New Versus Historic Mortars

So, what’s the difference and what should I use?

By Blair E. Bates, Building Renovation, LLC

In today’s fast-paced world, we are continually trying to make things better, stronger, and faster. The same can be said for mortar. In our rush for improvement, we disregard thousands of years of historic proof that lime-based mortars are more durable than Portland cement-based mortars. This paper intends to take the mystery out of mortar mixes.

Photograph 1:

The entry gates to a medieval 1500-year-old fortification in the Apian Mountains within the Tuscany region of central Italy. This historic masonry receives rainfall, experiences freezing and thawing, and is subjected to large temperature fluctuations. The mortar is most probably lime and sand. No expansion joints, caulking or Portland cement. It is in quite good condition.

What is old (historic) mortar?

It was discovered that when limestone was burned, and then combined with water, it produced a plastic material that would dry and then harden with age. The earliest documented use of lime in mortar was approximately 4,000 B.C. when the Egyptians used it to plaster the Pyramids. The use of lime in load-bearing masonry mortar was not well documented until the Romans used it in the construction of their empire. This old mortar (pre-1900’s) was commonly a simple blend of aggregate and binder. This binder was typically a pre-burnt lime known as calcium oxide. As a binder in masonry construction, lime is sometimes considered inferior to Portland cement-based mortars, though limestone (calcium carbonate) is a raw material for both. Lime may be weaker, take longer to set, set in a different way (carbonation), and require a higher level of skill to use properly, but it has distinct long-term advantages. These include: Greater compatibility with soft materials, good workability, increased adhesion, and better weathering properties. The Romans modernized mortar to a highly technical level. However, what will be referred to as modern mortar in this paper, is mortar which had Portland cement added to it, which generally started in the early 1900’s.

Old-time mortars were used in the masonry construction process as a separator or gasket between the individual masonry units. The purpose of this gasket (just like in an automobile engine) was to adsorb small amounts of movement yet keep the pieces together. During water-adsorption periods, the mortar allows moisture in as well as out. During times of minor movements (thermal, vibration, superimposed loads etc.) the mortar was able to adsorb minor movement without permanent distortion or yielding (cracking, crushing, spalling, etc.).

What is old historic construction?

For the past 8,000 years until the turn of the 19th century, when one was to build a masonry structure, it was just that; a solid masonry building! All loads placed on the building were transferred to the masonry whether structural or climatic. In stone masonry, the stone was properly laid to a dry laid standard as the first level of strength with the mortar being used as a secondary aspect to hold the stones in place and transferring the loads more uniformly.

What is new mortar?

New masonry mortars are commonly used in veneer construction for brick and stone buildings. New mortars are designed on a specific performance or proportion criteria known as ASTM C-270. This allows the mortar manufacturer to either use an exact blend of lime and Portland cement or can blend many things into the mortar to come up with a specifically required result. The additives are used to: extend or shorten the set time, add waterproofing, increase workability for the mason, and achieve ultimate strength earlier. Quite often, lime is not in the mix at all.

What is new construction?

When looking at a modern (early 1900’s and later construction) building, you see the masonry as cladding while the structural elements are hidden underneath, such as steel or structural concrete. With such a thin cladding masonry system, the requirements for the mortar are very different from historic masonry.

Mortar Properties

What are the properties of mortar that are most important in mix design considerations?

Coefficient of Expansion

The amount a material moves as a result of temperature change (i.e.: a 200 foot stone wall that sees an annual low temperature of 20°F and a maximum surface temperature of 160°F). That would be a resultant total temperature change of 180°F. Therefore, multiply the coefficient of expansion for random rubble stone 0.00035 (see Table 1) times the change in temperature (180) then multiply this by the wall length to arrive at the change in wall length for temperature changes. In this example, it would be 1.6 inches. Something needs to accommodate this movement.

Compressive Strength

The ability of the mortar itself to hold a compressive load without failure. This is normally tested at a 28-day strength, which is a standard for Portland cement-based products but has little relevance to lime-based mortars that need time to carbonate, which takes an inch per year. Therefore, a standard 2” square testing cube will take a minimum of 1 year to carbonate to the center of the cube.

Ductility

Basically, the mortar’s ability to deform without failure.

Porosity

The mortar’s ability to pass moisture through it. In new mortars, porosity is to be kept at a minimum. In old mortars, porosity is a good thing so as to let the masonry dry out or, “breathe.”

Bond Strength

The dry adhesive quality of mortar; its ability to hold to the masonry unit. Normally measured in psi (pounds per square inch).

Modulus of Elasticity

A factor in building without construction joints.

Table 1: 

Coefficient of expansion of different building materials and their amount of movement as in the 200-foot stone wall example in the definitions section.  Note the close relation to concrete and steel.  This makes them compatible with one another.  Whereas rubble masonry and concrete begin to have conflict in their relative amounts of movement.

Photograph 2: 

A 200 foot long mortared stone wall in the foreground and right side that was built using a Type “ N” mortar with no other provisions for movement.  The result is a full-depth crack at 10 foot intervals for the entire length of the wall.

