At a week old, your pieces are probably at their heaviest. As the excess water evaporates, they will lighten considerably.

You can use vermiculite instead of sand - however, the resulting piece will take longer to set to the point of demolding (it's more fragile for a significant amount of time), it will always be somewhat softer (an advantage for carving) as vermiculite is softer than sand and it will affect the color (I like the flecks of gold mica).

Most people would opt for perlite as a partial replacement for the sand. Perlite comes in different grades (including food grade for livestock) and sizes. Perlite is fluffier than sand and is riddled with microscoptic air holes (too small to be detrimental in concrete) which makes it lighter in weight and a good insulator. Common garden variety perlite can be used to replace 2 parts of your sand. You probably want to keep 1 part sand as the smaller, harder grains fill in the spaces between the perlite pieces which is a huge structural advantage. Some white sands also add a beautiful sparkle.

There is a special nylon fiber made for concrete which strengthens by reduciing microcracks (I think that's the reason). Adding that will make the piece stronger overall. Some people also add chickenwire, diamond mesh (used in stucco work) or special alkalai-resistant fiberglass mesh to the interior. I don't know if your plaques are large enough to warrant that.

The acrylic admixture will add strength and water resistence.

Good luck!

Thanks for your wealth of information Daisy. One more question please if you don't mind. If I use the perlite will it make pock marks in my pieces?? Most of my molds are no more than 2" thick.
Thanks again.

Hello Rhonda --- Vermiculite will indeed "lighten your load", but a portion of sand will add strength. If you are going for a "porcelain" look, try to find some white sugar sand, it will keep your mix from going tan or whatever color the sand might otherwise impart to the finished pieces. As for ratios, casters use formulas that are all over the board depending on many factors such as how detailed the mold is, unit cost, strength, finishing techniques, etc. If it's weight you are trying to minimze, you might start with something like the following, then experiment and adjust to your own needs and tastes...

(This is in relative "parts", so you can scale the mix as needed...)

Dry Mix Ingredients:
1 Part White Portland
3 Parts Vermiculite
1/2 Part Very Fine (White) Sand
Dry Integral Color as Desired
(For Added Strength, toss in a small amount of short Poly Fibers. There is a variety that's about 1/4 inch long that is excellent for casting and I've never had to "burn" anything off the finished surface.)

Wet Ingredients:
(Keep your total water to no more than 25% of the Cement by weight for a strong mix)
Water: max 25% of the cement weight (try for less if you can)
Latex Admix: 5 to 10% of the cement weight (suggested starting point)
NOTES: The Latex is optional and actual amounts used range from zero to 100%! High concentrations will interfere with the cements reaction to acid-etched stains if you plan to use them.
Liquid Color As Desired

Other Options:
You can also reduce the overall weight of any given piece by as much as 15% with the use of an air-entraining admix. While it was originally designed to reduce freeze/thaw damage, it is routinely used today to reduce weight.
NOTE...To get the benefit from this admix, it really needs to be mixed in a mixer. Hand-mixing simply does not distribute and activate this agent properly.

This is just meant to ba a starting point and there are a host of other formulas, options and admixes. Jump in and play around with some small batches to determine what works for you...but as I always prompt everyone here...keep rigorous notes on your mix formulations, curing processes, colors and so on. Besides keeping a "log", I maintain a shelf of actual finished samples with all the basic info written on the back of each in permenent marker or keyed to the notes in a journal. The only thing more frustrating than a "failure"...is a great success that you can't ever figure out how to do again.

Hope this helps get you started, just jump in, experiment and have fun. Best of luck.

No, the perlite will not make pockmarks as the cement fills in to give a smooth surface.

Also, the working properties vary a lot depending on the ingredients. When I've used vermiculite, I've had to add more water yet keep its clumping property while pure sand cement is a lot more flowing & slumpy (not a bad thing in a mold). Superplaticiser can make sand cement way too slumpy for sculpting.

BTW, listen to tango - his work, professional, is fantastic.

Thanks so much for your input. This is getting to be more of a challenge than what I was hoping for. I just thought I could mix up a little concrete in a tub, pour it into the molds and it would turn out wonderful (just kidding). I was hoping for
something simple. I was hoping for something simple. I'm feeling a little overwhelmed at this point. Just need to accept the challenge and get it done!! My goal is to get some things made in time for Mother's day. Time is of essence.
Thanks again.

Part of the beauty of cement/concrete is that it is both simple and complicated.

You really can just mix it up, pour it into a mold and get something very nice - especially with white cement & white sand (it took me forever, almost, to find white sand).

When you get bored with that, you can start experimenting with additives, pigments and, my personal favorite, polishing.

You have plenty of time until Mother's Day since you've already started. Just have fun and see where it takes you.

You mentioned you liked the porcelin look - after your plaques have cured for approximately a month, you can get a nice sheen by polishing with wax. Some people have mentioned beeswax, but I've used Butchers Wax (the family name is Butcher, nothing to do with meat). It is used on furniture, musical instruments and bowling alleys. It comes in both light amber and clear, so read the label. You can get it at hardware stores and supermarkets.

I apply it with a bristle brush, let it set a bit, wipe off the excess and then polish it to a high gloss with and old t-shirt or sock (repeat as necessary). I doubt it will hold up outdoors though.

Thanks for the helpful tips Daisy (again) I appreciate it very much. Your encouraging words help too!!

Hello Rhonda --- Vermiculite will indeed "lighten your load", but a portion of sand will add strength. If you are going for a "porcelain" look, try to find some white sugar sand, it will keep your mix from going tan or whatever color the sand might otherwise impart to the finished pieces. As for ratios, casters use formulas that are all over the board depending on many factors such as how detailed the mold is, unit cost, strength, finishing techniques, etc. If it's weight you are trying to minimze, you might start with something like the following, then experiment and adjust to your own needs and tastes...

(This is in relative "parts", so you can scale the mix as needed...)

Dry Mix Ingredients:
1 Part White Portland
3 Parts Vermiculite
1/2 Part Very Fine (White) Sand
Dry Integral Color as Desired
(For Added Strength, toss in a small amount of short Poly Fibers. There is a variety that's about 1/4 inch long that is excellent for casting and I've never had to "burn" anything off the finished surface.)

Wet Ingredients:
(Keep your total water to no more than 25% of the Cement by weight for a strong mix)
Water: max 25% of the cement weight (try for less if you can)
Latex Admix: 5 to 10% of the cement weight (suggested starting point)
NOTES: The Latex is optional and actual amounts used range from zero to 100%! High concentrations will interfere with the cements reaction to acid-etched stains if you plan to use them.
Liquid Color As Desired

Other Options:
You can also reduce the overall weight of any given piece by as much as 15% with the use of an air-entraining admix. While it was originally designed to reduce freeze/thaw damage, it is routinely used today to reduce weight.
NOTE...To get the benefit from this admix, it really needs to be mixed in a mixer. Hand-mixing simply does not distribute and activate this agent properly.

This is just meant to ba a starting point and there are a host of other formulas, options and admixes. Jump in and play around with some small batches to determine what works for you...but as I always prompt everyone here...keep rigorous notes on your mix formulations, curing processes, colors and so on. Besides keeping a "log", I maintain a shelf of actual finished samples with all the basic info written on the back of each in permenent marker or keyed to the notes in a journal. The only thing more frustrating than a "failure"...is a great success that you can't ever figure out how to do again.

Hope this helps get you started, just jump in, experiment and have fun. Best of luck.