Introduction: Faux Terrazzo Concrete Lamp

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This project features a concrete lamp made to imitate terrazzo, which is usually made with chips of granite, glass, marble etc. Instead, I used colourful plastic easily found in any kitchen. I opted for vibrant colours, but it wouldn't be too hard to find more subdued colours and make it suit your personal needs. I used a single 12V LED strip knowing that the lamp won't cast too much light. It can work as a night lamp or a small background light.

Supplies

  • 3D printer (optional)
  • corrugated cardboard
  • wide packing/packaging tape
  • hot glue gun
  • plastic bits (see step 4 for details)
  • vaseline, cotton buds, small brush
  • concrete mix
  • small amount (about 1/4 cup) cement or plaster of paris for slurry (see step 8)
  • sanding paper (60, 80, 120, 240)
  • marble/concrete polishing pads (optional, but gives the final product a nice shine and smooth texture)
  • concrete sealer or clear varnish
  • one 12V LED strip, soldering iron, 12V AC/DC adapter
  • epoxy

Step 1: 3D Printed Parts

I tried to find a round object around the house that would fit my specific needs but I couldn't find anything that size, so I simply 3D printed that part (make sure to print it with 0% infill). I also 3D printed three corners to aid me in making a mould, but its not necessary, you can simply use cardboard and notch it on one side to make sure it bends the way you want it. That last, small part is necessary if you want your LED strip to be sunken into the mould, but you can omit it completely or use something else to create it. Plasticine, air drying clay or even a thick layer of hot glue would be sufficient.

Attachments

Step 2: Assembling the Mould

Cut three strips out of corrugated cardboard, size depends on your personal preferences, but I made mine 4cm thick and 24cm long. Pay special attention while cutting it, make sure all strips are straight and even. It will be easier to assembly it later. Cover one side of the strips with tape. It will prevent the sides from absorbing moisture from concrete and help ease the finished object from the mould later.

I used my 3D printed corners, I hot-glued plastic and cardboard together and, as you can see in the last picture, I added additional strips of cardboard to all three sides (my cardboard turned out to be rather flimsy and you need the mould to be as rigid as possible)

Step 3: Assembling the Mould 2

Once I finished making the triangle I positioned it on top of a large piece of cardboard (this piece of cardboard was already plastic coated, so I didn't use packing tape to seal it) and traced the shape of the triangle (Don't skip this step, it will be useful later). I played around with my plastic ring to determine where to position it permanently. Once decided I traced the ring on paper. I used hot glue to connect that small 3D printed bit to the ring and made a line out of glue that will later be used to conceal the LED cable. Finally, I used hot glue to adhere the ring to the cardboard.

DO NOT GLUE THE TRIANGLE TO THE CARDBOARD AT THIS POINT.

Step 4: Plastic Bits

I used 3 plastic lids I found in the kitchen and cut them into small, irregular pieces. You won't need too much plastic, 2-4 different colours will be enough. Make sure to only use the top part of the lid, it's flat and easy to cut. I advise to use plastic lids because most of them are made of hard plastic and are thick enough to stay embedded in concrete.

Use acetone or rubbing alcohol to remove the expiration date from the lid.

That plastic confetti you can see in the first bowl was made with plastic straws I had lying around the kitchen. I simply flattened a few of them and cut them up. This part is optional as not many people have leftover plastic straws and, I admit, the idea I had to sprinkle the plastic confetti onto the mould backfired in the end as half of my confetti slid off the walls while I was pouring concrete. If I was to repeat this project I would cut the hard plastic pieces as small as possible and use that to create a 'confetti'.

Using a brush I applied a generous amount of vaseline (pink vaseline for better visual) onto the outside of the ring and then using a moist cotton bud I placed pieces of plastic onto the walls. Last step was to sprinkle the plastic confetti.

Step 5: Finishing the Mould

Once I covered the ring with plastic bits I did the same to the inside of the triangle (only two walls, I left the bottom plain) and the bottom part of the mould. Once that was done I positioned the triangle back on the cardboard and used glue gun to secure it.

Step 6: Concrete Mix

I can't tell you what type of concrete mix to use as they are vastly different in every country. I made my own concrete by mixing cement, sand and small pieces of aggregate. Ready to use concrete mix can be used, but if it contains big pieces of aggregate you might end up having troubles with sharp edges. If possible, try to pick bigger pieces of aggregate (gravel, stones etc) from the mix (use gloves).

Once the mix was ready I transferred it to a zip lock bag (optional) and poured half of it into a mould. To make sure concrete was distributed evenly I tapped it several times to level it out and get rid of any air bubbles. I poured the remaining of the mix and tapped it again. Left it to set for 2 days.

Last picture shows equations you might find useful to calculate the amount of concrete needed. Concrete's density is 2.3g/cm3.

To calculate the volume of your concrete lamp you need to calculate the volume of the cylinder and subtract that from the volume of the triangle.

My lamp ended up at around 700cm3 in volume, which means I had to prepare around 1.6kg of concrete (2.3g/cm3 x 700cm3=1610g)

Step 7: Mould Removal

Two days later I removed the cardboard walls, used pliers to remove plastic pieces. I washed it with soap and a stiff nail brush to remove vaseline layer and left it to dry.

Step 8: Filling Holes

I used 80 and 120 grid sand paper to lightly sand the whole surface, washed it again and left it to dry. Once dry, I was able to see how many bugholes (holes created by entrapped air) there were on the surface and how they were distributed. Originally I was going to use cement slurry to fill them up, but I changed my mind and went with plaster of paris to add some white colour to the lamp. I simply mixed 2TBS of plaster with water and used a small spatula to spread it all over. Plaster dried quickly, so I had to mix one more batch to cover the sides.
If you don't want to use plaster, mix cement slurry (1-2 parts cement, 1part water) and spread it all over the surface. Let it dry for a few hours.

Step 9: Sanding and Polishing

Once plaster was dry I began sanding. 60 grid first as plaster turned out to be quite stubborn, 80, 120 and finally 240 for a good measure. At this point you could sand some more with higher grit sand paper and varnish it, but I stopped at 240 and moved to polishing pads.

I have a set of seven polishing pads ranging from 50 to 3000 grit. I wet-polished the whole surface with those and ended up with a really smooth, shiny surface.

https://www.amazon.co.uk/Aussel-Diamond-Polishing-...

Last step was sealing. Normally I'd use concrete sealer or specialised polish, but this lamp is purely for decoration and won't be used as, for example, a table top, so I just used a water based varnish. One or two layers will be enough, it dries quickly and isn't too shiny.

Step 10: LED Strip

I wired one LED strip directly to an AC/DC adapter (my cable was extra long, but if yours isn't you will have to wire the LED to a different cable and wire the other side of that cable to a female DC connector ----> https://www.ebay.co.uk/itm/10pcs-12V-DC-Female-Mal... ). I used epoxy to glue the strip inside the hole, used more epoxy to hide the cable inside that ridge I created with hot glue (step 3 ) and finally I secured the cable to the back of the lamp.

Step 11: