Mortar Matters

Mortar is the bonding agent used in masonry construction. Stone and brick walls cannot stand without it; the individual cuboids will shift and topple in the absence of a bonding agent to hold them together. This means that this cement and/or lime + sand + water mixture is used to permanently bond stonework or brickwork. The common mixing ratios used for mortar are 1:3 for normal plastering or brickwork, and 1:4 for engineering bricks.

Mortar at work:

Here are some types of mortar commonly used in the construction industry.

• Lime Mortar

As its name implies, lime mortar is a mixture of lime, sand and water. It was widely used in the construction industry prior to the advent of cement. Its high plasticity offers a better chance of separating individual bricks or stones without cracking or failing as easily as cement mortar.

Today, lime mortar is considered inferior because of its slow rate of hardening and relatively low compressive strength. Not only does it harden slower than cement mortar (a process which can take years before it reaches maximum strength); it is also weaker than its counterpart.

However, its perceived weaknesses can also be seen as a boon, which is why lime mortar is still in demand today. The rationale behind this will be discussed later.

• Cement Mortar

Cement mortar is a relative newcomer in the construction industry, but its superior compressive strength and fast rate of hardening allowed it to succeed lime mortar as the prime bonding agent used in masonry construction. Cement mortar is a mixture of Portland cement, sand and water. The cement:sand ratio is usually either 1:3 or 1:4.

Cement mortar is also seen as a convenient option as it is suitable for all kinds of brick bonding. The inclusion of Portland cement makes it an extremely strong bonding agent, especially in comparison to lime mortar.

However, the use of cement also makes cement mortar a lot less plastic once it has hardened. This may not be a desirable trait, depending on the type of construction.

• Cement-Lime Mortar

What better stroke of ingenuity is there than to create a hybrid mortar? Cement mortar lacks the plasticity and water retentivity of lime mortar, whilst lime mortar lacks the strength and faster rate of hardening of cement mortar. The use of both cement and lime in a mortar mixture helps to somewhat overcome these disadvantages. The resulting mortar mixture has both high compressive strength and high rate of hardening, plus increased plasticity and better water retentivity.

Some commonly used cement:lime:sand ratios of cement-lime mortar include 1:1:6, 1:3:12, and 1:2:9, in order of decreasing strength. The Smeaton Project, spearheaded by John Smeaton - which researched on the effects of adding cement to lime mortar - discovered that the 1:1:6 ratio dissuades segregation (a situation where the cement colloid clogs up the natural pores in lime mortar, thus weakening the mixture), while a 1:2:9 ratio is at risk of segregation.

The picture to the right shows segregated mortar.

One possible way to avoid segregation when a chemical set is required is by using hydraulic lime. A possible alternative to cement, which causes segregation, is brick dust. It is cheaper and effective as a pozzolanic (like pozzolana, a porous variety of volcanic ash used to make hydraulic cement) additive.

• Gypsum Mortar

Gypsum mortar has its roots as far back as the Egyptian era, when it was used to build what the Egyptians were famed for: pyramids.

Since many pyramids still dot the earth today, it is relatively safe to assume that gypsum mortar is durable. Gypsum is a soft mineral with high calcium content. It is also more commonly known as Plaster of Paris, for which it is a key ingredient. Like cement mortar, gypsum mortar has a high rate of hardening. It is usually used for detail work in masonry construction.

The high cost of gypsum mortar discourages heavy usage, although in certain parts of the world it is still in demand, mostly fuelled by restoration projects more than new constructions.

Gypsum mortar is only recommended for use in masonry constructed for either internal or extremely dry climates, unless some form of protection is afforded to it. It deteriorates when attacked by moisture and as such, gypsum mortar is unsuitable for external masonry construction. The unrestored historic structures in Paris, gingerly held together with gypsum mortar, is testament to this fact.



Discussions


Reparation Efforts
It should be noted that any reparation efforts of masonry should be done with identical mixture ratios and type of mortars where possible. Cement mortar, for instance, is not a good substitute for lime mortar as it will cause the masonry to fail in a relatively short time frame.


The Most Suitable Mortar for Green Efforts: Lime Mortar
You might ask: "Why lime mortar?"

While it is true that lime mortar is weaker and takes much longer to harden than cement mortar, it is also considerably weaker - and this is why it is a more suitable mortar for use in green buildings. The bond between bricks are weaker, making it more possible for the bricks to be removed whole without much damage. The lime mortar can then be wiped off and the bricks reused: something which cannot be done when strong Portland cement is used as a binder, as is the case with cement mortar.

In fact, this is one of the reasons why lime mortar is seeing a comeback in several countries around the world.



References
1. O'Hare, G. (1995). Lime Mortars and Renders: The Relative Merits of Adding Cement. Building Conversation Directory. Retrieved March 14, 2012 from http://www.buildingconservation.com/articles/cement/cement.htm

2. Cemex Mortars (n.d.). Introduction to Mortars. Retrieved March 14, 2012 from http://www.cemex.co.uk/re/pdf/lt-intro_to_mortar.pdf

3. Lyons, A. (2010). Materials for Architects and Builders (4th ed.). Burlington, MA: Elsevier.