Hearth slab a little to dry

Just finished placing hearth slab but must not of used quite enough h2o. So was wondering if its ok to fill in the voids with a concrete patch of some kind.

Welcome to the BrickWood forum!

It would be helpful to know more about the current condition of your hearth slab. Where are the voids and how big are they? Do you have concrete crumbling away along the outside, or stones from the mix coming loose?

If you could take a photo or two and post them here, that would help a lot. Much of our advice depends on where you’re seeing voids in the slab, and how severe they might be.

Thanks in advance; we’re here to help!

They seem to be solid not crumbling just not a smooth surface.

Thanks for the photos!

Overall I’d characterize that as “minor.” It appears from the detail photo that you may not have sealed the joints between the slabs with mortar. You will be applying mortar on the top surface when you lay the hearth frame bricks. Be sure you moisten the surface before you do that.

You’ll also want to seal up the two joints inside the insulation void with mortar before you mix and apply your insulation (Portland cement/Perlite mix). That particular mix is going to be relatively fluid, so apply a good skim of mortar and let it cure before you start the insulation mix.

The two voids at the joints? Honestly, I’d also fill those with mortar, especially if you plan to veneer the vertical surfaces as part of finishing the oven. Mix it like you did for the concrete blocks below (those joints look fine), moisten the concrete, and use your pointing trowel to make sure the mortar gets pushed all the way into those holes. Trowel the surfaces flush to the side and top.

To my eye the slab looks right exactly where it needs to be—on the bottom. You’ve got everything square and even, and that’s most of the battle.

Good luck, and I’d really be happy if you keep checking in as you progress with your build!

Thank you for your advice and I will keep you updated as I progress with the build.

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