When repairing or restoring an old historical building, it is important to use materials that will maintain the original appearance and integrity of the structure. Portland cement and lime mortar have both been used for centuries for this purpose, but which one is better? Let’s take a closer look at each material’s composition, masonry history, and uses to find out more!
Historical renovations come with a variety of challenges and considerations and should only be done by an expert. Most historical buildings feature masonry of some sort, from small fireplaces to entire exteriors of stonework. An integral part of that masonry is the mortar, which holds the bricks together. Over time mortar can weather and degrade, leading to gaps in construction that are vulnerable to erosion and damage. When it comes time to repair or replace the masonry, how can you ensure the building will remain strong while staying true to its original appearance? Portland cement and lime mortar are the two most common mortars available, but they will perform very differently and will affect both the appearance and the strength of your building. Read on to learn about each material and where its use is most appropriate.
Portland cement is a man-made material that was first developed in the early 1800s. It is made by heating a mixture of limestone and clay and adding in gypsum. This is then ground into a very fine, uniform powder that forms the basis of what we know as mortar. This material has many advantages, including a quick, even set time and high strength. It has the ability to set even when used underwater and its small particles allow it to fill tiny pores and gaps. This feature also means that the concrete product is less permeable to moisture, which is an important consideration in the durability and weather resistance of construction. Portland cement is the most commonly used cement in mortar today and currently makes up about 95% of cement production worldwide.
Strong, quick to set, widely produced, useful in many settings – this has to be the best all-around choice, right? Well, it depends. For most modern construction the answer is yes. However, there are other considerations when working on historical masonry. Many people think that super-strong, waterproof moisture means a stronger construction, but in fact the opposite is true.
In a brick masonry structure, the mortar is meant to take the bulk of the building stress and allow moisture to pass through it. This means that the bricks are less likely to crack or erode. If the mortar is waterproof, that will raise the moisture content within the bricks, causing weaknesses and erosion. And if the mortar is stronger than the bricks around it the stress of the building will be absorbed by the masonry units themselves rather than the mortar. If the mortar is the softer and more permeable part in a wall, that means that the bricks will not retain excess moisture and that the mortar will flake, crack, or erode instead of the bricks. It is vastly preferrable for the mortar to fail instead of the bricks. Mortar is much easier to replace and match, as long as the right materials are available.
So why is lime mortar the all-around better material for historical masonry? For one, that flexibility and stress absorption is necessary in older structures that were not built with vertical expansion joints. Vertical expansion joints are installed along vertical faces to allow the masonry to move and flex with changes in temperature or humidity, weather stress, and settlement. Lime mortar does this naturally due to its softer composition and ability to “self-heal” or reseal hairline cracks with its greater water solubility. If a section of lime mortar in a building is replaced by a harder and less permeable mortar, the brick surrounding it is much more likely to crack, flake, or spall leaving visible sections of damage on a building’s face.
Remember how Portland cement’s tiny particle size made it desirable due to its ability to fill tiny gas and bond well to other materials? The particle size in lime mortar is even smaller, allowing even better fill and a stronger bond to the masonry units. Lastly, using a lime mortar will enable you to match the color of the rest of the structure, which is vital to maintaining the historical integrity and authenticity of the building.
Ensuring that you use a compatible masonry material is not only important for the aesthetic of the building, but also in ensuring that your construction will remain strong and your brick undamaged. At Black Goose we have completed repairs on numerous historical buildings, including 18th, and 19th century structures statewide including the Wren Building (1695) at William and Mary. We can source period-accurate materials and salvage bricks in order to maintain a live artifact. Contact us today to discuss how we can repair or restore your historic masonry with authenticity and longevity.