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Old 05-21-2012, 10:52 AM   #1
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2 years ago my wife bought some molds to make small concrete animals. She poured them and they came out great. She got ill and had to quit. She has since recovered and has been trying it again. They just crumble. She is using the same mix as before. She started with cement left over from 2 years ago, when that didn't work, we bought new. Any idea?

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Old 05-21-2012, 11:11 AM   #2
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What kind of concrete were you using before?

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Old 05-21-2012, 06:34 PM   #3
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Was the mix you bought really new or just new to you?
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Old 05-21-2012, 09:38 PM   #4
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The portland cement had just come in and was still wrapped in plastic on a pallet.
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Old 05-21-2012, 09:47 PM   #5
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rusty -

How long did the concrete cure before trying to strip from the mold and what was the temperature?

Could there have been any salt or sugar in the sand?

What kind of molds (rubber plastic, wood, etc,)?

Did you spray anything into or onto the mold surfaces before filling?

High early cement (TYPE III) or reasonable heat and moisture during curing will give you higher early strengths and make stripping easier if you get anxious to make more quickly.

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Old 05-21-2012, 10:33 PM   #6
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The sand is play sand, sold for use in sandboxes.

Latex molds.

Release agent is 2 parts castor, 15 parts rubbing alcohol. (As per mold maker)

Mortar is 3 parts perlite, 2 sand, 1 cement. (again as per mold maker)


This all worked fine 2 years ago. It has been a little cooler so she increased set up time from 24 to 48 hours. Temp is 80's high to 60's lows.
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Old 05-22-2012, 09:16 AM   #7
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Perlite is like vermiculite (but not as bad) can break down with over-mixing, causing a larger surface area and a greater need for cement. There could also be salt contamination since often a bagging plant will also produce sand for ice removal/traction.

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Old 05-22-2012, 04:00 PM   #8
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Thanks, I'll tell her.
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Old 05-22-2012, 04:35 PM   #9
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Quote:
Originally Posted by concretemasonry View Post
Perlite is like vermiculite (but not as bad) can break down with over-mixing, causing a larger surface area and a greater need for cement. There could also be salt contamination since often a bagging plant will also produce sand for ice removal/traction.

Dick
Used to get sand that was supposed to have been but was not from seaside environments in the landscape industry. If we did not catch it, the salt threatened newly planted plants.

Make sure you are buying washed sand.
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Old 05-22-2012, 05:27 PM   #10
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I make concrete statues. I have used dozens of mixes and products. what I have found works well is to use quikcrete non-shrink grout. Big yellow bag, use a 3/8" drill and paddle mixer. I also use the occasional sand topping mix with an acrylic admix, quikcrete or acryl-60 from Thorobond. Sometimes I use acryl-60 in the grout too.

It is fun to make your own mix, but buying bag mix is much, much easier and more consistent.

I have found the grout to be about the best, it is designed for precast concrete, captures detail well and holds up over time.

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