What are some aluminum compounds that are yellow in color?

Question

My science fair experiment involved non-ferrous (not containing iron) metals and their reactions with concrete. The aluminum created a yellow gunky substance. Does anybody know what this is? Even if you aren't sure, could you just give me a list of aluminum compounds? I could figure out the answer from there.

Answer 1

To begin, we need to know what is in concrete. Concrete is roughly made up of 50% tricalcium silicate, 25% dicalcium silicate, 10% tricalcium aluminate, 10% tetracalcium aluminoferrite, and 5% gypsum. The chemical equation for aluminum added to concrete looks like this:

Al + O2 + Ca3SiO5 + Ca2SiO4 + Ca3Al2O6 + Ca4Al2Fe2O10 + CaSO4 ===>> AlO2 + Ca3SiO5 + Ca2SiO4 + Ca3Al2O6 + Ca4Al2Fe2O10 + CaSO4

As you can see the concrete does not become rearranged after the reaction. During the reaction, oxygen from either the concrete or the air bonded with the aluminum to make AlO2. This is called aluminate and it may have been the yellow gunky substance you saw. It most likely formed by the bonding of oxygen molecules from the concrete, making this reaction a single displacement reaction.

PS: The equation above was made very roughly and is still not balanced.

Answer 2

Great answer from Q&A, but all the possibilities he lists are white. I think the yellow colour may have come from traces of iron-containing materials. This happens very easily, because Al3+ and Fe3+ have very similar chemistry in some ways.

If you want an aluminum compound that is yellow even when pure, the yellow colour needs to come from the anion. For example, aluminum chromate (though I am quite sure that's not what you made).