Using Hydraulic and Hydrated Lime Together

Window Jamb at Corner

During the past five years the use of lime plaster has increased many-fold in the natural building world because it is readily available, can be relatively inexpensive, is easy and forgiving to apply.  If you are doing research on what type of lime plaster to use you are probably aware of the different types of lime.  More specifically, the two main types of lime available for plastering are hydrated lime and hydraulic lime.  The differences are significant and chances are you won’t confuse one for the other when making a purchase.  What you usually are not told is that you can use them together in the same plaster.

Hydrated lime is a common type of lime that is available in large bags at most hardware stores and even farm and ranch supply outlets because it is used as a soil amendment (although the farm type of lime is not suitable for building). The reason for the term “hydrated” is because calcium oxide (CaO) has been hydrated with just enough water to turn the CaO into calcium hydroxide Ca(OH)2, which is the powder you are buying.  Once the lime is applied to the wall it is able to absorb carbon dioxide from the atmosphere to slowly transform into calcium carbonate (CaCO3), or limestone – the original material mined from the earth.  Hydrated lime has what we would call a medium set.  It does not set hard like concrete in a short period of time, but it sets harder than earthen plaster within a week or so.  It is a bit more durable than earthen plaster because there is a definite chemical reaction taking place making the plaster cure harder as time passes.  This is the type of lime you use for slaking and making putty.

Hydraulic lime on the other hand is more like Portland Cement.  It’s source is also limestone, but the types of impurities within the limestone formation make hydraulic lime act differently at the molecular level.  Hydraulic lime is fired at a lower temperature than hydrated lime and it has what we would call a fast set, just like cement.  It does not depend on absorption from the atmosphere to become hard in the short term.  It can set under water and in the absence if air.  This is not the type of lime you make putty with because it will begin to set upon the addition of water.  If there are hydraulic lime quarries in the U.S. they are not selling their product on the open market.  There are a number of hydraulic lime quarries in Europe all of with slightly varying levels of impurities and they all market their products as NHL (Natural Hydraulic Lime).  There is one main supplier of hydraulic lime in the U.S. – Saint Astier Natural Hydraulic Lime, which comes from France.  Due to it’s scarcity it is relatively expensive.  Because of the high level of quality control throughout the extraction and processing of hydraulic lime it is very uniform and of high quality.

We have plastered a number of buildings using mainly hydraulic lime.  Our clients were convinced that they wanted to use hydraulic lime because of it’s warranty, it’s uniformity and high quality.  The cost of a bag of hydraulic lime can be as much as four times the cost of a bag of hydrated lime.

Our first project using hydraulic lime was plagued with a quick set and we had to work fast.  We were not prepared for a product that gave us so little time to finish.  The designation of the material was NHL 3.5 meaning it is moderately hydraulic.  The finish was rough, but appropriate for the application, and the clients liked it.  On our next project we worked with a life-long plasterer who came from a long line of plasterers.  He taught us that mixing hydrated lime with hydraulic lime slowed the set down and made the product more workable.  It also decreased the overall cost of the lime material since each bag of hydraulic lime costing nearly $40/bag was mixed with the same volume of hydrated lime (less than a standard bag, which is roughly $10).

Lime plaster with fresco finish

Most of the time we would mix the two products at a ratio of 1:1, but also would vary the ratio down to 0.5 hydrated to 1 hydraulic for a quicker set.  The finished product is still incredibly beautiful.  The picture to the left is of a project using a 1:1 ratio with a fresco style finish over the third coat of lime plaster.  Aside from the workability and setting issues, mixing hydraulic and hydrated lime can save your clients money.  The results can still be as spectacular as if you are using hydraulic lime alone.

The suppliers of hydraulic lime will tell you that it is not correct procedure to mix the two types of lime, and they will make sure you understand that they will not warranty their product if you mix it with something else.  In my opinion this is very fair of them to say, but they are also biased and want to sell you more product.  The truth is that the hydraulic lime product is of so much higher quality than hydrated lime you don’t need to worry about their product being defective.  They go to great lengths to make a high quality, premium product.

The other way to think of it is, you are adding hydraulic lime, or a quick-set pozzolanic material, to hydrated lime.  One other obvious option is to add cement to hydrated lime to quicken the set and increase hardness.  People have been plastering for ever with hydrated lime.  Using an additive to quicken the set is not unusual.  Of great importance is that you are able to work the plaster just as much as you must before it sets.  Commercial plasterers prefer a product that sets within a day or two so they can finish their work and move on to the next project.  Whatever you choose to add to your lime will affect the permeability of the finished plaster so do your homework before throwing things together.

If you are the type of person that likes a challenge, and are willing to do some tests prior to using the experimental materials, adding pozzolans (Perlite, Rice hull ash, Volcanic ash, Brick dust) to hydrated lime is a way to create “poor-mans” artificial hydraulic lime (AHL).  Real AHL is made by mixing hydrated lime with special clays and then heating it to create a truly reactive quicklime with hydraulic properties.  Whatever your taste, there is a way to create hydraulic lime yourself.

A brief summary of the history of hydraulic mortar can be found here.  Good luck with your hydraulic lime experience and let us know how your experiments turn out.

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