How does mortar get hard?

Lime Carbonation

Carbonation is the process of forming carbonates and, in this context, the formation of calcium carbonate from calcium hydroxide when a lime develops its set. Basically, the carbon that was removed in the burning process is re-adsorbed in the carbonation process to achieve a stable element.

Cement Hydration

Soon after the sand, water, and Portland cement are combined, the mixture starts to harden. All Portland cements are hydraulic cements that set and harden through a chemical reaction with water called hydration. During this reaction a node forms on the surface of each cement particle. The node grows and expands until it links up with nodes from other cement particles or adheres to adjacent aggregates. This process requires the Portland cement to remain moist or the process stops. If properly hydrated, the Portland cement portion of the mix allows for a dense low porosity-mortar.

Table 2: 

28 day vs. 365 day compressive strength comparisons of typically used mortars.  Note large differences in long-term compressive strength over time when lime is in the mix.  Reason:  carbonation vs. hydration.  Note that the higher the lime content, the more inaccurate the compressive strength if the 28 day values are only considered. Also note the historic mortar (NHL 2) compressive values of 420 psi vs. common modern mortar (Type “N” & “S”) which are 3 and almost 6 times higher, which is not always a good thing.

Commonly Used Binding Materials

Now that we have an understanding of how the mortar mixes work and what the important mortar properties are, here are the most commonly used binding materials:

Portland cement Type “1”

Portland cement is manufactured by blending limestone and clay, burning this mixture above “clinkering” temperature (greater than or equal to 2,300°F)  and when cooled, grinding the resulting clinker. The compounds present are formed by the interaction during burning of the lime, silica, alumina and ferric oxide compounds. The principal setting compounds in Portland are tricalcium silicate (CS), dicalcium silicate (CS), tricalcium aluminate (CA), and tetracalcium aluminoferrite (CAF). These compounds are present in known controlled proportions. In the 20th century, the desire for a higher strength product led to increased CS and reduced CS proportions. The setting process is the hydration of these four compounds, but it is the CS that contains all the essential properties of Portland cement.

There are, as one would expect, both advantages and disadvantages in blending (gauging) non-hydraulic (lime) mortars into Portland cement to make them hydraulic.

Advantages

• It imparts a chemical set which occurs before full shrinkage occurs, thereby reducing the risk of cracking.

• Layers may be built up more rapidly, without the need to wait a long time for one to set fully before applying the next.

• It hardens rapidly, thereby providing protection from rain before carbonation has been completed. This helps to beat inclement weather.

Disadvantages

• The rapid setting time limits the time available to the user in which to work with the gauged mortar.

• Some Portland cement contains appreciable amounts of soluble salts, particularly potassium sulfate, which may become a source of salt damage to the host masonry.

• The use of Portland cement tends to lead the user into treating the gauged lime mortar as if it were a fully hydraulic lime or cement. Too much reliance on the initial chemical set leads to neglect of the importance of the longer-term carbonation of the non-hydraulic component present.

• The danger that segregation occurs, whereby the cement separates from the lime as the mortar dries and hardens.

Mixing Portland cement with lime (gauged mixes)

It has been common practice for some time to use mortars containing a mixture of cement, lime and sand, e.g. Type “N” 1:1:6 or Type “S” 1:2:9 or Type “M” 1:3:12. These mixes are well established and conform to British Standards; hence they are specified a great deal. They’re all harder and less porous than pure fat lime mixes. The English Heritage Smeaton project has shown that gauged mixes with small amounts of cement (less than 1:2:9) are in fact less resistant to frost than straightforward fat-lime mixes, and I would suggest that mixes of 1:2:9, or stronger, are rarely appropriate on old buildings. Some may consider that gauged mixes have fulfilled a useful role in the absence of available hydraulic limes. We now have a (hopefully) increasing range of hydraulic limes available which should replace the need for cement/lime/sand mixes.  One note of caution is that it has been tempting for people to specify mixing hydraulic limes with non-hydraulic to modify their properties. It is difficult to modify properties that are not fully understood, and it seems that diluting a ‘weak’ hydraulic lime tends to lose most of the hydraulic traits, although it may be more appropriate with stronger hydraulic limes  

Segregation is a major hazard of gauging lime mortars with cement. As the mortar sets, the cement colloid tends to migrate into the pores of the lime mortar as they form, clogging them and leading to a greatly reduced porosity. If the proportion of cement is high enough, segregation is much less likely to occur, but the resulting mortar will be hard. If the cement proportion is low, the mortar will be less hard, but segregation is more likely to occur. The resulting mortar will be seriously weakened, with a poorly formed pore structure leaving it very susceptible to frost damage and deterioration, even after carbonation of the present non-hydraulic lime has taken place.

The Smeaton Project, a research program commenced by English Heritage indicates that a 1:1:6 mix, containing a 50% cement binder, is unlikely to segregate, while a 1:2:9 mix, containing a 33% cement binder, is almost certainly at risk. Until recently it was considered good practice to gauge lime mortars with as little as 5% cement, just enough to impart a chemical set but not enough to make the mortar appreciably harder. However, all of the Smeaton Project test samples containing less than 25% failed.

