We are going round in circles here so … time for a basic lesson in concrete to help you achieve your goal.
Cement- the grey powder in bags which is the magic ingredient you add to aggregate (along with water) to make concrete.
Aggregates—Are the sand and gravel you use.
Gravel—Stone added to the mix can be round, or angular and available in different sizes eg 6mm 10mm or 20mm.
Sand- Also added to the mix—IMPORTANT there are essentially 2 types of sand:
1. Soft sand, also known as yellow sand or building sand…..This is the stuff you use to make mortar for laying bricks—usually a ratio of about 5:1 Sand :cement- it is essentially a gap filler with little or no structural strength. If you have ever knocked down a brick wall you may have noticed you can pick up the bits of mortar from between the bricks and snap them with your hands.
2. Sharp or coarse sand--- The sand you use for making concrete—as its name suggests it has larger grains and hence is moving towards having some structural strength.
When you mix concrete the idea is that the sand fills the gaps between the gravel to make a solid mass.
(Reading between the lines from your last post, you have used your left over soft sand ? You really need sharp sand )
Water- Concrete does not dry, it cures…it is a chemical reaction. The cement you add to the mix reacts with the water. You should really add exactly the amount of water to react with the amount of cement you have added to get maximum strength… Otherwise the excess water which is left over will create voids in the concrete reducing its strength. (the excess water will sit in the mix until it dries. leaving a void) Usually if you only added this much water you would end up with a mix that is far too dry to work with…ie to tamp, or compact into complicated moulds. Hence practically we add enough water such that the mix is of the consistency or ‘workability’ that we want this will be too much water. (ok we have lost a bit of strength… but we are building flower pots not the Humber bridge !)
Plasticisers-- So this is where plasticisers fit in… They are an additive which you can put in your mix to improve workability—By adding some plasticiser workability is improved hence you don’t need to add as much water so strength is maintained. As tizala rightly points out you can go to the builders merchants and buy some plasticiser , usually in 5 litre containers—if your going to make a lot do this, otherwise fairy liquid will do. Add about twice what you would put in a washing up bowl to a bucket of water, stir the water and use this to make your concrete.
The ratio of aggregate to cement will determine strength--- If you work on 5 aggregate to 1 cement you will be about right. (your supplier told you 4:1 so not that different)
Now as a twist- render which you might face a wall with is made with say 4 sharp sand: 1 soft sand:1 cement…..The sharp sand provides strength whilst the soft sand gives a nice smooth finish.
So what mix do you need for your plant pots ? Well you need strength, so mortar is no good. You need to work it into a small mould---20mm aggregate probably wont fit (what is the wall thickness of your pot ?) I think you can buy 6mm chippings in bags from builders merchants. So how about 2.5 chippings:1.5 sharp sand:1 soft sand:1cement , ….this keeps your 5:1 aggregate:cement ratio, soft sand will help give a nice finish,chippings and sharp sand will give strength.
Add your water and plasticiser or fairy liquid as little as gives a nice workable mix to get into your mould.
You can buy concrete dies to effect colour as well as using different coloured sands.
Don’t forget to poke your concrete lots as you fill the mould, this compacts it and gets the air out---It is an essential part of the process.
Ive never actually made pots and havent seen what size the mould is you have, but i think this should be somewhere close. Hope the info helps.
Stu
Give a man a fish he eats for a day.
Teach a man to fish he eats for a life time.
Topic: Thus could all end very badly (Read 7441 times)
