When restoring historic masonry, it is important to identify the composition of the original mortar used in the construction of the masonry structure. While this may sound like a suggestion and an attempt to sell you an idea, there are many reasons why you need to test your mortar and determine the composition of the original mortar used.

Older or historic masonry units were made by hand and tend to be softer or more brittle than the fired clay bricks used in construction today, and the stone will have different varying factors. Depending on the type of stone, it will have different crushing pressures, and will require a mortar that is softer than the rock. This is also true for handmade bricks. The mortar should be softer than the masonry unit being used to avoid damaging the bricks or stone. This part carries over to today’s mortars, but is often overlooked by today’s builders.

Each type of mortar is based on a variable range of composition or compressive strength. An example of this is type n masonry cement (most commonly used today) has a compressive strength of 750 psi and is usually made using one part Portland cement mixed with one to one and one-quarter parts of lime and about two and a quarter to three parts of sand by volume, and a lime mortar has a compressive strength of about 75 psi.

The various types of mortar are lime mortar, historic lime mortar (containing historic lime), k-type mortar, o-type mortar, n-type mortar, s-type mortar, and m-type mortar. And the crushing pressure of these types of mortar are 75 psi for lime mortars, 150 psi for k type, 350 psi for o type, 750 psi for n type, 1800 psi for s type and 2800 psi for m type mortar.

As you can see, there is a big difference between each type of mortar and it will certainly affect the results of any masonry repair.

Historic mortars tend to range from lime mortar to mortar type or partly due to the cost of Portland cement, but also to protect masonry units. The lime used often loosened in place and tended to have large chunks. This is known as historic cal. These large pieces are the difference between historical lime and the type S/SA lime used today, which are sieved to ensure smaller particles. The only real effect these pieces have on the mortar is visibility. They change the appearance of the color and texture of the mortar.

Since it was often necessary to add strength to the mortar, but not exceed the compressive strength of the masonry units, fibers were often mixed into the mortar. The most common fiber used was horse hair. Horse hair and different herbs were also used to make the bricks and hold them together.

In order to preserve the historic stonework, close attention to detail must be paid. To do this, you must first collect the available details about the mortar and masonry units. This requires testing the hardened mortar and/or bricks or stones.

Through mortar analysis, you identify the composition of the mortar, the compressive strength of the mortar, the type of lime used, whether fibers were used, and the type, color, and particle size of the sand used in the mortar. old mortar.

These are the details needed to make a matching mortar for grouting.

There are now some sophisticated and often expensive test methods used to analyze mortar samples that are simply not very reliable. Some of these tests take nice pictures of the mortar sample that identify different elements of the mortar by color. This image is then observed visually and the best guess of the proportions is determined. This method has many limitations, the worst being the understanding, experience, and knowledge of the technician testing the mortar samples. If they don’t understand masonry, how it’s used and how it’s built, they can’t accurately interpret the data.

An excellent example of this is calcium, and this carries over into many of the mortar test methods and the results are analyzed only by chemists. Throughout the industry, emphasis is placed on the amount of calcium in the mortar. This has very little to do with the composition of the mortar and makes little to no change to the way the mortar works.

You will find calcium in sand usually in small amounts, but some sands have more calcium than others. In this use calcium is important. However, the use of calcium added to the mortar tells us more about the temperature and weather conditions during the time of construction.

As lime mortars cure, the lime acquires and binds with CO2, causing it to cure and harden. Portland cement releases CO2 as it cures. This makes them work well together. However, in cold weather, the chemical reaction of Portland can slow down or stop. When this happens, the mortar cures more slowly and may not allow the Portland to add strength due to its chemical reaction, causing the mortar to cure like a lime mortar. Calcium can be added to the mortar to increase the chemical reactions of the Portland and the lime and cause more heat. With the added heat and the fast CO2 removing Portland, there is enough CO2 for the line to stick to and the mortar will cure even in colder temperatures.

As you can see, a mortar analysis is needed to identify the composition of the mortar, but the results should be interpreted by someone with masonry knowledge and experience. This does not mean that you can rely solely on the knowledge and experience of any mason. You need accurate test data to obtain mortar composition for masonry preservation. The understanding and use of that data can only be interpreted by a person with knowledge of how it works, and recommendations for variance should be made with this understanding.

When all of these considerations of the mortar, its composition and performance tests, and its use have been met, you will have a good starting point toward the preservation and restoration of historic masonry. And the most important fact in historic building restoration or historic mortar grouting is to remember that while you may be the owner of a historic building, you are simply the caretaker of the property. This structure was here before you were born, and if properly cared for, it will last long after you are gone.

So for the people involved in restoring history, remember that the historic building is the actual customer and not necessarily the person who signs the check.

To put it in a more understandable and less offensive term, consider this. When you visit the doctor, your insurance pays the bill, but the professional (the doctor) must put you as a patient or client and your needs first.

Don’t damage, destroy, or alter history just to get a paycheck. Take the time and do it right.