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I mixed a wheelbarrow load of concrete (sand, cement, water) this morning, but before I could use most of it, I got called away and didn't get back until this evening, like 7-8 hours later. By now the concrete is half set - very stiff and dry and has to be almost scraped out so it can't really be used.

But I find if I add a little water and mix, it's back to the same consistency as when I first mixed it this morning. This saves a bit of work and cement / sand.

My question is, is it any good at this stage (being used for laying patio slabs) or I should I just dump it and mix a new batch?

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    In future, don't lay your patio slabs on wet concrete. Mix up a dry mix of sand and cement, and lay the slabs on this. Ground moisture will set the dry mix soon enough. – Martin Bonner supports Monica Oct 7 at 12:49
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Depends on what it will be used for as the reaction has already started. If it is for fence posts it will be fine, if it is for a smooth drive then not.

  • The OP has specified the application - patio slabs. Given you can lay patio slabs on plain gravel, I'd say it would be fine. – Martin Bonner supports Monica Oct 7 at 12:47
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One thing to consider is that concrete doesn't really "dry" like mud. The water is part of a chemical reaction that hardens the mixture over time. Once the concrete starts to get thick and lumpy, that chemical reaction is well under way.

Adding more water and mixing will possibly make it smooth again, but all those bonds that have started to form will be broken apart and will not form again. Of course many new bonds will form and the concrete will get hard again, but once it is over-watered it will never be as strong as it could have been.

Does that matter? Well if you really need the strength of the concrete it matters a lot. When pouring structural concrete there are well defined limits of how much water can be added and how long the mix can sit in a truck. After that point, the whole load is ruined and won't be strong enough.

For a back-yard job of setting pavers, it's probably fine. Just thought I'd give you some background of what was really going on.

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In a variety of materials, you have two different things happening on two separate tracks.

  • Drying, reducer (e.g. Water) evaporating, soaking into the substrate or being consumed by curing, and
  • Curing, the chemical reaction turning it from materials X and Y, into material Z.

Wetness of concrete does need to be carefully managed during the curing process, just look at Blancolirio's reportage of the Oroville dam reconstruction and all the backflips they had to do to get reliable cure in the high heat and low humidity.

The problem is you reset the drying process, but the concrete is already well on its way through the curing process, and now there's a mismatch. Concrete is sensitive to that. Expect poor binding and the stuff to not act more like packed earth than concrete. In the future, don't ever expect that to work!

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