I recently moved into a new flat, the larger amount of space finally allowed me to put some nice plants around the house. I picked up a nice palm from Ikea but couldn't find a pot that I liked. I've always wanted to make something using concrete so this seemed the perfect opportunity.
Step 1: Materials and Tests
I've been thinking about working with concrete for a long time now, I loved the idea of including some coloured aggregate in a concrete mix. My understanding of how these materials interact is probably much better now having actually worked with them on a large project. If I were to do this again I think I would try and find a coloured plastic to use as aggregate (i'll explain why later).
I collected a large quantity of sand and white shells from the local beach and bought two bags of fish tank gravel. I performed several tests with the materials and they seemed to perform well.
Step 2: Constructing the Form
I decided that using the existing plastic plant pot as part of the form would be an excellent way to ensure I made the new pot a suitable size for this particular plant.
A hexagon seemed like a relatively easy shape to build around the existing pot. I googled 'How to draw a hexagon' and sketched a suitable size hexagon around a trace of the existing pot. (You can see this in the first image). I used 5mm MDF (salvaged from a skip) to create each side of the hexagon.
Using wood glue I used some wooden offcuts to bracket the sides of the form. You can probably see in the forth image that I had marked a suitable depth for the base of the plant pot. I had planned a complicated drainage system that involved casting a separate base however I eventually decided against this when i realised how much work it involved.
Step 3: Sealing the Form
The form effectively needs to be watertight in order to contain the cement. I used some really cheap roof sealant to fill all the gaps around the edge of the form.
You can see in image three that I had planned for a small overlap on each corner of the hexagon. I wanted this to create a slightly rough unfinished edge.
I gave the entire interior of the form several coats of PVA glue in order to help seal it. The final image shows the whole construction before poring.
Step 4: Preparing to Pour
As this is the first time I had used concrete I was a little uncertain of it's overall strength. I decided to encase some large nails within the concrete during pouring, these would act as (very cheap) reinforcing rebar. I planned roughly where they would go so that i would be happy placing them as I poured the concrete into the mould.
Image 3 shows the plastic plant pot inside the wooden form. As you can see I didn't really leave myself much space to actually pour the concrete into the mould. This would soon prove problematic.
Image 4 and 5 show the various concrete constituents being combined before adding the water. I roughly followed the instructions on the quick drying cement bag but to be honest I think this stuff would set under any conditions.
Step 5: Pouring (and Making a Mess)
I haven't included any images from during the pouring process as it turned out to be an ENORMOUS STRESSFUL NIGHTMARE!!! during which I was unable to touch my camera due to the massive quantity of cement all over my hands. The pouring process went pretty much as badly as possible so I'll recount it here in the hope that you might be able to avoid some of this yourself.
First I mixed the dry concrete ingredients together in my cement mixer (actually a bucket). I was almost certain that i had MORE than enough mixture.
I started adding water (The directions on the bag said 'add as required' which threw me a little...) and mixing with a wooden spoon. Predictably the bucket & spoon technique was absolutely useless... nothing was getting properly mixed and i wasn't using anywhere near enough water. After 10 mins of panicky stirring I gave up with the spoon and stuck my hands in... another mistake. As I mixed the cement together the sharp stones were scraping against my hands in a really painful way... but it did the job. When I thought the consistency seemed about right I started to plop it into the form. I quickly realised that trying to pour it down the TINY GAP i had left myself was not going to happen. I frantically scooped it into the form using my bare hands and quickly began to realise that I hadn't used anywhere near enough water!
I quickly ran out of concrete about half way up the form. My first batch had included all of my exciting coloured aggregate so i was forced to very quickly mix up a new batch using only stones and sand. This time i added LOTS more water and managed to make something that much more closely resembled the sort of thing you see on programs like 'Grand Designs'. I poured this on top of the other mix and hoped for the best.
The big nails seemed to work quite well and fitted nicely throughout the form. I shook the form for a few minutes to remove bubbles and then spent about 10 minutes trying to wash my hands.
Here is a list of the things I have learned:
-Mixing large amount of concrete in a bucket with a wooden spoon is stupid and very very difficult.
-You should really mix concrete outdoors and not on a small tarpaulin sheet in your flat.
-Cement is really bad for your skin!
-Use plenty of water when mixing cement
-Don't bother with rapid set cement... its far to stressful.
-Ensure the form has plenty of space to pour concrete.
-Mix twice as much cement as you think you will need!
Step 6: De-Moulding
After the disastrous pouring process I was pretty sure that the whole thing would just crumble into pieces upon de-moulding... LUCKILY it didn't. During the time it took to set I had convinced myself that the two separate layers of concrete would actually look completely intentional and I could just tell people that I did it on purpose anyway.
After de-moulding i decided that it didn't look as bad as it could have been.
Step 7: Skimming and Curing
Due to the massive gaps in the lower section of the plant pot I decided that it would be best to skim the entire thing with a mortar mix made up from the same cement. I used a filling blade and it didn't look that bad.
I sprayed the whole thing with loads and loads of water and wrapped it in cling film to cure. Curing can apparently take about a week with some cement however the instructions on mine didn't give any guidance about this. I left it wrapped up over night before unwrapping it to work on further.
Step 8: Sanding, More Skimming and Polishing
I started smoothing out the whole thing using a large file. This took the worst of the rough edges off. Next i used a little electric sander with loads of water and some course paper in order to begin to smooth the sides down a bit.
It soon became clear that I had done a bit of a crap job skimming it the previous day so I mixed up a very very thin mortar mix and re-skimmed each side using a filling knife.
After drying I recommenced sanding. I worked my way from course to reasonably fine until I had reached a reasonable finish. I didn't want a proper polished concrete finish so I settled for a nice smooth finish.
Step 9: More Curing...
I really wasn't sure about how long to leave this thing to properly cure. I had read lots of stories about peoples projects cracking after improper curing times but by this point I decided that a couple more days of water spraying would probably do the job. I gave the whole thing a final wash (in the bath) after about 3 days.
Step 10: Base Construction
Whilst the pot was busy curing I noticed that unfortunately our carpet is almost the exact same colour as the finish of the concrete pot. In order to separate it slightly from the floor I decided to add a thin MDF base.
I quickly traced the shape onto some MDF and sprayed the resulting shape black.
Image 3 shows how this helped to break it up from the carpet a bit. It also shows the plastic bag that I used to line the pot before it was waterproofed.
Step 11: Waterproofing the Interior
I left the pot to finish curing/drying out for a few weeks before waterproofing the inside. I wanted to make sure that the concrete had properly finished doing all of its strange chemical reaction stuff before applying more chemicals.
I used a selection of products to waterproof the interior of the pot. I really didn't want water soaking into the concrete because I knew that it would only be a matter of time before the nails inside rusted and expanded... leading the concrete to crack.
I filled most of the large holes with some interior sealant and then coated the whole inside with several coats of good quality damp sealant. After this I also put two coats of white gloss paint on the inside.
Step 12: The Finished Article
Here is the finished behemoth of a plant pot. Whilst the strange double layer appearance is a little odd I have decided that I don't mind it that much. Perhaps if I make another I will use this effect to my advantage somehow.
All in all the whole project went reasonably well considering it was my first ever project using concrete. It weighs in at an impressive 18.5KG so it would probably make a very good exterior plant pot, nobody would be running off with it in a hurry.
If I were to try this again I would replace the coloured gravel with coloured plastic beads. Unfortunately the gravel was only coated in a blue layer which meant that during the sanding process much of the colour colour speckles on the edge of the pot were lost. Plastic would san away nicely and remain visible.