Hi Im needing some advice what would be causing a rusty looking stain on 4 month old unsealed exposed aggregate. It has only coming through on the back deck and sides of property nothing on driveway and front half, there’s obviously a couple different batches.
Hi Paul (@Adao1978)
Welcome to the Bunnings Workshop community. We're really pleased to have you join us and trust that our ever-helpful members will be able to assist you with this problem and many other projects in the future.
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Your stains look suspiciously like tannin which has leeched out of leaves. They could be rust if there was an iron-rich mineral in the aggregate, but I feel the point of origin of the rust would be more specific.
I'd start but using a stiff-bristled scrubbing brush and some hot soapy water and see if you can scrub away any of the stains. If this is not effective then you can try bleach, chlorine or potentially the Hydrochloric acid mentioned in another discussion. It could also be worth trying CLR 1L Calcium Lime Rust Remover which will remove a portion of the stains if they are due to rust. Please ensure you wear adequate protective equipment including goggles, gloves, a respirator and long sleeve pants and shirt when handling corrosive liquids.
If the stains were from leaves or another source outside of the aggregate then once removed and sealed it will be a permanent fix. If the stains are from a contaminant in the aggregate then once removed and sealed they might return.
Please let me know if you need further assistance or had any questions.
Thank you Mitchell I’m leaning towards a contaminant in the aggregate as it’s only four months old and no leaves have been sitting on the slab
If it is a contaminate then it is most likely some iron-rich particle in the mix. I'd start with the CLR product to try and remove the stains and then possibly a Hydrochloric acid on the particle if you can find it. You might be able to dissolve the particle with the acid. Another option might be to try a rust converter product.