Friday, August 24, 2012

Project: Making Cement

I have always wanted to mix cement, pour it and see my hard work in a permanent state. Odd, I know.  But, it has been one of those things (like driving a speeding car up a moving truck ramp, that I always wanted to do.)  Mixing cement is far more sedate.
My friends and I are getting together tonight to do an art project and one of the things we needed was cement filled cups. I volunteered to make them as it gave me the chance to finally mix some cement.
First, I did some research and found that I really wanted to make concrete -- which is a composite of water, cement, (binder),  and aggregate (sand, gravel or other stone).
Home improvement and hardware stores sell cement mixes that can be used for making grout for tiles or cement for outdoor projects, flooring and countertops.  Cement powder is sold in large bags (94 pounds) and I only needed a small portion of a bag.  (Luckily, Jon had a bag of cement that was going unused, so he gave me all the cement I could use.)
I found that there is no formal recipe for mixing cement, just proportions - and they are also found on the bags of cement.
Simply put - 1 part Portland Cement + 2 parts sand + 3 parts gravel (mixed with water) = concrete  
For my project, which is NOT making a foundation or going to be outdoors, I didn't require the additional expense of gravel.  So, I used additional sand, instead of gravel.  This gave my finished concrete a smooth finish.
There are safety warnings with cement mixing.
Wear safety glasses and a mask to prevent the cement powder from getting in eyes or nose.
Do not let the powder touch your skin or it could burn.
I mixed my proportions of cement and sand into a bucket.  I quickly realized that I needed more room that one bucket allowed. I slowly poured the one bucket into a larger bucket and stirred to mix the two textures.  (In a lot of ways, this was like combining flour and baking soda in a standard recipe.)
I poured the mixture back and forth a couple times more.  When I felt it was combined, I slowly added water.
This part was a bit of a guessing game, as I couldn't find out how much water to add.  The instructions that I found suggested adding enough water to moisten and then add more until the cement mixture was the consistency of cake batter. So, I did this by feel... (I would guess that I used 1/2 part water, 1 part cement 3 parts sand as my formula.  I suspect that this formula would change if the temperature varied from 80 degrees and sunny.)
I took the wet mixture (like if you were whipping egg whites to the stiff peak stage) and stirred it for 2 minutes more.  (I read that doing this extra mixing activates the cement for curing.)
I then divided the wet cement among the 12 cups that I needed to fill for the project and set them aside to harden.
I checked the cement about 6 hours later and the cement hardened quite nicely.  Stay tuned for updates on the project!

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