Hi all--I am wondering your experience in using concrete? I am reading Sylvia's book now and she says to be careful of this. What are your experiences? Of course some variation to the pH but here in China it is extremely cheap and could help make the whole system more economically sustainable.
The acidic nature of your aquaponic system will pull the lime out of your concrete and buffer the pH up constantly. It will be very difficult to maintain an optimal pH (6-7 depending on who you ask) because the concrete will be constantly leaching into your water. You can solve this by adding some sort of sealant to your concrete, to protect it from the water. Paraffin wax is something that I know has been used successfully.
Our tilapia source puts out 14,000 lbs of tilapia a week, and he uses concrete tanks. He has to resurface them every few years however because the concrete gets eaten away over time. He does not coat his tanks with anything.
Yeah the limestone in the concrete leaches and messes with the pH. Now, I've always thought that you had to seal the concrete with wax, epoxy, whatever to create a physical barrier with a third substance between it and your water...but there are some folks on this site who claim that an acid wash is enough to do the trick, and have done so. I don't know because I've never tried it, but maybe they could chime in if this acid wash technique has worked out in the long term...I think one guy had a decades long history of using concrete like this for some purpose or another... DO NOT take my word on this because again, I have no direct experience using this acid wash technique, but hopefully someone who has can answer...and/or you can research it further...
You need to carefully check with concrete manufacturer on what the aggregate is that they use as well as the bonding agent. Shotcrete is primarily what has been used in commercial aquaculture tanks but in needs to not contain limestone or marble, both are very commonly used in making all concrete products, as well as being very expensive when marble is added. They are not strong enough for structure but are used as a coating to pools covering standard concrete or steel.
You also need to watch out for those that use granite as it is can leech toxins into the water. I would say stay away from concrete in general but we used a quartz/sand shotcrete that used a hydraulic cement to encase concrete 20k gallon spawn beds for trout. It was a very expensive project. The university spent well over $2 million on 2 beds.
You can put a liner in the concrete bed, tank, whatever. Saves the messy acid washing and leakage due to cracks later on. If you should decide to use concrete, keeping it damp for slow curing,up to 30 days is recommended to avoid developing any cracks further on down the road.
Thanks so much for all your help and suggestions! Very helpful. We will try and see :-) We need lowest cost material to make it work.
Pool paint works on concrete as well as steel livestock tanks.