Perilite concrete not setting up--normal?

[BarryKeith]

I have had a great time with this project. My boys are 16 and 17 and talked me into building a smoker and a grill on each side (in firebrick) and setting my pizza oven in the middle. What a great way to spend time with my boys. We went to set my insulating concrete layer and after 3 days it is still soft. Is this normal. We used 5:1 mixture of perilite to concrete and when a peices brake off it is wet on the underside. Do I need to let it set longer? It has been in the 60s to 70s as a high. Was I suppose to add anyting to the mixture? I can't see this being effective for the outer layer of insulation, but I already bought the perilite. ANy help or ideas?

[Neil2]

Re: Perilite concrete not setting up--normal?

"5:1 mixture of perilite to concrete "

Perlite to cement. Keep it damp and under cover for a few days and see what happens. It will always be a bit "crumbly" on the surface.

[BarryKeith]

Re: Perilite concrete not setting up--normal?

I used sakrete(concrete mix) from home depot, 5 parts Perlite to 1 part sakrete. Is this wrong? Should I have used something else?

[Raffy]

Re: Perilite concrete not setting up--normal?

I agree with Neil. Give it time to set but keep it hydrated for it to cure properly. Perlite on the surface has a tendency to chip off. It happened to me too. DOn't worry too much about it. The fireclay mixture will help smoothen the uneven surface of the perlite for you to lay the bricks on.

[BarryKeith]

Re: Perilite concrete not setting up--normal?

I have researched the difference between the concrete and cement. I feel like an idiot. Do you think that the perlite and concrete will set up and be useful or should I scrape it off and go again with perlite and cement?

[BarryKeith]

Re: Perilite concrete not setting up--normal?

Raffy,
Thanks, I was typing while you responded, Sorry. I feels almost wet to the touch and when I chip a peice off (which is not hard to do) it feels wet. How long should I wait?

[Tscarborough]

Re: Perilite concrete not setting up--normal?

The amount of water required to mix perlite and cement is very high in relation to what is required to actually hydrate the cement. I also noticed that it required a long time to dry. It will always be soft and crumbly, but the water will dry out.

[tfasz]

Re: Perilite concrete not setting up--normal?

Mine took a while to setup. I think I mixed it too dry, and every day I would spray about a liter of water over it. After about a week or two I could actually stand on it without a problem. I'd wait a bit more and see if it has strength - you just want to make sure it can hold up the weight of the oven.

Even if you do have to redo it, at least it is a really in-expensive mistake to make.

[BarryKeith]

Re: Perilite concrete not setting up--normal?

I need to have little patience. My boys are begging me to be able to go out in the dark and take the pick axe to it. I just want to make sure that it is going to work. I would hate to build the oven and have it not work.

[Tscarborough]

Re: Perilite concrete not setting up--normal?

The bottom line is that 3 or 4 days will put it at 75% ultimate strength. No need to wait to continue with the build. I placed the perlcrete on Saturday and wet-laid the floor the next day, soaked though the perlcrete was. I did leave the crappy wooden forms for the perlcrete on so I wouldn't destroy the edges, though.

[RTflorida]

Re: Perilite concrete not setting up--normal?

I think it will set fine. Any portland based product needs time. The general rule of thumb is to keep it damp and covered...for a week. This includes the support slab, hearth slab, or pouring a sidewalk. Some will argue waiting a week is excessive...why risk it? Tearing out any concrete or mortar may sound fun, but is hard work and leaves you with a huge mess to get rid of...if you have to re-do it.

The biggest issue in your case is the use of concrete instead of portland cement. You are going to be considerably shy of portland because of the sand and aggregate that is contained in the Sakrete. The sand and aggregate won't hurt anything, but the lack of portland may make your mix more crumbly (there may not be enough to hold all those materials together). Not sure what your 5:1 ratio is actually reduced to, maybe someone else can take a stab at it.
The good news - many forum members have had success with ratios of 8:1 to 10:1.
I would give it a few more days, see how crumbly it is and see if it supports a weight load.

RT

[Raffy]

Re: Perilite concrete not setting up--normal?

Raffy,
Thanks, I was typing while you responded, Sorry. I feels almost wet to the touch and when I chip a peice off (which is not hard to do) it feels wet. How long should I wait?

All us newbies to concrete and mortar have this urge to pick and scratch the newly laid concrete/mortar to see if it sets or has hardened. I know I was very anxious about it and in the end I gave in and I picked at the perlite. It chipped off and added to my anxiety. Patience is a virtue as in all things but even more so in oven building.

The strength of concrete is not derived from the drying process. There is a chemical reaction which I cannot fully describe to you because I'm not a concrete expert. But, from my rudimentary understanding, the curing process requires hydration over a period of time for the concrete's bond to strengthen. Having said that, it's okay if it is wet to the touch. Just give it some time to set. Humidity is also a factor which may explain why it is still wet.

Another example is my inner arch. I was very worried when I thought it was not setting properly causing me to fiddle with it and thus ruining an otherwise "okay" mortaring job. After cleaning up the mess and scraping off the old mortar, I tried again but this time left it alone to set properly. I kept it well hydrated (damp) and over time I could even stand on it. It was very sturdy to say the least and my patience paid off.

Sorry for the long reply but it's because I understand the anxiety and second guessing you are going through because it mirrors my feelings during my build.

P.S. I agree with Tom (tscarborough). You should leave the forms on for a while until it sets completely.

[Neil2]

Re: Perilite concrete not setting up--normal?

"5 parts Perlite to 1 part sakrete. "

You want to use normal portland cement (also called Type 10 in some areas). If you used 5:1 sakrete than you ended up with a very weak mix and probably not as well an insulator. I would take it out and start over.

[BarryKeith]

Re: Perilite concrete not setting up--normal?

Thank you for all the responses. I went outside last night and stood and then jumped on the slab and it seemed solid. Only on the corners is it really weak and crumbly, but even that is firmer. By the way I am 6'3" and weigh about 3 and a half bills (my boys lingo for 350 pounds). I am going to lay my floor and if I have problems I may remove around the floor, form it up again and pour concrete outside of it.

[egalecki]

Re: Perilite concrete not setting up--normal?

My concern would be with the insulating quality of the perlcrete since it has sand and aggregate mixed in. If you could get some insulating board to lay on top of it, and then lay your hearth, you'd be certain that it won't be a giant heat sink.

I have insulating board on top of my vermicrete- total overkill, but my slab stays cool the whole time.

If you are leaking heat through the sand and aggregate, it will take longer to heat your oven and it won't retain heat well. You can order insulating board from Forno Bravo, and you can find it online other places as well.

Be certain, not sorry!