I was riding my bike on the Cherry Creek Trail the other day and noticed a honeycomb texture to the wall.  It looks kind of cool, but it’s always a bad sign for the wall.  This happens when the masonry wall is built with strong mortar.  As you can see in the photo below, this stone wall was erected on top of a concrete retaining wall.  There are plants or grass on the other side of the wall, allowing moisture to easily get into the wall.

Moisture is inherently lazy: it seeks the path of least resistance.  When water enters a wall, it will seep out of cracks or open mortar joints.  But if it becomes trapped, it will migrate into the masonry or mortar, whichever is the weaker material.  As the moisture builds up just beneath the exposed surface of the masonry, some moisture will evaporate causing little damage to its host.  In wintertime, however, the moisture freezes and thaws within the pores of the host masonry or mortar, and because ice has greater volume than liquid water, the ice microscopically breaks apart its host.  Repeated freeze-thaw cycles cause the surface of the wall to erode.

In this case, the stone is weaker than the strong mortar, and the stone has eroded deeply in many places.  Notice in the photo below that the mortar is the pink and has pink, black and white speckles.  Did you also notice that’s the same color and texture as the concrete retaining wall?  It is likely that the mortar contains only cement, which is too strong for a sandstone wall.

To prevent this honeycomb deterioration, most masons use mortar that is softer and more porous than the masonry.  They do this for two simple reasons: is far easier and far less expensive to replace eroded mortar than it is to cut new stone and rebuild the wall.

For those of you who are curious about appropriate mortar mixes, here’s a very quick overview: The American Society for Testing and Materials (ASTM) categorizes mortars based on overall strength.  Using every other letter from the phrase “mason work”, ASTM standardized each type of mortar and its components (all by volume), from strongest to softest.

• Type M mortar has 1 part cement, 1/4 part lime, and 3 to 3 3/4 parts sand.
• Type S mortar has 1 part cement, 1/2 part lime, and 4 to 4 1/2 parts sand.
• Type N mortar has 1 part cement, 1 part lime and 5 to 6 parts sand.
• Type O mortar has 1 part cement, 2 parts lime and 8 to 9 part sand.
• Type K mortar has 1 part cement, 3 parts lime and 1o to 12 parts sand.
• There is also a non-standard “Type L” mortar, which contains no cement, 1 part lime and 2 1/4 to 3 parts sand.

Historically, masons did not use cement in their mortar or only used a small amount of cement or cement-like material as a strengthening agent.  Therefore, it is rare to find Type M or Type S mortars used on historic buildings.  In fact, the earliest buildings in our country would not have had any cement in their mortar as Portland cement was not manufactured in the US until 1872, and natural cement was not discovered until the construction of the Erie Canal in the early nineteenth century.  (Before cements came on the market, masons often used additives like pozzolans and ash to increase the strength and setting time of lime-rich mortars.)

Most buildings built with soft stone or low-fired brick manufactured before the industrial revolution would have had a predominantly lime-based mortar (Type O, K or “L”).  Buildings with higher-fired brick from the late-nineteenth and early-twentieth centuries, or buildings with stronger stone could withstand equal amounts of lime and cement (Type N).  Keep in mind this is a vast generalization, as building materials and climates differ.  For more detailed information about selecting mortar for use on historic buildings, check out the National Park Service’s Preservation Brief 2: Repointing Mortar Joints in Historic Masonry Buildings.