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.

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 with true lime mortars is to understand the management of water in plastering and pointing. Assuming they understand the basic concept of the carbonate cycle and the need for the management of water in the cementitious phase as it transforms the lime from the hydroxide to the carbonate state, then the problem is how to achieve that balance.

Unlike hydraulic materials such as ordinary portland cement (OPC) that have a chemical reaction when added to water that ensures a hard set, lime mortars cure by reabsorbing the CO2 from the atmosphere in a slow and even process. As long as the mortar remains wet, a lime-sand mortar will never set. In fact, stored in a sealed container with excess water, pre-made lime-sand mortar can be stored indefinitely.

On the other hand, if water loss occurs too fast after installation, there will be no cementitious binding. What’s 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.

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.

The second condition to mimic was the application of a lime-sand mortar as a stucco on well-hydrated masonry.

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.

I began by making a batch of mortar with a typical concrete sand using a high-calcium, high-surface area lime. (We will cover the importance of these two criteria in future posts.) I let the batch rest a few days and then remixed it. (More on the importance of pre-mixed mortar in a future post too!) Then 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. I let them sit for a week in the shop and then moved them outside and left them in the weather for a month. (I did this test in the fall when temperatures were cool but not freezing.) There were several good soaking rains during that month. (We can discuss the affect of rainwater on curing and curing techniques in general in a future post as well.)

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. He enthusiastically agreed and added that he had some new modeling software that he could use to graphically demonstrate what we would see. As he said later, the results even surprised him.





Click on each to enlarge




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 44% of its mass as CO2 is drawn in). Without countervailing force of pressure and time, it can expand 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 over enough time.

Sample C: No puffballs and a homogenous crystal structure. The upper left shows the sand particles full coated and fused together with the carbonated lime.

This simple test indicates the importance of water duration and pressure in developing durable mortars and plasters/stuccoes. We will return to this subject later when we discuss field application methods.

The false color plots show the increasing homogeneity and fusing of the mortar (blue):


Under compression and with adequate water and time, mortar crystals fuse into a tough new structure in the voids


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