Concrete mix advice

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Hi,

I am making a flat slab (like a paving stone) to sit on top of a BBQ chimney.

It will sit on 4 half bricks, 1 at each corner. Therefore the slab needs to be quite strong. I will put rebar inside, but I am a little confused about the mix.

Usually, I mix 1:4 cement and ballast. But sometimes the stones are quite large and visible in the finished slab. If I mix 1:4 cement and sharp sand - would the slab be weaker?

I don't understand why ballast (bigger stones) would make it stronger than sharp sand.

Advice appreciated
 
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Yeah it’d be weaker
You could float the surface as it’s setting to lose the stones, not that I reckon you’d get loads of knobbly stones visible anyway
Or look at “grano dust”
 
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Hi,


I don't understand why ballast (bigger stones) would make it stronger than sharp sand.

Advice appreciated
The smaller the aggregate the more binder(cement or lime) you need to cover each particle. With concrete the smaller sand grains fill in the voids.
 

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