Reactions between Aluminum and Lime

A. Hi, Joseph. Although the word 'aluminized' may have a more general meaning to some people, to many readers here it has the very specific meaning that the pipes are steel that has been hot dipped in molten aluminum to produce a sacrificial coating. So, assuming these are steel pipes, I don't think the attack of the concrete on their aluminum coating should be a significant problem for a non-critical application like a goal post. But it is frowned upon and certainly would be no good for a nuclear power plant :-)

If the pipes are made of aluminum, I wouldn't do it. Good luck.

Regards,

A. I know that when aluminum reacts with lye, the product is sodium aluminate, so that when calcium hydroxide reacts with aluminum, I'd expect calcium aluminate. Hydrogen is the by-product.

I plan to deliberately apply calcium hydroxide as limewash to the AAC house I want to build. Aluminum flashing, windows, etc. are RIGHT OUT...

A. Hi. Calcium is very active and certainly will not be produced as a free-standing metal from a reaction of this sort, so Mark's reply above sounds correct to me. Please see the Wikipedia articles on sodium aluminate & calcium aluminate to learn that aluminates are generated in many different forms; and the temperature and degree of hydration probably lead to very complicated reactions, but it can probably be written as approximately:
2Al + 2Ca(OH)2 + 6H2O (l) ----> 2CaAl(OH)5 + 3H2^ -- in other words, aluminates and hydroxides of various forms plus hydrogen gas.

Regards,

A. If you did already know, Al is an amphoteric (soluble/reactive in both Bases or in Acids) element, as is Zn.

*Bases do act like a "catalyst" in "Aqueous/moist environments" on the amphoterics like Aluminum.

** Hydroxide ions of bases, as a starter of reaction, do attack at the unstable passive oxide layers of Aluminum, and from the weakest oxide layer spot it starts dissolving in water through ion complex. At this pit hole, hydroxide ion of water attachs to the de-oxidized aluminum pit and forms a jelly-like, sticky, viscous insoluble Aluminum hydroxides/bohemite together with hexegonal calcium/sodium-katoite (depends on the base used)

***Here are reaction/molecular substitution phases together with their inter phases:

Ca + H2O >> CaO + H2 (CaO Unstable Reactive)(I)

CaO + 2H2O>> CaOOH(-) + H(+) (CaOOH Unstable Reactive) (II)

At this point reaction can go in 2 different ways:

a. Ca(2+) + OOH(+) + H(+) >> Ca(2+) + H-O-OH >> Ca(OH)2
b. CaO + OH(-) + H(+) >> CaO-H(+) + OH(-) >> Ca(OH)2

CaOOH(-) + H(+) + H2O >> CaOOH(-) + H3O(+) (III) >> Ca(OH)2 + H2O

Ca(OH)2 + Al(s) + 8H2O >> Ca3Al2((OH)4)3 + 6H2 (IV)
Note: Ca3Al2((OH)4))3 is called as KATOITE and is stable.
As you see here the Aluminum Hydroxide (Boehmite) diluted in water (Al(OH)4 ion) in basic conditions and then made hexagonal crystal complex with calcium ions which is Ca3Al2((OH)4)3: the Katoite

*Active Lime: Calcium Oxide (CaO)dust in non-moisture environment has no meaningful corrosive effect on aluminum. But under moisture conditions, even under atmospheric moisture, Active lime thermodynamically (exothermic reaction) dissolves in moisture (water) resulting with its base which is Calcium Hydroxide (Ca(OH)2) and this base reacts with passive outer oxide layer of Aluminum. As the Calcium Hydroxide opening/striping the oxide layers of Aluminum, Water attacks to the oxide layerless bold aluminum pits and forming jelly sticky white colored Aluminum Hydroxide precipitate.

Here we openly see Ca(OH)2 acts like a 'catalyst/catalysor' paving the way for H2O (moisture/water) to attach to stripped aluminum in Basic conditions.

**Normally this jelly-sticky precipitate of aluminum produced at the pit of oxide layer of aluminum acts like inhibiting the process that blocking contact/impact of both Ca(OH)2 and Water to the surface pits of aluminum. But this valid for the conditions that when Ca(OH)2 amount is too little.

As long as the basic terms of environment continue (that means big amounts of Ca(OH)2 and/or continuous addition of Ca(OH)2 to the system) insoluble aluminum Hydroxide precipitation will not settle down but will be dissolved in water ionically. Let us see how this happens:

Al(OH)3(aq) + Ca(OH)2(aq) >> Al(OH)4(-) + Ca(OH)(-) + H(+) + OH(-) (I)

Al(OH)4(-) + Ca(2+) + H2O + OH(-) >> CaAl(OH)4(+) + OH(-) + H2O (CALCIUM ALUMINATE ION)

let's get the balance:

it is 3 times Ca(2+) and 2 times (~twice)Al(OH)4(-) ion

3Ca(2+) + 3((Al(OH)4)): (6+) --- (6-)

>> Ca3.Al3(OH)12 >> Ca3(Al(OH)4)3 (unstable)

RESULT:

3Ca + 2Al(OH)4 + 8 H2O >> Ca3Al2((OH)4)3 + 6H2 >> Ca3Al2(OH)12 + 6H2

HERE 6 OH(-) IONS OF 3 MOLECULE CA(OH)2 HAS PROCESSED FOR OPENING PITS ON OXIDE SURFACE FOR WATER. IN ANOTHER TELLING THE 6 OH(-) ION WORKED FOR BREAKING ALUMINUM AND OXYGEN LINK IN ALUMINUM OXIDE ON THE SURFACE OF ALUMINUM.

WHEN OXIDE IS DILUTED IN OH(-) H(+) ION WEB (HERE H(+) COMES FROM WATER!) THE PIT IS OPEN FOR WATER TO ATTACK NAKED ALUMINUM AND FORMS AL(OH)3 THEN AL(OH)4 WHICH DISSOLVED IN WATER (AND SO IN CA(OH)2 AGAIN TO FORM CA3AL2((OH)4)3 CALLED AS KATOITE AND TOTAL 6 H2 GAS MOLECULE.

Hi cousin Kaplan. Thanks for the detailed and exacting answer!

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
August 2014

Hi Jagannadha. It seems you're asking a manufacturer of AAC blocks for detailed information on what you need to do to compete against them and take some of their business :-)

Here's your posting but I'm not expecting a lot of response -- maybe people will surprise me :-)

Good luck,

A. Hi. Lime will probably not hurt steel if that's what your pool is made of. If your pool framing is made of aluminum, it can't rust since only iron/steel can rust, and just having lime in the general area probably isn't a major issue. Good luck.

Regards,