What about RapidSet Mortar Mix?

So you may be wondering, if you’ve been following the concrete garden box series over the years, what about the RapidSet Mortar mix that I was using in the wooden thin panel molds?

man in workshop in front of concrete molds — The high-strength pea gravel concrete will work in my existing thin wooden molds

If you’re new to my channel I’ll get you up to date, I made a series of thin panel wooden molds and poured a fast-setting pre-blended mortar mix that’s available from that orange big box building supply store.

And, I’ve been getting great results in those molds and in the Concrete and Red Cedar Bench and Potting Bench Projects.
Now the RadidSet Mortar mix, as the name implies, sets very quickly. And in doing so, it generates a lot of heat. With the wooden molds, the wood absorbs that heat from the concrete and the end result is a very good casting.

Heat Buildup and Cracking Issues

With my plastic molds, the heat really has no where to go so the concrete gets too hot. And when its curing in those first hours the heat causes the concrete to expand and form some cracks on the surface. This happens even as I’m spraying it with water as the instructions dictate.

concrete casting and wooden mold on bench — Surface cracks are easy to see in the mortar mix cast in the plastic molds

I have tried this a number of times and get the same surface cracks each time. And it’s very noticeable hot as it’s hardening and beginning to cure.

surface cracks on concrete casting — There’s surface cracks in the concrete due to heat and expansion while hardening

I’ve unmolded these less-than-perfect panels and they are intact. The cracks appear to be only on the surface and I’ll still use them in my own garden to make some beds.

concrete casting and wooden table on workbench — The RapidSet Mortar mix panels are intact and the cracks are only on the surface

The pea gravel and portland mix I showed in this video is a great alternative to that RapidSet Mortar Mix. It’s gives you a strong and smooth thin panel. And, those ingredients are easier to source for most folks. And it works out to be cheaper per panel as well.

And if you have some of the thin panel wooden molds then the pea gravel mix in this video will work in those as well.
Finally, in some areas you can buy bagged pre-blended concrete countertop mix. And this mix has a plasticizer already added. But, I’ve found it to be even more expensive than the RadidSet Mortar mix. So, I haven’t tried that option but, it should work.

So thanks for reading this blog post!

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