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Are Concrete Blocks Toxic?

If you have or plan on building a home garden you may be considering using concrete blocks for the foundation. Many raised gardens use them because they’re very strong, cheap and resistant to the elements. But are they safe? Are concrete blocks toxic to the environment, you and your plants.? The answer is yes and no. Concrete blocks, a.k.a cinder blocks, can be made with materials that could potentially cause health issues if consumed. Fly ash, which is a byproduct of burning coal and used to make some types of cinder blocks, can leach toxic substances like calcium oxide, aluminum oxide, and magnesium oxide into the soil. Other heavy metal byproducts such as lead and arsenic may also be present and could also leach. In addition, lime can leach into the soil which can raise pH levels over time.

Fly ash and other heavy metal elements found in some concrete blocks can essentially be hazardous if those materials leach into the soil and are consumed. Scientists have conducted tests to find out if concrete blocks are harmful to humans but so far the tests are inconclusive.

Luckily there’s a solution. Some gardeners recommend using a concrete block sealer which provides a waterproof seal between the blocks and earth. You can also line them with plastic or some other protective layer.

Even though all cinder blocks do not contain these harmful elements, it’s almost impossible to know if yours do. I recommend always staying on the safe side and either including a protective layer between the blocks and garden or not using them at all.

Concrete Or Cinder Blocks

First, you need to find out if you’re working with actual cinder blocks or cement blocks. Many people call all concrete blocks cinder blocks but that’s just a nickname.

  • Cement blocks are made with Portland cement and aggregates. They are heavier and cost more.
  • Cinder blocks are made with Portland cement and fly ash which is a byproduct of the coal industry. They are light weight and usually cheaper but still very strong.

Fly ash is the problem. It’s a byproduct of coal burning electric plants. The ash is what’s left over after the coal is burned. It’s collected and used with cement to save money and lighten the blocks.

The recycling of coal ash technically makes cinder blocks a green building material. However, the fly ash leaching potentially toxic materials into the ground can be a problem. Coal contains heavy metals and other substances known to be toxic. Some of these metals and substances remain in the ash after the coal is burned and are then found in the cinder blocks.

Essentially, using cinder blocks is just like putting chunks of coal in your garden.

Garden beds built with cinder blocks may be fine for flowers and plants but avoid them if your planting vegetables, fruits and herbs. Toxic materials found in the fly ash can leach from the cinder blocks into the soil and then into the food.

Fly Ash

Fly ash a.k.a. “flue ash,” is a byproduct of coal burning plants. It’s the stuff that’s leftover once the boilers burn off all the fuel from the coal. In the past, fly ash was released into the atmosphere. But these days, due to new pollution laws, it’s collected and recycled.

There are two classes of fly ash.

Class F occurs when plants burn anthracite or bituminous coal. This type of coal is harder and older than ignite or sub-bituminous coal. Class F fly ash is considered pozzolanic which refers to where it comes from. Pozzolan is a type of volcanic ash that comes from Pozzuoli, Italy. It contains more alumino-silicate glass and quartz which has very little value as a cement ingredient.

Class C fly ash contains higher amounts of lime, alkali, and sulfate in its composition. Class C fly ash is considered cementitious and hydraulic. It contains calcium sulfate, calcium-rich glass, and magnesium oxide. These properties make it very good for making cement.

Class C fly ash can become cement when added to water just like Portland Cement. This is why Class C fly ash is sometimes used as a replacement or partial replacement for Portland Cement in many concrete mixtures. It’s cheaper and lighter but still very strong.

Concrete blocks that are made by using all cement instead of fly ash are much safer for use near a garden and are generally not considered toxic. However, if your block’s used fly ash instead of cement, they could be a problem.

Concrete Isn’t Generally Considered Toxic

Concrete is a fantastic building material that’s been used all over the world for centuries. Without concrete there would be no bridges, dams or large buildings. And most modern residential homes couldn’t be built either. Almost everything built these days uses some form of concrete and almost all of those things are perfectly safe.

Most concrete is made by mixing cement, sand and aggregate stone with water. None of which are considered toxic. Although there is some evidence pH levels in soil can become elevated due to the lime used in cement which is not generally considered harmful. It’s only when fly ash is added to the concrete mix that there’s a potential problem.

There are other additives that can be used in a concrete mix like colors and chemical hardeners but these aren’t typically used to form a cement block. You can check with the manufacturer who made the block to be sure.

