Cement render: safe cure period?

[andyroo]

Greetings,
Our girl's cement pond is nearly done; how long to let final render cure before it's fish-safe?

Better to cure with water or dry? IE can head-start the plants, amphipods/ostractods & swordtails (etc) prior to oscars & arowana?
Does charcoal in the filter help with reducing chemistry impact of the cure? I appreciate that charcoal likely won't help with pH spikes.

If must be dry & more than a few weeks, I may do a one-coat with leftover WestSystem (epoxy) to keep the water&cement separated for the meantime. Our girl's well & truly outgrown her aquarium & getting desperate for her new digs.

V= 5'x10'x5'deep to the main tank, with an 8" submerged rim for emergent plants in pots.

Thanks in advance,

 

[deeda]

What type of cement did you use? Just curious if you used bags of cement or had a pro 'shoot' the cement to form the pond.

My understanding of curing cement is it needs to stay moist for at least a week either by wetting it and covering with plastic or daily sprinkling depending on the temperature. I've also seen references to filling the cement pond with water for a week, drain, refill for a week, drain, refill, check pH and repeat as necessay but that may depend on the cement type/mixture used.

A couple Koi forums I quickly checked out offer a couple different suggestions on the proper way to cure cement ponds before adding fish and the timeline varies from 2 weeks to 4 weeks.

The problem with using cement for ponds or for that matter cement DIY backgrounds is the pH goes sky high as the lime is released.

 

[andyroo]

Good ol' Portland type-1 with an igneous river sand & sharp crushed limestone for the matrix. Bottom's poured & 2weeks cured (with BRC steel) and walls are plywood framed, BRC'd & 3/4 filled with cement. We'll finish the pour through first part of this week, then final render & steel-float to lighten the colour/tone & make watertight, hopefully by the end of this week. I may add a fumed silica to that final render if I can get it, and if I was doing this myself I'd to a layer of fibreglass under the final & steel-float, but this part ain't my show... and I'm famously paranoid Our guy's been #2 mason on my wife's team for a decade & done plenty of swimming pools & potable water-tanks, so I'm not worried.

The lime & pH spikes are my worry, yes. Three-weeks can work - I expect filling to cure then drain to refill will be the way, which will also give time & waste-water sinking-soak to the logwood & bamboo. Rainy season now, I doubt i'd get it dry enough to set an epoxy layer anyways.

Shotcrete would have been far easier & far more design-flexible, but the machinery was ungainly to & for the site. Costly also.
The wetting you refer to keeps it from drying before curing to crack (as I understand it), but we're pretty humid. We also get around that with the final render.

 

[andyroo]

Yes, sorry, bag cement that we're mixing adjacent & bucketing into the forms.

 

[deeda]

Thanks for the info, very detailed!

What is your #2 mason saying in regards to curing and how they deal with potable water tanks curing or is that not part of his repertoire?

 

[andyroo]

I'll ask him, but he's not likely to have an opinion beyond first water tasting funny as it won't be a human-health issue (maybe I ring Ministry of Health).
I'll give it a fill & drain through that first fortnight+, maybe run the rain-gutter overflow to & through. Final(?) fill will get tested for a few days before fish & root-wood, bamboo & almond leaves will buffer regardless.
Note: we cast cement anchors for a recent coral nursery install that kept bubbling (one-one tiny bubbles) for best part of a month. I do'know if this was curing off-gas or bubbles in my mix