be growing food to eat?  If yes, what would you do to the blocks before filing the bed with soil to plant in?”  My response to her – was to seal the blocks with concrete sealant and polymer paint before adding dirt. An extra layer of safety gives “Peace of Mind”.




11 Responses so far »

  1. 1

    But is polymer paint and plastic safe for food gardening?

    • 2

      Hi Ms. Smith,
      Thank you for your question. Let me forward your question to the author of the articles, and then I will send his response to you.

      regards, kh

    • 3

      Here is George’s response to your question:
      Plastics and polymer paints should be very stable when used on concrete blocks for raised beds. UV and high temperatures are used in the industry to test polymer breakdown. Since the soil side of the blocks would have little to no exposure to UV and no high temps, the lifetime of the polymers can be expected to last for extreme lengths of time with little degradation, like the polyethylene liners of landfills. Even near the soil surface, polyethylene will break down into harmless ethylene which is part of the plants organic metabolic process (Carbon dioxide cycle, growth factor, etc.). Additives are in polymers, like clay, silica, antioxidants, and slip agents, but they are either inert or in such low concentrations that the soil itself, would be more toxic.

      Polymer paints have been used for years in basements as a water sealer. I have not read any reviews or warnings of any toxins released after curing.

      • 4

        Nigel Simpson said,

        I would never grow edibles in soil that is in constant contact with plastic, especially in the Southern California heat. BPA, phlatates, Deca, etc — I’m concerned about feeing my family food laced with hormone disruptors. To learn more, consult the website of the Environmental Working Group:

        It may be inconvenient to avoid contaminating our food supply, but serious adverse health effects are much more inconvenient!

  2. 5

    […] waste.  Yes, there are small amounts of heavy metals in concrete products.  Typically the main composition of Class C fly ash contains 3.5 to 40% calcium oxide, 0.5 to 40% alum….  However smaller percentages of strontium, chromium, nickel, lead, arsenic, cadmium, and other […]

  3. 6

    Keith hawkins said,

    This concerned person is stereotyping plastics. Plastics come in all types of chemical compositions along with individual additive packages. One manufacturing facility may produce 200 different types of polymers with ethylene, a common monomer which is also extensively used by plants. That is why I recommend referring to the MSDS about specific polymers.

    Another factor is plants are specific about molecules passing through the root membranes. Not any metal or molecule from the soil will be absorbed and utilized in plant veggies and fruits.

    Also soil organisms will break down molecules and organic soil molecules bind with contaminants. Therefore risk levels associated plants absorbing a matrix molecule from a plastic in soil is remote. Here in very hot and humid Louisiana, polyethylene in soil used as a landfill liner is guarenteed usually for 30 years, but they may last hundreds of years underground.

    My recommendation is for a really concerned person would be to first ID the polymer, then look for solid studies that support an associated risk level.

  4. 8

    […] you’re concerned unwanted substances might be leaching from concrete into soil, you can protect your […]

  5. 9

    gardening said,

    I enjoy reading a post that will make men and women think.
    Also, thank you for permitting me to comment!

  6. 10

    Elektroauto said,

    Great article! We are linking to this particularly great post on our site.
    Keep up the good writing.

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