Toxicity

Not all concrete blocks are made with fly ash. Most concrete is made by mixing Portland Cement with sand, aggregate stone. The mixture hardens when mixed with water through a process called “curing.” These types of blocks are generally considered safe and toxin free.

Concrete blocks that included “cinders” in their ingredients is where the nickname cinder block came from. Any blocks that contain fly ash have the potential to leach toxic heavy metals into the soil.

Consuming toxic metals from contaminated soil, or plants grown in it, is pretty extensive.

So how to tell if you have blacks containing fly ash? Blocks that are only made from Portland Cement are heavier and more expensive compared to those made with fly ash. One of the main reasons people use fly ash is because it’s cheaper and lighter than cement but still very strong.

If you’re buying cement blocks I recommend you get them right from the concrete company because they can tell you exactly what’s in them. I’d also recommend always buying them new and not using old blocks because there’s no reliable way to know what went into the mixture.

If you already own cement blocks then I’d err on the side of caution and not use them or at least seal them really well before you do.

What Should You Do?

If your garden contains flowers instead of foods then toxic concrete blocks are less of a worry. Although I still wouldn’t want potentially harmful and toxic substances leaching into my yard. One day you may decide to grow vegetables or fruit in the garden and the soil could be contaminated.

If your garden is surrounded by concrete blocks that contains fly ash and has fruits and vegetables, it’s possible that toxic substances are seeping into the soil and possibly into whatever you grow. This is in addition to the potentially higher pH levels from the lime. Luckily you can monitor the pH levels of your soil to see if this is happening. But I’m not aware of an easy at home test that checks for toxicity.

Instead of using concrete blocks to build an elevated garden, I’d recommend more natural materials like wood or real stone.

But if you want to use blocks anyway then buy them brand new from the concrete plant and ask what ingredients were used to make them. I also recommend sealing the blocks with a waterproof sealant and using a liner just in case.

Don’t plant edibles inside the hollow chambers of the blocks. This will bypass the sealant layers. Roots will grow surrounded by block so chances of leeching and absorbing will be higher.

Try growing cleansing plants, such as sunflowers, for a few seasons if your not sure about the health of your soil. Some species of plants clean or neutralize the soil by removing toxic materials. At the end of the growing season make sure to destroy the dead plants. Adding contaminated plants to a compost pile can contaminate the pile.

Make Your Own Concrete Blocks

Did you know you can make your own DIY concrete blocks with a mold. Molds are sold online or you can make your own at home. By making your own concrete you can control exactly whats in it.

There’s an all natural form of concrete, called Roman Concrete, which uses crushed and baked limestone or seashells mixed with sand and aggregate stone. Every ingredient comes from nature so it doesn’t contain any contaminates. You can even source the materials from your own land so there’s no chance of foreign chemicals or metals.

Simply make the concrete, pour it into the molds and wait for it to dry.

Another benefit to making your own concrete blocks is that you can vary the shape however you like. Blocks are designed for building foundations, not garden beds, so the shape can be changed. I like to remove that center piece of cement located in the middle of the block. This creates one big opening that I can fill with soil and plant. That center strip is there to strengthen the block for foundation work but gardens don’t need to support tons of weight so it’s unnecessary.

Summary: Are Concrete Blocks Toxic?

If you have or plan on building a home garden you may be considering using concrete blocks for the foundation. Many raised gardens use them because they’re very strong, cheap and resistant to the elements. But are they safe? Are concrete blocks toxic to the environment, you and your plants.? The answer is yes and no. Concrete blocks, a.k.a cinder blocks, can be made with materials that could potentially cause health issues if consumed. Fly ash, which is a byproduct of burning coal and used to make some types of cinder blocks, can leach toxic substances like calcium oxide, aluminum oxide, and magnesium oxide into the soil. Other heavy metal byproducts such as lead and arsenic may also be present and could also leach. In addition, lime can leach into the soil which can raise pH levels over time.

Fly ash and other heavy metal elements found in some concrete blocks can essentially be hazardous if those materials leach into the soil and are consumed. Scientists have conducted tests to find out if concrete blocks are harmful to humans but so far the tests are inconclusive.

Luckily there’s a solution. Some gardeners recommend using a concrete block sealer which provides a waterproof seal between the blocks and earth. You can also line them with plastic or some other protective layer.

Even though all cinder blocks do not contain these harmful elements, it’s almost impossible to know if yours do. I recommend always staying on the safe side and either including a protective layer between the blocks and garden or not using them at all.

If you have any questions or comments e-mail us any time. We’d love to hear from you.

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