sugar

Not sure when you say retarder, do you want the surface to be 'removable' exposing something underneath or do you just want the surface to take longer to set.
if the purpose is to expose something underneath, consider Coca cola, molasses
If the purpose is to allow for you to be able to, say carve into the surface, consider a tufa type mix that in general takes longer to set. There are also commercial 'retarders'........
What might your project be?
Dena

Thanks David and Dena. Right enough, I had a feeling that I'd read somewhere that sugar would do it, so I'll boil up a syrup and give that a go. Never heard of Coca Cola and molasses, though, that sounds interesting. Washing the fines off the surface is what I want to achieve. What kind of mix do you do with the Coke and molasses?

I did some googling before posting my question on here, but could find only commercial retarders. For the amount that I'd be using it's not worth tracking down a concrete chemical supply company, and I'd probably have to buy a gallon drum of the stuff.

I do a lot of exposed aggregate.

I use a fine mister spray bottle and 2 cups of sugar in a quart. After about 10-30 minites I mist the surface letting the sugar water soak in but not to point where the water runs off.
For me there is no hard and fasr rule because I use a power brush to expose aggregate and I use less for say course copper slag and more for small stone. With the sugar I'm just trying to prevent cement from setting hard on the surface of the aggregate. All my ags are less than a 1/4 in size so I don't need much surface removed.
You can buy pea gravel (used for tar and gravel roofs) really cheap. It's nice and round. Thereare also many fancy round stone you can buy in 80 pounds bags that are made for exposed aggregate work. There are many colors.

Syrup or molasis might be cool because you could paint it on and it would stay in place

I should add this.
It's hard to be an expert on retarding the surface in our hobby because of the huge variation of recipes.
Suger chews right through a peat and vermiculite recipe (you can destroy it with sugar)and barely touches a super strong portland sand recipe.

Test first if you aren't sure. I make small 2x8 bricks and test on those first. Even then there can be problems.
Peat and vermiculite will suck sugar deep into a piece if you aren't careful and because it's already a weak recipe sugar can damage it to the point it crumbles later.
Perlite is much better.

It's best to avoid peat/vermiculite if you are using sugar.

This is why I do almost all my work in two or more layers. Make a strong inner layer/shell, let it cure a bit then apply my surface/ finish layer.

David makes several excellent points. Concrete and 'Tufa are very different and the "wicking" effect of sugar below the surface could easily be catastrophic. I also agree with the "strong core" concept for something like this as it circumvents the problem of weakening anything below the surface altogether. Good catch, Dave! And Roger the test proceedures. It is essential for establishing good formulations and practices in concrete work. And considering how far this group (myself included) pushes the envelope regarding mixes and such, it is even more important to test methodically and keep good notes.

Hi Tango
Ive had two bad results with sugar peat and vermiculite before it occured to me that the sugar was probably being sucked in by the peat and vermiculite.
I've never had a problem with sugar and perlite and I routinely spritz my stuff with sugar water if only to make removing the portland haze from the surface of the aggregates easier.

Vinegar is handy for scrubbing off "haze" as well. It's just a mild acid that's good for cleaning up all kinds of stuff.

I use gallons of strong pickling vinegar on my stuff in order to get the glitter back in the aggregate. My shop smells like a pickle factory most of the time.:)

Tango88 - thanks for the vinegar tip. I used to scrub the de-moulded surfaces with a strong hydrochloric acid solution (you have a different name for the acid, I think), but I'm sick of using such strong chemicals. I've been thinking about painting the inside of the moulds with a retarder instead.

For smaller stuff I'm going to try the vinegar and a scrubbing brush!

Reading over the above, I think my best option for larger work would be the pressure washer immediately after de-moulding. I got the big (over 5 foot high) heads in the attached photograph commercially shot-blasted, but it cost an awful lot, and caused a huge amount of mess. I'll be making these heads to order from April onwards, so the pressure-washer will be the most economic way.

{{gwi:66394}}

OMG I'm sitting here with my mouth open! Thanks so much for showing us your work!

Mercedes

Wow, those are BIG. Very impressive.
Rent a pressure washer first. I wish I had. I found it gave very uneven and SLOW results on curing concrete.
IMO you would be much better off with a sand blaster. Messy but does a great job.

BTW Pickling vinegar is much better thsn the regular stuff IMO.