Conserving Earthen Structures by Using Fluids in Motion
Edward Crocker

Speaking with a client about protecting the top of an adobe parapet, I used the word hydraulic in a context she did not understand. Not snapping to her confusion, I left her to reflect on my proposal to use a vapor-permeable hydraulic lime as an admixture in mud for a wall cap.

A few days later she called for clarification, having apparently lost patience in a diligent search for my meaning in dictionaries and on the Internet.

(In an effort to replicate her search, I found her frustration to be well founded. Webster's Third has an entry for hydraulic cement that simply says it can set under water. Google gave me lots and lots of hits, but not until I got to about the 30th did I find a reasonable explanation. Most simply provided information on the makeup of hydraulic limes and cements, assuming, obviously, that the reader already knows what they are.)

The word hydraulic is, I agree, a little confusing. When the term is invoked, we tend to think of brakes and cylinders and oil and high pressure. Again, the dictionary is of little help but does provide one elegant, if redundant, phrase stating that the word relates to fluids in motion. In the context I used with my client, this explanation is a bit spare and recondite, but is ultimately apt.

Hydraulic cements and limes are those that can set under water because the water used in the mixing becomes part of the crystal of concrete in the first instance, and calcium carbonate in the second.

In the simplest terms, hydraulic cement does not need to be exposed to the atmosphere in order to harden and achieve its strength because the embedded water does not need to leave; it joins the other substances to form a new molecular structure. This is fluid in motion, leaving its liquid state and becoming part of a very durable solid. The word "set" refers to this process, and in this context is the antonym of the word "dry" that refers to such nonhydraulic mixes as mud and most limes in which the water is liberated into the atmosphere.

Hydraulic cements were first used, as far as we know, by the Romans. That is why the Pantheon and the Coliseum are still standing. The Romans discovered that by adding volcanic ash to lime they had a much more durable mortar than otherwise. The ash came from the vicinity of the southern Italian seaport of Pozzuoli and is therefore called pozzolan. The resulting mortar or plaster is called pozzolanic, a term that has become generic and refers to any material that, though not cementitious in itself, contains constituents that will combine with lime at ordinary temperatures, in the presence of water, to form a stable, insoluble compound. The key word here is insoluble.

When I recommended to my confused client that she use hydraulic lime to cap her parapets, I was calling upon the insoluble properties of the substance to resist the onslaught of direct rain and melting snow. But, you might logically ask, why lime instead of Portland, an extremely durable and readily available hydraulic cement? The answer is vapor permeability.

The low permeability of Portland will allow moisture to gather just under the cap, causing the mud to spall and cove just beneath. The high permeability of lime, on the other hand, will allow the moisture to retreat into the atmosphere without causing the plaster, or the wall, to revert to a plastic state. This, perhaps, is the more salient aspect of fluid in motion apropos hydraulic lime.

Very useful, but who has ever seen a bag of hydraulic lime on the shelf at Home Depot? No one, guaranteed. But it is available and I will be happy to provide the sources to anyone interested.

You can also make your own using mason's lime, which is on the shelf. Fired bricks crushed into dust often contain the reactive aluminum silicates that are the key ingredients of pozzolan. So do the red volcanic cinders that can be bought locally and are used (unfortunately) for landscaping and driveways. You will need to experiment with the quantities of either substance when mixed with commercial lime. Results will depend in large part on the purity of the ground-up material. Vitrified glazes on the bricks as well as sand and other impurities in the cinders will affect the reaction. Nevertheless, I have used both successfully.

So, to my client go apologies for being presumptive and unclear. And to those learning about the properties of pozzolan for the first time, keep in mind the counterintuitive notion that fluids in motion may be key to the conservation of earthen structures.