Refractory materials: Magnesium/Calcium Oxide/Zirconate/Silicate Cement Kilns.

Cement has been for several millenia, an important building material, and an important consumer of energy.

Cement is actually (generally) a chemical reaction that is allowed to run in two separate directs, the decomposition of calcium carbonate (limestone) into calcium oxide (lime) using high temperatures, the addition of water to give calcium hydroxide (slaked lime), and finally absorption of carbon dioxide from the air to give calcium carbonate generally in a suspension of sand.

If you have ever had occassion to add water to pure lime (not slaked lime) you realize that this reaction generates considerable heat, so much heat that done right, one can actually make water boil. I need tell no one that this reaction is just a demostration of the first law of thermodynamics, energy conservation.

Generally the decomposition to make lime reaction is accomplished at very high temperatures and so one should ask of what, in fact, is the kiln made? Keep in mind also that the reactor material must be corrosion resistant, as lime is a fairly powerful base, and is thus corrosive.

This topic is discussed in a relatively recent paper in the journal of the European Journal of the European Ceramic Society.

Here's the abstract: 27 (2007) 79–89 http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6TX0-4JGJJ3X-4&_user=10&_coverDate=12%2F31%2F2007&_alid=1033357811&_rdoc=1&_fmt=high&_orig=search&_cdi=5576&_sort=r&_docanchor=&view=c&_ct=1&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=14f1fb2f160e21a7a3f599a549a1afd3">Journal of the European Ceramic Society 27 (2007) 79–89

The paper has interesting four dimensional phase diagrams.



Some very high temperature phase work was undertaken, possibly in a diamond anvil type device, since high pressures were alluded to, although the exact experimental details are given by reference to another paper that I happen to not have in front of me right now.

Corrosion was checked in the following way:



There's some cool electron micrographs of the system.



My interest in this paper was not stimulated by a particular focus on cement chemistry, by the way, although the energy implications of the paper do give some insight to the external costs of building "all new stuff," including the wonderful solar houses people are always prattling on about. Sometimes it is very worthwhile to keep the "old stuff" and make it last as long a possible, but that doesn't mesh well with consumer mentalities.

What is the typical manufacture process of hardware-store purchased lime? I believe it's often referred to as type-s (though I could be wrong). Where in the cement chemistry transformation process does this product lie?

When including such a purchased lime in a portland-cement/sand mix, it increases the plasticity of the resulting product (makes it softer), but not only softer, also stickier. Particularly in stucco, this is commonly used in greater proportions to portland-cement/sand for outer layer topcoats such as the final stucco coat, and it is also used in mortars used between bricks.

Anyway, the reason I ask is that often it seems there is a disconnect between what is taught by chemistry-teacher types, versus common products typically purchased at the store and used everyday by folks in the trade. When attempting to understand these various differences, I've never been able to get a lucid understanding of the basic differences due to differing names used by various groups.

From what you've written above, and from what I've been able to gather, the hardware store 50-lb bags of lime have been heated to high temperatures?

However, slaked lime has not been so treated? Curiously, it's precursor doesn't seem available at common stores such a building material would typically be found, though judging by construction-history discussions I've read, it seems to have been commonly available and used in the past by artisans in the building trade, particularly prior to the invention of portland cement sometime in the 1800s (IIRC).

Now, the building trade seems more designed to sell pre mixed bags of particular mixes said to be for a specific purpose. Much of the "artisan" has been lost for some reason.

On edit, it seems that this information on lime type-S and -N is now easier to find.

Lime (CaO) is just burnt limestone (CaCO3), bagged up for retail sale. Besides using it as a mortar addition, you can mark off a football field or correct your soil pH. "Slaked" lime has water added, so that it is, for the most part calcium hydroxide (Ca(OH)2). If you want to "slake" lime yourself, you need to add (slowly) 1 part (by weight) of water for every 3 parts of lime. Lime pretty much slakes itself (reacts with water to become the hydroxide) when you add lime to a mixture containing water.

When adding lime or slaked lime to a mortar, it helps to adjust the calcium concentration in the final product. Concrete is a mixture of calcium alumino-silicates where lime provides the calcium, pozzolans provide the alumino, and sand provides the silicate. the ultimate strength depends on a large number of variables including amount of aggregate, aggregate size, amount of pozzolans, pozzolan particle size, entrained air, temperature, chloride concentration, etc.

The chemistry of cement and concrete is complicated by the fact that it is a heterogeneous mixture; water, some water soluble ions, and solid surfaces (like the sand particles) where the reaction takes place. That is why artisans are still needed and it is not an exact science.

As far as a low-energy building material, it seems adobe clay may be preferred to any of the lime or cement mixes due to the latter's need for the kiln energy input, though I guess adobe's strength is probably much lower.

I was apparently misunderstanding that lime (pure) wasn't heated in a kiln for building purposes. Apparently it is heat treated.

Driving off the water and then letting it rehydrate makes for MUCH stronger materials. Unfortunately, that does require LOTS of energy, making it not a very 'green' process. Consider sun-baked adobe (low strength and low energy inputs) vs. bricks -- same material, but lots more strength from the kiln firing process.

:P I know they're useful, but they really helped turn me off to PChem because it wasn't made clear to me just what was being plotted. Now that I know how to use 2D phase diagrams, I need to learn 4D ones??

Looking forward to hearing what adding a little CaHfO3 might do. :crazy:

The fourth axis is the apex of a tetrahedron. Thus each plane is a composition diagram for the other three components.

There is a whole journal - a magnificient journal - devoted to phase diagrams, CALPHAD.

http://www.elsevier.com/wps/find/journaldescription.cws_home/254/description#description

It is marvelous, once you get used to it. I discovered it about two years ago. I took a week's vacation and spent every damn day in the library looking at phase diagrams.

There was a time I hated that stuff as well, but I've been thinking an awful lot about phases these days, and I've learned how to solve some difficult problems.