How to use oxide to colour pre-mixed concrete?

[Bucko]

I need to build a small plinth to mount a memorial plaque, at a cemetery far from home. I don't want to carry all of the materials long distance, so was planning to make a box from form ply that I can quickly assemble on site. I was thinking of using quick set concrete so I could quickly dismantle the formwork before I left the site. Can I use oxide powder to colour the concrete, and how would I make sure I have a uniform colour given that I think I'll need 3 bags of pre-mixed concrete?

[MitchellMc]

Welcome to the Bunnings Workshop community @Bucko. It's sensational to have you join us, and many thanks for your question about mixing concrete.

Yes, you can absolutely use oxide powder to colour your concrete. Oxide pigments are commonly used in pre-mixed concrete to achieve a variety of colours, and they can be mixed into both wet and dry concrete. To achieve a uniform colour, it's crucial to add the same quantity of oxide to each bag of concrete you're mixing, especially if you're using multiple bags. A general rule of thumb is to add around 2-5% of the weight of the dry mix in oxide, but you’ll need to refer to the instructions on the oxide packaging for specific guidance. When mixing, it’s essential to blend the oxide powder thoroughly with the dry concrete before adding water. This ensures the pigment is evenly distributed throughout the mix, avoiding colour blotches.

Since you're mixing multiple bags, it’s a good idea to mix all your bags at once, if possible, or ensure that the oxide is evenly distributed in each batch. One way to help with consistency is to pre-mix the oxide powder with a portion of the dry concrete in a separate container before adding it to the rest of the mix. This helps break up clumps and ensures an even spread of colour throughout the entire batch. Remember that the mix will set very quickly, and that quickset concrete is typically used for post holes. The instructions require you to add water to the hole and then add your concrete to it. You generally don't have much time to mix the concrete in a separate container and pour it into a hole or a mould.

As for the setting process, it’s important to allow the concrete to cure fully before removing the formwork. Although quick-set concrete can set fully in a few hours, it’s best to leave the formwork in place overnight or for at least 24 hours, depending on weather conditions. If you remove the form too early, the concrete might not have gained enough strength to hold its shape properly. Once the concrete is set, you can then carefully remove the form and inspect the finished product. You’ll have a nicely coloured plinth ready for the memorial plaque.

Remember to consult with the cemetery operator to ensure you have the appropriate approvals for the work. Perhaps they can arrange for someone to remove your formwork the next day.

Please let me know if you have any questions.

Mitchell
 

[Bucko]

Thanks Mitchell, the cemetery operators are on board with what we are planning, but I will see if they would be OK to remove the formwork after the event. That would mean that I could use ordinary concrete rather than quick set and give more working time.

One thought - would it make sense to mix the oxide in the water before adding the water to the dry mix? Thinking that might make it easier to control the consistency of the colour.

Cheers,

Paul

[EricL]

Hello @Bucko 

One of the issues with mixing the oxide wet is you get inconsistency in the mix along with the possibility of spot colouring. This is where the oxide does not mix properly with the concrete, and you end up with a section that is slightly darker than the rest. It would be better to mix the oxide with the concrete while it's dry and to mix it as a single batch as Mitchell has advised.

In regards to the concrete type, yes you can use ordinary instead of the quick set. Just keep in mind that it will take longer to dry off and cure.

If you need further assistance, please let us know.

Eric