#11  
Old 04-26-2010, 09:01 PM
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Default 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
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  #12  
Old 04-26-2010, 09:23 PM
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Default Re: Perilite concrete not setting up--normal?

Quote:
Originally Posted by BarryKeith View Post
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.
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Last edited by Raffy; 04-26-2010 at 10:50 PM.
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  #13  
Old 04-27-2010, 02:44 PM
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Default 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.

Last edited by Neil2; 04-27-2010 at 02:46 PM.
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  #14  
Old 04-27-2010, 03:34 PM
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Default 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.
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  #15  
Old 04-28-2010, 11:14 AM
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Default 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!
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  #16  
Old 04-28-2010, 12:39 PM
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Default Re: Perilite concrete not setting up--normal?

I agree with the coments from Elizabeth and Neil. I am concerned about the insulating properties of the layer you have created. If I were you I would either scrape it off and start again, or spring for some insulating board. This is an area that is pretty much impossible to correct after construction, so better to err on the side of caution...

Drake
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  #17  
Old 04-28-2010, 12:50 PM
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Default Re: Perilite concrete not setting up--normal?

Whether you used concrete or cement, the same amount of solids exist within the mix as long as the ratio is the same, and the same amount of perlite exists.
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  #18  
Old 04-28-2010, 01:59 PM
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Default Re: Perilite concrete not setting up--normal?

Quote:
Originally Posted by drogers View Post
Whether you used concrete or cement, the same amount of solids exist within the mix as long as the ratio is the same, and the same amount of perlite exists.
Although the ratio of perlite used is the same, 5 parts perlite / 1 part concrete contain large amounts of solids/aggregates which can negatively affect insulating properties compared to 5 perlite / 1 portland cement.
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  #19  
Old 04-29-2010, 09:25 AM
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Default Re: Perilite concrete not setting up--normal?

In regards to the 5/1 perlite cement, when mixed should it be the same consistency as if I was pouring a sidewalk or patio?
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  #20  
Old 04-29-2010, 09:41 AM
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Default Re: Perilite concrete not setting up--normal?

More like cottage cheese.
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