How does cement work?

How does cement REALLY work?
I know it involves a complex chemical reaction in which water plays an
important role.
However, it seems to be a common misconception that setting cement is a
lot like baking a cake or making paper mache, with the assumption that
water is an unwanted ingredient that has to be dried out.
So I've seen all kinds of wacky bahaviour among DIYers who want to
"accelerate" the "cement drying" process, such as using heaters or
fans, or pouring cement on really hot days. Perhaps someone could
clarify, or point me to an authoritative explanation?
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Everything you always wanted to know at the portland cement association:
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Scroll on the left to "cement basics" and knock your lights out. ______________________________ Keep the whole world singing . . . . DanG (remove the sevens)
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I agree; there is much confusion out there.
First of all, "cement" is the glue that holds everthing together. "Concrete" is the product which is composed of cement, water and aggregate (small and large) plus admixtures. .
Concrete needs water or moisture to gain strength over a specified period of time, normally 28 days. This process is called "curing". Then once it reaches its specified strength it can be dried out.
There are various ways to cure concrete: keeping it wet, wetting it and covering with visqueen, curing compound, leaving formwork on, etc.
Even though curing may take place over 28 days, you only have to keep it moist for part of that period. ACI says 5 days at 70 degrees F, or 7dats at 50 degrees F.
There are special precautions that need to be taken during hot weather, cold weather or hot and windy days. Protect the concrete from freezing. Bleed water should be removed with a hose rather than worked back into the concrete. Do not use calcium choloride in structural slabs or concrete with reinforcing.
I hope this helps!
Reply to
Jim K.
Sub-freezing temperture lend to poor curing of concrete, mortar, and similar based mixtures. Just a fact of life.
If the air temperature is too hot and not appreciably humid, the surface will dry too quickly for effecting a proper surface with a trowel of any kind.
"Cement" is a generic term. Pardon me if I did not interpret its meaning correctly. I would be interested in exactly what you mean by "cement".
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The only reason that you would use a heater is in the winter to prevent it from freezing until it has time to cure properly. You are correct that the water is required for the chemical reaction. ...but you don't need much water - excess water makes the concrete easier to work with because it is more fluid, but all of that water will eventually evaporate, which will leave the concrete porous and weaker. The trick is to use minimal water (within reason) when the concrete is mixed, and then keep that minimal water from evaporating by either sealing it, spraying with additional water after it sets, or better yet, daming it up and flooding with water to allow it to cure under water. The longer the better. A few days is good, a week to 10 days is a good compromise, a month is great. longer than a month and you've probably gone beyond the point of diminishing returns.
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