How to Make Terrazzo

Three components decide how the finished slab looks. White cement for a white base. Marble sand at 0 to 2 mm for the fine fill. And decorative chippings as the aggregate you actually see.

The marble sand is not fussiness. Ordinary sand carries dark specks, and because terrazzo gets ground right down, those specks surface and turn a white slab a dirty grey. White sand keeps the background white.

The aggregate is where the look comes from. Different stone, different colours, different sizes, mixed however you like. This build uses white chippings with black for contrast.

Three parts sand to one part cement by weight. Add a little microsilica, which makes the concrete harder, more flowable and generally easier to work with.

Water goes in at roughly 400 ml per kilo of cement. Mix it through with a drill and paddle. A concrete mixer works too, but for a batch this size cleaning the mixer afterwards costs more time than the mixing saves.

Mix plasticizer with water first, then add it to the batch.

It drops the surface tension in the mixing water, so the concrete turns fluid and compacts almost on its own. The difference is obvious the moment it goes in, and it is what saves you fighting the mix at the pour.

Tip the chippings in and mix them through. Coarse aggregate makes the batch noticeably harder to stir, which is normal.

One rule matters here: every stone must be smaller than the thickness of your slab. Oversized stones bridge the formwork, refuse to settle, and make a level top surface impossible.

Coated chipboard screwed together makes a good form. The coating gives a smooth face and survives contact with wet concrete.

Brush release agent over every surface the concrete will touch. This is what lets the slab come out cleanly instead of taking the form apart around it.

Pour the terrazzo mix in and compact it by hand as you go. Stop about a centimetre below the top edge.

Then mix a plain sand-and-cement batch with no stones in it and fill that last centimetre. This is the step people skip and regret. Coarse stones at the very top cannot move downward, so they fight compaction and you never get a level face.

Strike the fine layer off with a straight edge or trowel.

Take the form apart and lift the slab out once it has set.

The face does not need to be flawless. Every bit of it is about to be ground away, so small blemishes at this stage genuinely do not matter.

One timing note: do not let the concrete cure too hard first. Grinding is significantly easier while it is still green, and waiting only adds work.

A diamond cup wheel takes the cement skin off and exposes the stones. A dedicated concrete grinder with dust extraction is the comfortable option, but a cup disc on any angle grinder does the same job.

Keep the cup flat on the surface. Tipping it cuts grooves that are painful to grind back out. Work with even pressure so the whole face comes down uniformly.

This is dusty, slow work. Wear a mask.

Grinding opens up air pockets across the face. Left alone they stay as visible pits.

Mix a very fine mortar, two parts marble powder to one part white cement, and work it over the whole surface to push it into every pore. Marble powder is just finely ground marble sand, so it matches the background colour and leaves no patchiness.

Wipe back with a damp cloth and let it harden overnight.

Polishing is the step that turns a flat grey slab into terrazzo.

Use a wet polisher and work through diamond pads from about 50 grit up to 3000, roughly doubling the grit each pass. Working wet cuts the dust and gives a better finish.

Do this outside if you can, and use a splash guard. Wet polishing throws slurry a surprising distance.