Things to know about concrete/cement/mortar

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Things to know about concrete/cement/mortar

Postby wryan » Mon Nov 30, 2015 12:48 pm

It occurred to me after I had read some posts in the forum while I was working on my first background that those undertaking building a backdrop may lack some basic familiarity with working with products (concrete/cement/mortar) that use Portland cement as the binder.

Here's a few simple things to know that may help make your project a success:

Portland cement products have really great compressive strength when cured properly. That means they can support a tremendous amount of weight (the product is in compression)

Unfortunately, they aren't really too forgiving as far as tensile (pulling apart) strength or flexing (bending) goes ...

As a practical matter what this means this that a backdrop made out of pieces of foam siliconed together needs to be treated fairly gingerly, to avoid flexing it ... even after it is cured ... otherwise it will crack.

I don't have a lot of experience with acrylic fortifiers, so I'm not really sure how acrylic effects the above, but if I had to guess I'd say: not all that much ...

Choose your foam wisely:

Extruded Polystrene Foam is better than Expanded Polystrene Foam for this reason ... because it tends to be denser and have less flex.

There are several reasons that Portland cement products will fail:

1. Too much water vs. cement ratio. Excessive water makes the mixture weaker.

2. Too fast a cure time. This occurs when the product is allowed to become too dry, too quickly ... and the curing process is cut short. Hence it's better to allow the product to cure at lower temps (but not below freezing) and higher relative humidity.

Keep the above in mind as you go to apply those second and third coats ... might be wise to mist those previous coats with a little water in a spray bottle (or use a brush) ... so that the (now largely dry) first (or second) coat doesn't suck all the moisture out of what you are now applying.

All products using Portland cement will continue to cure and gain strength indefinitely ... as long as some moisture is present to facilitate the curing process. It's true tho' that the law of diminishing returns applies (most of the strength is gained in the first few days/weeks)
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