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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”.