dennisfrancisblewettiii - 21-2-2007 at 06:09
Sometimes various stains are noticable on concrete/cement floors in basements. I can understand things such as paint, but why does cement absorb
things such as oil, grease, or other types of compounds and solutions?
[Edited on 22-2-2007 by Genecks]
Levi - 21-2-2007 at 06:58
Concrete is a pretty complicated thing, chemically speaking. In general, "cement" is the term used to bind concrete and it is usually something like
lime. There are, however, different "recipes" for cement and some are quite complex, involving minerals with 5 or more elements ( Al, Ca, C, O, Na,
K, Mg, Ti, etc.). Aside from the cement binder, concrete is also composed of an aggregate like sand and small stones. "Sand" can be just about
anything depending on where in the world you obtain it. The cement and aggregate are mixed with water and the concrete is then allowed to cure.
During this time the water reacts with the lime (CaO) to produce slaked Lime (Ca(OH)<sub>2</sub>
The slaked lime then aligns itself within the mix and the result is the crystaline stucture we call "concrete."
Therefore it is quite hard to answer your question without a specific reference to the type of concrete and its mineral composition but in general,
concrete ends up being quite porous; like a big sponge. Unsealed concrete will absorb gases and moisture from the air and (unfortunately) it also
absorbs spilled liquids like oil. The oil is soaked up and gets stored inside the concrete sponge which makes it extremely
difficult to clean up.
not_important - 21-2-2007 at 07:05
Cement, after it sets and cures, is a network of calcium aluminium silicate and calcium crbonate crystals. It's rather porous stuff, even the newer
high density grades are very rough and open under the micrscope.
So any liquid you put onto cement can flow into those pores. Once there it may dry, or simple hold inself in place through capillary foorces.
dennisfrancisblewettiii - 22-2-2007 at 14:56
How come bleach doesn't clear out stains?
Would using a torch on the cement heat up the stains and making them evaporate, thus fade away or disappear?
[Edited on 22-2-2007 by Genecks]
12AX7 - 22-2-2007 at 15:00
If it's a bleachable stain, it'll clean as bleach clean any other stain.
Obviously, rust isn't going to come out as it is already oxidized. For that matter, iron oxide is a common cement pigment.
Torch + cement = explosion.
Tim
Mr. Wizard - 23-2-2007 at 09:13
Some stains are the result of insoluble metal carbonates and their breakdown products. When you use a soluble iron salt to add iron to your lawn, it
will stain your concrete, as will washing the corrosion off your automobile battery steel bolts. These stains are not removeable.
Oil stains , such as engine oil can be removed with cat box litter, a form of clay. Scrape off the bulk of the oil with a scraper, and then pile cat
litter on the oil spot and dampen it. Let it sit and dry for a day or so. You may have to repeat a time or two, but it works very well. I don't know
exactly why or how. It will remove enough oil from the concrete so that the concrete can be wetted. I have never been able to clean enough oil from
concrete with a solvent so it wouldn't bead up water placed on it.
halogen - 23-2-2007 at 10:04
How does heating concrete cause an explosion? Is it that there is water trapped in the structure that is vaporised?
Mr. Wizard - 23-2-2007 at 14:14
halogen, you've got it exactly right. The water vapor is trapped inside the concrete and is generated at a rate that exceeds it's escape. The chunks
of concrete can be as small as one or two inches (2.5-5 cm) when using an acetylene torch over a concrete slab, or they can be over a foot (30 cm)
across and sound like a land mine going off when a building burns down over green (a few weeks old ) concrete. Both are dangerous. 
Think of popcorn. When the pressure exceeds the strength of the container you get a BLEVE (Boiling Liquid Expanding Vapor Explosion)
[Edited on 23-2-2007 by Mr. Wizard]
Levi - 24-2-2007 at 01:41
I expect that the evaporation of water from the moist cat litter lifts the oil from the concrete and the now dry clay absorbs the oil.