Cement/concrete in decorations & habitats

[Andyroo]

Has anybody used good ol' Portland Type1 to make habitat/decoration? I've got two ponds of bare rendered cement and have built at least two more for clients over the years including for planter-pots, but not really heard of anybody using cement for habitat.

I appreciate the leaching, pH issues & boundary layers through to full cure (30days), and I have had all-tank kills from rocks that turned out to be the wrong stuff (concrete), so

is there a cure time and/or cure method/environment to be able to safely use this material?
eg: submerged in rainwater for 6-weeks, or leave in the garden until mossy?

is there an admix allowing same?
eg: Fumed silica, perlite or wood-chips?

Option #2 is to seal it (with epoxy), but I'm hoping for a rougher, more complicated finish than that, though not so sharp-rough as actual coral.
This is intended for a malawi themed system based around Cyrtocara moorii (blue dolphin).

 

[Midwater]

I have made a few concrete ponds, both with portland cement , and blended portland and fly ash. Fly ash - the superfines seem better for waterproofing. I have never used silica fume

Concrete can leach up the ph for a while. The way it is generally done here is on first fill of the pond, while it is still curing, throw in a banana tree stem and let it rot away for a couple of weeks. Possibly any vegetation would do, but the banana is readily available and works well.

I have had success with concrete ponds, and think the rough surface is a good medium for beneficial bacteria too. Concrete ponds seem to need much less filtration than lined ponds.

 

[Andyroo]

Volume is going to be a contributor, too - cement's cheap & strong, so the system will tend to be larger.
Thoughts on decorations, though? At aquarium volumes?

I'm thinking to build/form habitat modules over a balled paper core for cave/hollows, plus to keep weights down.
if an 1800 gal pond gets to be nearly-fully "fish-safe" at 6-weeks, how long for a couple of 5lb habitat modules (of very high surface area) in a 250gal?
And why is nobody using this, including for backgrounds etc?

 

[Midwater]

The greater the surface area (proportional to the volume), if submerged, the quicker the 'banana' cure will be.

I think concrete is the way to go.

But for caves, maybe used something prefabricated. Preformed concrete sewer pipes - not reinforced - but be careful of the thin walled ones, they tend to be asbestos cement.

 

[Andyroo]

perfect-world I'd be buying a block of nicely innert food-grade light grey clay, making my habitats & getting them fired...
however, these are a headache on a small-island... whereas cement is everywhere.

Loooong cure time in a water-filled (plastic) drum with banana stem - doable, particularly as Beryl smashed our bananas into a regrowth phase

 

[jjohnwm]

I've never built a concrete pond, but I do use a lot of concrete blocks (whole blocks as well as broken fragments). Almost all of these blocks were new and I simply soaked them in buckets of water with about a cup or two of vinegar mixed in to neutralize them. Years ago when doing this I actually measured the pH of the soak water periodically and changed the dilute vinegar solution whenever I observed the pH going up rather than down. Very scientific, I know.

I still use vinegar but I don't bother testing anything anymore. I just soak the stuff for about a month, change the solution, give it another month, rinse thoroughly and soak in fresh water for a few days and then plop them into the stock tanks. The blocks provide nest sites, cover/concealment, sight barriers, and surface area for bacterial growth...all at once. They serve as underwater stands to raise planter pots closer to the surface for emergent vegetation as well. They do just about everything one could ask...except for actually looking good.

In many cases, a 200-ish gallon stock tank will contain 4 or 5 whole blocks or the equivalent in broken shards. It's a lot of concrete in a small water volume. No problems.

I also use a lot of concrete drainage pipes (tiles) that were unearthed when an old house nearby was being demolished. These were from the drainage system under and around the foundation, which carried groundwater to the basement sump to be pumped away. They were specifically not part of a septic drainage field. I use them both in outdoor stock tanks and also indoor aquariums; they make great caves and retreats. After spending a few decades buried underground, I figured they were as neutralized as they would ever be; have had zero problems with them. I have at least a dozen in a 360-gallon tank; they have been in there for several years now, and all is well.

The banana tree trick sounds interesting, but my backyard banana trees live a pretty hard life and I don't like to traumatize them so I won't be trying it. Stay tuned, though; somebody should be along shortly to explain the intricacies of concrete block boiling techniques.

 

[Midwater]

I use malt vinegar ... with fish and chips.

 

[Milingu]

I used Portland cement and reinforcement fabric to build artificial rocks.
I kept them in the tank for approx 2 month and did weekly 90%+ water changes. The pH went up to approx 10 (test scale ended at 9.5) in the beginning. At the end the pH stayed close to the tap water's pH and never changed later besides the small up and downs typically for a cycled tank.
But I guess the concrete still leaks some minerals on a very small level because certain algae grew very intensely on the concrete rocks.