Matching mortars by recognizing the fractal nature of geology

To finish up this series of articles about mortar, I wanted to put the awesomeness of fractal geology into perspective, while putting it in the context of how to go about making matching mortars. The answer is often right beneath your feet!

Blue Ridge Mountains
Blue Ridge Mountains

In the previous post we talked about finding the necessary aggregate on site, recognizing it may have to be washed some first (but not to completely clay free as with modern commercial sands since then it will not match the earlier mortars.) While working on our mortar match, let’s try to understand more about the geology of this area as well as explore the fractal nature of geological deposits.

For this project, we were digging in the foothills of the Blue Ridge mountains. They are part of the Appalachian mountain chain which stretches from Newfoundland to Alabama. These are one of the oldest mountain ranges on earth. At one time, these were as high as the Rockies or the Himalayas are now, but they have been weathered down over hundreds of millions of years. This Piedmont province is primarily underlain with granitic rock, quartzite, phyllite, and acrostic sandstone which for me is an interesting and unfamiliar geology for building materials, very different that what I encounter along the coastal plains.

granite outcroppings

Driving through the area one gets glimpses of the underlying stone at roadside cuts. Yet few in the area may recognize the sand and rocks that populate their backyards are this same stone after its been worn down.

Once a mortar has been digested, you don’t need to be able to identify or name the minerals, but you can begin recognizing the same aggregate under your feet. You are standing on the very thing you’re looking for!

Aggregate from sand in Little Washington VA 2015 John Greenwalt Lee

As an aside, when we began rinsing the aggregate from this site, we realized we had seen these same minerals in DC. But there they were more rounded from tumbling river action as they passed over the falls, with more soluble components dissolving, before being deposited where the Potomac River fans out into the marshes on which the nation’s capital city is built.

Acrostic sandstone – which comes from the breakdown of granitic rocks – is found throughout. Typically it contains loosely clay-bound stream bed depositions of quartz, feldspar, potash and granitic rock fragments that can be worn down into the components by crushing or river action and dissolution.

Shoveling soil through screen to collect only what passes 1/4-inch. Little Washington VA 2015 John Greenwalt Lee

I pushed the soil through a 1/4 inch screen to remove loose clay particles.

First sieving of local sand is beginning to reveal the minerals. 2015 John Greenwalt Lee

After initial sieving, but before crushing, the aggregate is starting to become more visible, but the clay clings to everything, causing there to be large clumps of aggregate. I crushed them as best I could and further screened them. Many were too hard to crush this way so I set them aside in case we later decided to use some mechanical means to break them up.

Low-tech separation of clay from aggregate and sand in Little Washington VA 2015. John Greenwalt Lee

In the second round of sieving, I further crushed the soil while trying to push and grind the aggregate across the screen. Already the minerals are beginning to show.

Beginning to wash clay from the soil to get cleaner sand for repointing
Beginning to wash clay from the soil to get cleaner sand and aggregate for repointing

This is the aggregate retained on the screen after sieving and ready for washing.

When you are looking at a pile of rocks and dirt, it is easy to loose perspective on what makes up those aggregates. I decided to take a handful of the rocks that had not gone through screen and put them in a rock tumbler so I could polish them up for easier identification at a scale I could see with the naked eye. Of course I’m now thinking of a side career in jewelry making these things are so beautiful.

Polished rocks removed from soil at Little Washington VA in 2015 while cleaning soil to make matching historic mortar. John Greenwalt Lee

Regardless of scale, whether looking at mountains or soil beneath your feet, notice the shapes and percentages (particle distribution) remain roughly the same. This is a thumbprint of the local geology, part of the fractal quality of geology.

Whether in the distribution of big boulders seen along Skyline Drive or the size of rocks people dig out of their gardens which match those in this picture, or the grains of “sand” in the mortar of historic buildings in the area, the same proportions of the same minerals are showing up, just in different stages of the weathering process from mountains down to soil.

What we are looking at here is what geologists would refer to as poorly sorted. In other words, there is a wide variability in the size of particles next to one another.

Since there was such care given in the joints up high, I now wonder whether the lower wall was always parged with lime mortar.

Again, the mortars do look very different, but the aggregate is the same, only much more of the silts have been washed from the mortar above the watertable.

Now that the soil has been screened we will determine how much washing are needed to match the mortar both above and below the watertable.

Beginning to wash clay from the soil to get cleaner sand for repointing
Beginning to wash clay from the soil to get cleaner sand for repointing

First you will notice just in beginning to shake or disturb the water that the silts are lifted as the aggregate falls. We will use this tumbling of water to wash the silts away, pouring off the silts from the top, before adding and rinsing with fresh water. The question is how many rinses will be needed. (Of course in the future we will be doing this rinsing by the wheelbarrow-load.)

After three washes, the volume of the aggregate is shrinking as the silt washes out. Individual aggregate grains are now visible.

Making a 1:3 lime:aggregate mortar produced a good match for the lower wall. By contrast it would take ten washings to get enough (nearly all) silt out of the soil to match the upper wall.

Coming up next: a change of topic introducing the uses of carbon fiber technology for repair and reinforcement in historic structures.