The strength of mortar: an introduction to lime mortar

Around 1995, while in charge of all woodwork and finishes for a preservation project in Philly, the head of the masonry restoration team left and I was asked to oversee this challenge as well. This is where my interest in masonry conservation started. Since lime mortars, plasters and finishes have been the mainstay of buildings all over the world for thousands of years, it was important that I learn as much about them as possible. It wouldn’t be enough to just read book, I would actually need to learn how to work the materials and understand the underlying physical and chemical processes that mean either success or failure. I won’t limit this blog to discussions of lime mortars, but I think it is an important place to start with our first post.

lime putty in an Imer mixer

I’m not going to start at the beginning with lime 101. I can can come back to that topic many times and in different ways in future postings. The fun part of this blog for me is that I can take it anywhere I want each day as I work on different buildings. The idea is that each posting should come out of actual work on buildings during the week.

One of the hardest things for would-be restoration masons or experienced masons trying to move to preservation work using true lime mortars (not hydraulic lime) is to understand the management of water. I’m not going to spend a lot of time on the carbonate cycle but this graphic helps explain the phases. What I have repeatedly observed with all masons is the need to reinforce the importance of water management in lime’s cementitious phase as it transforms from the hydroxide to the carbonate state. Understanding it theoretically and implementing it in the field seems to challenge many people at least initially, or requires reinforcement if a mason is regularly switching back and forth to hydraulic mortars.

Graphic of the lime cycle

Lime mortars do not behave like hydraulic materials such as ordinary portland cement (OPC). With hydraulic limes and cements there is a chemical reaction when water is added that ensures a hard set. By contrast, lime mortars cure by absorbing carbon dioxide or CO2 from the atmosphere in a slow and even process that requires water content be managed.

(It should be noted that this is a closed cycle so that the same quantity of CO2 lost when limestone is calcined into an oxide is reabsorbed as the putty and sand cure back into manmade limestone. This is a benefit of using lime putty mortars for the environment that cements do not afford.)

If a lime putty mortar never lost any water, it would not cure or get hard. Stored in a sealed container with excess water, pre-made lime putty-and-sand mortar can be stored indefinitely.

On the other hand, if water loss occurs too quickly after installation, there will be no cementitious binding. In other words, the mortar will fail if not hydrated after installation.

So how does one determine the right amount of water? And how long do you need to maintain it? How do you achieve this in a porous material subject to swings in temperature and humidity, wind and sun, all of which directly affect the physical properties of water?

I decided to conduct a simple experiment to see if I could understand from a laboratory perspective the results of three typical conditions of mortar in the field.

  1. The first condition to replicate was pointing of a brick wall with a lime-sand mortar on a hot summer day. Working successfully in these conditions is not easy, but is can be done. Yet is also the area where we regularly see failures, mostly because the masonry substrate is not well hydrated beforehand.
  2. The second condition to mimic was the application of a lime-sand mortar as a stucco on well-hydrated masonry.
  3. The third condition is the use of a lime-sand mortar in constructing a brick wall, where the masonry is put under a compression load, rather than just laid into a mortar joint indifferently.

For all three tests I would use the same mortar made from a sand with an ideal bell curve (more on this in upcoming posts) and a high-calcium, high-surface area lime. I let the batch rest a few days and then remixed it (or “knocked it up”).

Lime putty-and-sand mortar should always be prepared in advance. I would strongly recommend it be prepared a month in advance of use, and then knocked up again before installation in something like an Imer mixer (lie the unit in the image above) that allows the mortar to be mixed dry without causing strain on the motor.

Mixing the mortar too wet because you only have a typical mortar mixer that mixes by lifting will probably still burn out the motor quickly — in as little as a half hour in my experience — and still cause lots of extra labor trying to dry the mortar before installation. Or if installed too wet, lead to lots of labor to address shrinkage. None of that is necessary if the correct type of mixer is used. Sunbelt Rentals carries these units in some parts of the US if you prefer to try before you buy.

Likewise, not mixing the mortar in advance will make a less workable mortar that is not creamy and easy to trowel if a clean sand is used. If however you are trying to match an historic mortar using the same local soil as the builders originally used, not mixing the mortar in advance means you may have completely unusable mortar until the lime has had the chance to stabilize any clays. It would not be much fun to show up on a site to work and find the mortar wants to fall out or peel off when applied as stucco/plaster because it wasn’t aged. And it is incredibly embarrassing in front of a client. (Yes, I’m speaking from experience.)

More about why you would want to mix using the same site-specific aggregates (washing soil in most cases) as the original builders is available in this post.

Okay, back to how we treated these three field simulations with the same mortar and got very different results…

I took one brick and heated it up to 95°F and covered one face with the mortar and set the brick on a shelf in the cool shop. Next I took another brick and soaked it in a bucket of water for a few minutes and then covered one face with the same mortar and set it on the shelf. Finally I soaked two bricks, covered one face with the same mortar, pushed them together and set them on the shelf and stacked six more brick on top to put the mortar in compression.

The bricks were allowed to cure in the shop for a week before being moved outside and left in the weather for a month. Importantly these mockups were done in the fall when rain was common and temperatures were cool but not freezing. There were several good soaking rains during that month.

This article at the website provides some data about why exposure to rainwater as opposed to water from a faucet accelerates curing.

I then took samples of mortar from each brick to send to the laboratory. Mortar from brick A was weak and crumbly. Mortar from brick B was firm and hard. Mortar from brick C was hard and dense. I asked my friend Richard Wolbers to sample each using a Scanning Electron Microscope (SEM) to see if there were visible differences. As he admitted later, the results surprised him.

Sample A SEM: Looking closely you will see the crystals have expanded to look like dandelion heads before the wind whisks them away. At the upper left, you see a powdery lime just sitting on the surface of the sand. As lime carbonates, it expands (reabsorbing CO2 with a related increase in mass of 44%). Without appropriate pressure (compaction) and exacerbated by the lack of hydration, it expanded into loose disconnected particles.

Sample B: Shows fewer puffballs and a more complete crystalline structure. In the upper left, the crystals are seen starting to flow over and around the sand. This is a good example of what happens with enough water, even without considerable compaction.

Sample C: No puffballs are present and the entirety exhibits an homogenous crystal structure. The upper left shows the sand particles full coated and fused together with the carbonated lime. This is what good workmanship achieves. This is how a repointing mortar or replica plaster can last a hundred years.

This simple test indicates the importance of water duration and pressure in developing durable bricklaying and pointing mortars installed with proper force, as well as plasters and stuccoes if they are well compacted with a wood float as the initial water is lost into the substrate.