Types of Binders for Mortar

Binders:

A binder is a material that will hold aggregate (sand) together when making a material called a mortar. Mortar is the word used to describe the material between the bricks and stones of a building. There are three basic categories of mortar binders that are available for use.

These are: Portland Cement, Natural Hydraulic Lime (NHL) and Carbonating Lime Putty. These three materials are divided into two categories: Hydraulic and non-hydraulic. The term hydraulic in the masonry field means the mortar will set up underwater. Non-hydraulic means the mortar will never set under water. Hydraulic binders are like a two-part epoxy. Part A being the water and part B being the dry powder. Mixing the water and the powder together starts the setting process. Hydraulic binders are predictable in their setting times and in their compressive strengths. Non-hydraulic Lime Putty only sets when it absorbs carbon dioxide. If lime putty stays wet, the carbon dioxide in the atmosphere cannot enter the lime; therefore, it will never set. If lime putty is kept wet, it will never set even after thousands of years. Portland Cement and NHLs are hydraulic. Carbonating Lime Putty’s are non-hydraulic.

Important characteristics of the three types of binders:

Carbonating Lime Putty:

Carbonating limes are some of the earliest of buildings materials. Most buildings built in the USA before 1900 were built using carbonating lime putty as the sole binder. Carbonating limes on a broad/commercial scale have gone extinct in the USA and are relatively unknown and not widely available.

Carbonating lime is made up of 100% free lime and able to continually offer flexibility. The free lime is the part that dissolves when it gets wet/damp but will reset/re-carbonate when dry. Lime Putty Mortars use 100% free lime, they are also the most prone to dissolve when they stay wet/damp. The PSI of carbonating lime putty is impossible to quantify. This is because their PSI is constantly changing depending on what stage of the wet/dry cycle they are in. Are they in the dissolving stage because of recent rain, or are they in the re-carbonating/resetting stage as they are drying? When they are damp, they will be softer, and when they are dry, they will be harder. The ability of lime putties to dissolve/reset/dissolve/reset seemingly forever is what gives the lime putty mortar its flexibility.

Flexibility and Expansion Joints

Think of a large modern building that is built using Portland cement. These buildings have expansion joints every 25’ or so. This is because Portland cement is brittle and not self-healing; therefore, unless there are expansion joints every so often, the buildings would quickly crack and fall apart. Lime putty mortars do not need expansion joints because every mortar joint acts as an expansion joint. When the mortar gets damp, the lime dissolves slightly which allows the sand to “resettle” to the movement of the building. When the mortar dries, the lime resets and holds the sand in the new position.

The sand required for Lime Putty mortars is extremely important as they gain most of their compressive strength through the sand and not the lime putty. Great care must be taken when choosing sand for use in a lime putty mortar. The more wet/dry cycles a lime putty mortar goes through, the more durable they become without an increase of the PSI. Lime Putty mortars have the highest permeability. The perm rating is usually in the 70’s. The perm rating is not reduced over time. Lime Putty mortars require the most cognizance, alertness and skill to use. They are not difficult once learned, but they are different than Portland or NHL mortars. 

Natural Hydraulic Lime (NHL):

Introduced widely in the USA starting in the 1990's, NHL was rarely, if ever, used before that. It has only been manufactured in the UK until just recently. The largest manufacturers of NHL’s are in the UK.

Characteristics of NHL: 

NHL’s set slower than Portland cements, they have predictable setting times and PSI’s. They come in three different strengths: NHL 2.0, NHL 3.5 and NHL 5.0.

• NHL 2.0 has the lowest PSI and is used for soft bricks, soft stone, plaster, stucco and limewash.
• NHL 3.5 has a higher PSI than the 2.0 is used for harder bricks and harder stone.
• NHL 5.0 has the highest PSI of all three and is used for extremely hard stone, modern brick or block, constantly wet locations such as bridge abutments or wells, and skyward facing mortar joints.

The type of sand used in NHL’s is not overly important because like Portland cements, NHLs are a glue that both holds the sand and the masonry units together. NHLs are much more pervious/breathable than Portland cements. NHLs are less pervious when compared to lime putty mortars.

The perm rating on NHL’s varies depending on the NHL used and how long they have been installed. NHLs continue to get harder with time and lose breathability as time passes. Reports from the UK say that after 25 years NHL’s reach the same and greater PSI as Portland cement mortars. NHL’s have a slight ability to “self-heal.” This means that when they get wet/damp, the free lime in them will dissolve slightly. When they dry and carbon dioxide is absorbed into the mortar, they will slightly reset.

The free lime is the term used to describe the type of lime that can reset when they re-dry. Each of the three types of NHL’s have varying amounts of free lime. NHL’s are generally easier for masons to learn because they are like Portland cements in the type of sand used, ratios of lime to sand, installation methods, workability and setting times.

Ordinary Portland Cement (OPC) Mortar:

Portland was introduced widely in the USA around the year1900.  1900- 1930 mortars in the US were often a mix of Portland and carbonating lime. From 1930-today, Portland Cement is the only binder used in new construction.

OPC's set fast and hard. The commonly used Portland cements are Type O, N and S. The PSI of OPC mortars is usually between 1,500 and 2,500. The type of sand used is not overly important because it is basically a filler. Portland is a glue that holds the sand and the masonry units together.

Portland mortars do not easily allow moisture to move through them. They are very impervious when compared to lime putty mortars. The perm rating of OPC's is usually in the 20’s and are not self-healing. This means that once they crack, they are cracked forever. Expansion joints must be used in buildings built with Portland mortar. The expansion and contraction of the building will cause cracking, destroying it. Portland cement mortars are widely used throughout the USA. Many masons, architects, engineers and building inspectors don’t know that any other mortar besides Portland cement mortar is available.

Is it a Lime or Portland binder?

Here are broadly accepted ways to determine if lime was the binder in a historic mortar:

•  If the mortar is from a building that was built before 1900, it is likely lime.
•  If the mortar is soft and crumbles when scratched or squeezed between fingers, it is likely lime.
•  If there are white lime blebs this indicates a lime putty mortar was used. Lime putty turns into a cottage cheese consistency when it freezes, these blebs appear in historic mortars.
•  If the aggregate used is weathered deeply, it is likely lime.
•  If no “tool marks” are visible, it is likely lime.
• If the mortar digests/reacts rapidly when acetic acid is poured onto the crushed mortar, it is a lime mortar. Portland cement has very little or no reaction to acetic acid.