is concrete safe?

[shenton25]

Hi everyone, looking for a bit of advice.
I am making some raised beds using slabs on end for the sides and a concrete base to prevent the roots from going into the soil, as the soil is contaminated.
The slabs are old 3x2 ex council slabs and the cement is hand mixed using sharp sand and cement.
Would this to be safe to grow veg in? I have been reading some confusing things on the internet and I am just looking for some clarification.
Thanks in advance.

[Bigmallly]

Only if you fill it with compost first............... when you grout the sides, I would put a 1" piece of wood in a couple of the joints at the bottom then when the joint is set, knock out the wood. This should give you some drainage holes. Another option would be to put some tubing in the joints and leave it there.

[Greenleaves]

Totally agree with BM, failiure to put in drainage wil potentially give you raised bogs. Depending upon your fill I would also consider putting an expansion joint or two in. My MiL had some brick built planters installed which were filled with soil. As the soil expands and contracts depending upon the weather cracks appeared. Good luck

[Nicos]

Are you concerned about the alkalinity of it?

To be honest I think you'll find that the only plants affected would be around the area of the run off ( good point there BM and GL!)

There are loads of alkaline loving veg and flower plants you could grow in that area

[Potstubsdustbins]

Two of my beds stand on concrete never had a problem, like others have said make sure you have good drainage.

[shenton25]

Thanks so much for the replies.
I was considering boring holes in for drainage but you've given me some good ideas, thanks
I have been looking through concrete raised beds on the internet and I have seen some things about fibre blocks (may be wrong). Just got me thinking about if concrete would be OK to use or if anything was likely to seep out into the soil.

[Bill HH]

If you worried about toxic effects of concrete i think the entire human race would be dead by now if there were any problems.

[starloc]

Every garden in Bulgaria is full of the stuff surrounded by the stuff and has piles of it for repairs, and loads of lime rendering that has crumbled everywhere, the plants seem to love it and look very very healthy....

Even acid loving ones grow well so it cant be getting into the soil much

The only problem with the Bulgarian concrete is lots of it is very old so if you smash it up it has asbestos in it, no harm for growing in or near though just the dust

[Valleyman]

Given the majority of our entire water supply is stored behind concrete dams, water treatment tanks are made of concrete, and half the water pipe infrastructure under the ground delivering your water everyday is made of concrete... I think we can say we're pretty safe.

Seriously though, as a structural engineer and material scientist, concrete is pretty close to inert once cured. Concrete doesn't dry to get hard, it's a chemical reaction. Some old stuff, which used old power station clinker tends to bleed sulphuric acid, but that's more of an issue to embedded reinforcement and steel-work in buildings than human/plant health. There are also a few old mixes out there that cause other problems as well, such as the ASR which can cause concrete to spall (Break apart) awfully. But modern mixes though certainly satisfy that "almost inert" thing i mentioned previously. Wet concrete will give awful alkali burns, but once cured the amount of alkaline that can leach out is minimal.

If you had a low density mortar mix and passed water over it constantly in a sealed system (i.e a pond) you might get a notable increase in the pH, but that's about it.