Given the possible hazards of segregation, a non-gauged lime mortar relying solely on carbonation is likely to be more resilient in the long run than one gauged with a small amount of cement. This will require care in its application and careful nurturing to ensure that it carbonates properly. If a chemical set is required, a safer alternative would be to use a hydraulic lime. In these, the hydraulic components are so closely associated with the non-hydraulic that segregation does not occur. These tend to be hard and impermeable, but not usually as hard as a 1:1:6 mix. Brick dust is a cheap and highly effective Pozzolanic additive (see below), providing a useful alternative to cement.

Hydraulic Lime

The essential difference between modern hydraulic limes and Portland cement is that hydraulic lime does not contain tricalcium silicate CS and contains lime. Various types are available, and they are produced in various grades. Limestone containing clay and/or silica is burnt in a kiln at below clinkering temperature (less than or equal to 2,190°F) and the resultant product is hydrated with just enough water to convert the calcium oxide to safer, less reactive calcium hydroxide, but not to hydrate the CS, which is slow to hydrate. The setting process is a combination of the hydration of CS and the carbonation of the lime. In most hydraulic limes, a proportion of uncombined reactive silica and alumina is also present, and these will react with the lime in the mortar to also produce calcium silicate hydrates and calcium aluminate hydrates. Hydraulic limes are classed as:

Feebly hydraulic         NHL 2 
Moderately hydraulic     NHL 3.5
Eminently hydraulic        NHL 5

Pozzolanic Lime

Pozzolanas are defined as materials which, though not cementitious in themselves, contain constituents which will combine with lime at ordinary temperatures in the presence of water to form stable insoluble compounds possessing cementing properties. Natural pozzolanas are mainly materials of volcanic origin. Artificial pozzolanas are mainly products obtained by the heat treatment of natural materials, such as brick dust, fly ash and china clay.  

Non-Hydraulic Lime 

Limes (less than 95% calcium hydroxide) made by hydrating or “slaking” the quicklime (burned calcium carbonate) of relatively pure limestone, which sets by carbonation. Two forms are available:

Lime Putty

Ordinary (non-hydraulic) lime produced by slaking fresh quicklime in an excess of water to form a putty. Lime putty is matured for several months in pits or under a thin film of water to prevent carbonation, and during this process the Portlandite (lime) crystals change shape, becoming smaller and flatter, thus aiding workability. 

Dry Hydrated Lime

Ordinary (non-hydraulic) lime produced as a dry powder by hydrating the quicklime (just burned) with sufficient water only to convert calcium oxide to calcium hydroxide. Also known as “bagged” or “masons” lime.  This must be kept away from the air or it will carbonate in the bag.

Now that we know how mortar works and what is available, how does one choose what to use on their project?

The Appendix of ASTM C-270, developed in 1951, provides a reference to assist in deciding which mortar type should be used in some general applications.  Refer to Table 2 for an understanding of each mortar type. A synopsis of these modern recommendations are:

Exterior, Above Grade 

(Load-bearing wall, non-load bearing wall, parapet wall) Type “N”.

Exterior, at or below grade

(Foundation wall, retaining wall, manholes, sewers, pavements, walks, and patios) Type “S” or “M”.

Interior, load-bearing wall

Type “N”, “S” or “M”.

Interior non-load bearing wall 

Type “O” or “N”.

Chart 1: 

The great compromise. This graph shows the relationship between the amount of cement vs. lime in a mortar mix and its relationship of water retention (board life and workability) to compressive strength. Where the two cross, (Type “N”) is, in theory, the best of both worlds for the mason and the engineer. Not necessarily the durability needs of the structure, such as ductility and porosity.

Photograph 3

The fallacy of comprehensive strength requirements.  This staircase enclosure weighs 112,000 pounds is 10 feet in diameter and 25 feet tall with 1 foot thick walls. Since the wall base is 1 square foot per foot, the 144 square inches of bearing area are only holding up 24 pounds per square inch.

These ASTM recommendations are based on the philosophy of “stronger and denser is better.” They are also based on the newer construction standards of making structures thinner, thus having to carry larger loads and have less room for moisture transfer. What is not shown is the recommendation to place many expansion joints (commonly at 20 foot vertical spacing and one at each floor level) throughout the structure to accept the movement that the mortar now can not accept due to its rigidity. These expansion joints are then sealed with caulking compounds that have a maximum life expectancy of 20 years which makes no sense when the potential durability of a properly mortared structure would be 100 years until maintenance of the mortar is required. Remember that history has shown us the realistic durability expectations. Lime mortars have shown us 4,000 years of durability expectations, while modern mortars have had a poorer showing of durability over the past 100 years and are still being modified and understood.

If you have a need for a quick setting durable mortar for real stone masonry, consider using a hydraulic lime mortar. If not, consider properly using a lime mortar. History is on your side.

Photograph 4

A deteriorating section of the Great Wall of China.  This lime sand mortar has had very little maintenance and is still holding its own after 700 years, but it is time for some maintenance.

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