Cast Refractor Failure?

Here is my first attempt at casting refractory cement. Product is called Meeco's Red Devil Castable Refractory. This is for the firebox roof in a Shorty core, which I split into two parts because one continuous slab would be over 60 lbs, which seemed unreasonable to cure and handle.
I made the form 2.5 inches tall and coated it in wax for form release.
I dry mixed the refractory, and then carefully added the appropriate amount of water and mixed for about 7 minutes (instructions said I should mix for between 3 and 6 minutes but I needed more time.)

From there I packed it tightly into the form. Then I TRIED to vibrate, and this is where things started to feel off. I had 2 massage guns and an oscillating tool on hand and tried them all but the cement did not visibly settle much at all and no water rose to the top as I'd seen in videos of this process. I figured I'd just packed it well enough. I sprayed the top with a squirt bottle and covered it in plastic.

48 hours later I released the form and I'm seeing dry cement around the edges that brushes right off. Also seeing some cavities.

I sprayed down the dry areas and covered it back up, hoping they absorb and firm up.

But... did I totally botch this attempt???

Well Matt;
I agree with you; those do not look like they will hold up in the long run (perhaps even in the short run!).

I used a variable-speed saws- all with no blade. My forms were sitting on plywood with a tire bolted to it.
It worked great.  https://permies.com/t/254174/Casting-Large-refractory-Slabs

I suggest pouring the box slab in one piece to reduce the risk of issues later.

thomas rubino wrote:
I used a variable-speed saws- all with no blade. My forms were sitting on plywood with a tire bolted to it.

I very much appreciated and studied that post before I made this attempt, but I suppose I was unclear on how you used the saw to vibrate the form.
So the form was sitting on a tire which allowed the whole thing to "jiggle" more than it ever could sitting on a solid surface (my first mistake.)

How did you use the saw though? Just pressed the front of the saw against the table and pulling the trigger engaged the piston inside enough to vibrate the table?

The plywood was bolted to the tire and the form sat on top.
Yes, no blade, rest against plywood, and apply throttle.
When you get the speed right you will know,  it will smooth out with no bouncing. The few bubbles there are will quickly rise around the edges.
I was warned, possibly by Fox, who has vast experience vibrating cast items, to be careful "Not" to over-vibrate!

thomas rubino wrote:
I was warned, possibly by Fox, who has vast experience vibrating cast items, to be careful "Not" to over-vibrate!

I took the warnings not to over vibrate or over water too far!

So now I'll put these slabs through a torture test just for fun. And probably end up re-casting (a $100 mistake.)

Or I could sandwich the slab in kiln shelves. I could have used kiln shelf from the get-go but that's more expensive and I wanted the thick cast refractory slab for mass since the top of the firebox will be the bottom of a white oven.

But on the bright side, sharing failures is what helps other succeed!  

Oof, that 100$ is the kinda lose that makes looking for alternatives so appealing.
I think you should use it as is, and then if it fails, put the pieces on top of the kiln shelf and stick them together /parge them with some cheaper, lower temperature refractory.

Well that is unfortunate for you, something obviously went wrong!
The top looks like it has been deprived of ‘water’ or ‘cement’ not something I have seen very often but it looks like the top layer dryed out to quickly however, without actually being able to witness how it was mixed and how it was vibrated, I can only guess at what happened.
The correct amount of water (normally around 4lt to 25 kg depending on manufacturer ingredients )  is really critical for two main reasons.
One is to cause the chemical reaction to cure the cement and the other is to ensure a strong end product that won’t carry too many voids.
So whatever the amount of water added, that volume of water will be replaced by air as the water evaporates forming microscopic voids through out the cured end product. That is why it is mixed so dry, if you double the amount of water you will double the amount of voids!
So the mixing of relatively small amounts of water must be very thorough to ensure every monocle  of cement dust is dampened.
Vibrating offers several benefits, it allows all the grains and cement in the mix to settle down tightly and in turn expel any excess water.
However if you over vibrate all the smaller grain and particles (dust) will fall to the bottom leaving a dry top and dense bottom!
With a proper vibrating table, with a shallow mold it will take about 10 seconds.
With DIY tools you just have to guess and keep your fingers crossed.
With a proper table, the mix is compressed very rapidly forcing the water to the surface, the water on the surface will then partially sink back down but also ensure the mix stays moist and does not dry out to quickly.
However too much water on the surface is a sure sign of, to wet a mix or over vibrating.
Your results look like a mix that has been left to dry in the sun and has dried out too quickly. I not saying you did that but that is what it would look like if somebody did do that.
Refractory cement is not an easy product to work with and the end results are very much based on correct mixing, vibrating and curing.
As it only offers about 20-30 minutes working time you need to be quick and make sure the mold is covered as air tight as possible as quick as possible. IE as soon as it is vibrated, it must be covered to keep the moisture in for the next 8 hours or so.
A big plastic bag would be ideal but anything that absorbs moisture like dry sacking or plywood will have the opposite effect !
The lower part of your cast may be ok, I would scape off the top and test it out.

An update: I opened the mold and sprayed the tops and edges with water. Then re-covered in plastic again for another 24 hrs. The top edges that got squirted from both exposed top and sides seemed to have firmed up a bit. The bottom edges were still awful soft (dry/powdery) so when I took a wire brush to all the edges, the bottom edges gave up more and exposed some aggregate. A metal scraper knocked off any loose aggregate.

No idea why those bottom edges were so dry. The soy wax I used was plenty thick and souldn't have robbed moisture. But the bottom face is nice and smooth at least.

Now they're suspended over a dehumidifier to begin the drying process. I think I'll keep playing with these and not count them as a 100% loss just yet. I will do the gradual heating ramp up curing and see how they look and feel after all that.