How to Create Very Thin Concrete or Screed

No idea whether it would work but you could try adding PVA glue to the mix.

The OP's floor leveling screed is essentially concrete with PVA already added. The manufacturers use Silica sand and Dolomite flour as the aggregate and add citric acid to control the setting times. The amount of water added also needs to be tightly controlled. Buying as a premixed slurry is better than the type where you add the water because the Coefficient of Variation in the premixed is very low (All the ingredients are evenly dispersed to get optimum results. Almost impossible to achieve without the correct mixing technology and mix timing). Also check the use by date on the packaging, these products have a very short shelf life. Epoxy resin would work but I suspect that with the amount required to make 3mm thick concrete robust the OP would end up making powder filled epoxy jewellery rather than epoxy loaded cement jewellery.

Concrete as a material is very good in compression but lousy in tension and shear. It is the wrong material for making thin durable items like jewellery. Any designer, be they an engineer or an artist must make him or her self aware of the limitations in the materials used and either modify their designs to work within those limits or use a different material. Considering the aesthetics of a piece in isolation is not enough.

Apparently this is more common than I thought, though I'm still wondering why anyone would want a chunk of concrete hanging around one's neck.

Cast Concrete Pendant Necklace - Instructables

Concrete makes a cheap and attractive medium for embedding colorful stones on a rigid surface....These are very popular...

Caution: Shipping weight 40#. Extra freight charges will apply.

Actually these are small....350 grams...

https://www.etsy.com/listing/237066775/indian-metal-clutchhand-craftedinlaid?ref=market

Have bought many like these....

I would have thought you were more of a pearl type, living by the sea and all.

Har har har.....

I have to agree fully, and believe the OP would be better off using tile cement mixed with bonding liquid.

cement is desirable for many reasons including it's compressive strength. you aren't going to get good resultshttp://www.understanding-cement.com/strength.html

You need an epoxy type hardener mixed in with the concrete....they sell artisan concrete kits...you also may want to take a look at polymer clay for your artistic endeavors....

https://www.youtube.com/watch?v=Cvi9JpcjH8o

EnCapture Artisan Concrete kit...

https://www.youtube.com/watch?v=zpU74xQOOAM

The finished product is usually coated with a clear glaze....

EnCapture Artisan Concrete™ Jewelry Making Kit
EnCapture Artisan Concrete™ is a unique, texture-rich medium that offers a wide range of new possibilities for designers of every skill level. Embed a variety of treasures such as stones, glass and metal to create visual interest and contrast in your finished pieces. EnCapture is very versatile. It can be dyed with pigments or pained or stained when dry.
Items included in kit:
- EnCapture Concrete Base Material
- EnCapture Activator
- Disposable Mixing Cups
- Palette Knife
- Wooden Mixing Sticks
- Syringe for Measuring Activator
- Dust Mask
- Disposable Gloves
- Brass Brush
- Straight Carver
- Double-ended Carver
- Colored Powder Pigments
- Measuring Spoon for Pigment
https://www.etsy.com/listing/257543703/encapture-artisan-concrete-jewelry?ref=related-1

Mix some dust with a little J-B Weld. And don't forget to use the hypoallergenic, organic J-B Weld and the free range concrete dust.

Chopped Glass fiber in Mix will help a lot

Silica Sand - graded 3mm down is all the aggregate you need.

Add 3rd. Gen Super-Plasticizer

Add Minimum water i.e. w /c max 0.3

Use within 30 minutes of adding water

Tell us your experience-please

Your project may hit a snag if it comes to market in the form of cement. (Concrete contains aggregate or rocks).

Skin contact may be an issue.

The MSDS for portland cement contains the following information: "Dry portland cement contacting wet skin or exposure to moist or wet portland cement may cause more severe skin effects including thickening, cracking or fissuring of the skin. Prolonged exposure can cause severe skin damage in the form of (alkali) chemical burns."

MATERIAL SAFETY DATA SHEET (MSDS) FOR ... - Cemex

JE in Chicago, may have come close with his tongue-in-cheek suggestion of an epoxy matrix surrounding the cement powder. Or, maybe a cement-like grey filler/glass fiber mix in clear epoxy would work.
Something to consider.

Glass fibres? The answer is both Yes and No. Yes, glass fibres go well with concrete, in fact, for the projects you have in mind, much better than steel as reinforcements, but NOT E-type glass, which is the type commonly used with Polyester and epoxy formulations. Portland cement is highly corrosive to E-type glass. The only glass filaments that will work are the AR-Type. AR being Alkaline Resistant. For small quantities look where backer board is sold or other cementinous board products used to support tile work. A cement countertop with steel reinforcement needs to be at least 2" thick, whereas one with no steel and only AR glass fiber reinforcement can be made 1" to 1 1/4' and still have the same bending resistance and no surface micro fractures. I have produced many glass reinforced bathroom fixtures, countertops and sculptures using this method with much success once I got rid of the steel and started using AR glass reinforcements.

Many concrete contractors are using FiberMEsh mixes and leaving out the rebar in certain non- shear and compressive applications . Sound like the same stuff. Also, non -shrink cementitious products with fiber..Asbestos is / was great for that, but alas, not to be found in powdered state any more.

I have made small statue from white cement. It worked well for me as I got smooth surface and it accepted white glazing oil paint.

I remember (?) a topic years ago for producing very strong material from CEMENT (presumably Portland type).

You need the very fine CEMENT and water only, and the trick is in the curing.

Experiment and let us know.

If it is properly homogenised, one can make springs out of cement. So homogenised material looks to be the way forward for smaller items.

Look at small-size homogeniser machinery, perhaps?

Thank you, thank you, thank you for all of your comments! I can't tell you how much I appreciate the help and advice you've given me. We all have busy lives so it's great when people give up some of their time for you. I'm truly grateful.

The glass fibres (AR of course!) are duly ordered and are expected today hopefully. I am going to look into the prepared screed mix as someone suggested and also lots of other ideas mentioned - too many to list.

I will keep on experimenting though and if I crack it, will come back and share the mix.

Brilliant site, you've all given me loads of hope!