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Mortar

Overview
Mortar is the bonding agent that integrates a masonry wall. It must be strong, durable, capable of keeping the wall intact, and must create a water-resistant farrier. Mortar as recommended by the Brick Institute of America and other masonry experts is made from the following ingredients.

Mortar Ingredients

Portland Cement
A hydraulic cement, contributes to durability, high early strength and high compressive strength. It is one of the principal cementitious ingredients for mortars. Only three of the eight types covered by ASTM C 150, Standard Specifications for Portland Cement, are recommended by the BIA.

Three Types Recommended by the BIA:

  • Type 1 - For general use when the special properties of Types II and III are not required.
  • Type II - For use when moderate salt water resistance or moderate heat of hydration is desired. Sometimes used in hot weather as an alternative to adding a retarder.
  • Type III - For use when high early strength is desired, may be used in cold weather.

Hydrated Lime
Type S, covered by ASTM C 207 sets only upon contact with air, contributes to workability, water retentivity (preventing water from evaporating too quickly for the mortar), elasticity , bond strength and resisting water enervation through the brickwork. It is also a cementitious ingredient, however, it takes much longer than Portland Cement to develop its strength.

High lime mortars create a phenomenon called autogenous healing. When rainwater, which always contains carbon dioxide is absorbed into the mortar, it either absorbs directly into the mortar or dissolves minute amounts of hydrated lime or limestone. This solution penetrates into cracks or voids while the hydrated lime combines chemically with the carbon dioxide to form limestone, which becomes slightly larger then the original hydrated lime. The limestone hardens as the air dries it, and bonds to adjacent surfaces in the voids. Eventually the voids or cracks may become filled. This slow process continues for years. The higher the lime content, the more effective the healing process.

Sand specified by ASTM C 144 acts as a filler, providing the most economic mix and contributing to strength.

Water is the mixing vehicle which creates plastic workability and initiates the cementing action.

Color can be achieved in a variety of ways:

  • Grey or white Portland Cement, when mixed with local sands will create shades of grey or white mortar.
  • Other colors should only be achieved, using metallic oxides, such as iron, manganese and chromium oxides, carbon black and ultramarine blue.
  • Too much pigment will reduce strengths and durability, therefore maximum weight of pigments should not exceed 1% of the weight of the Portland Cement except carbon black, which should not exceed 2 to 3% of the cement weight.

Admixtures
Sometimes added to the mortar, but are not normally recommended are the following:

  • Air Entraining admixtures are added to improve mortar workability and durability. However, research has shown that air contents in the mortar of over 12% show increased likelihood of moisture penetration and deceased bond strength.
  • Antifreeze compounds or other similar substances are sometimes used to lower the freezing point of the mortar, so that brick can be laid in cold weather. However, in order to be effective, a considerable amount must be added to the mortar, and this greatly reduces bond strength, and contributes to efflorescence and spalling in the brickwork.
  • Accelerators, such as calcium chloride are used to speed the hydration of the mortar in cold weather, so that heating of the mortar materials is not necessary. Calcium chloride tends to corrode reinforcement embedded in the mortar. This steel holds the brick to the building. Accelerators, even those that do not contain calcium chloride, tend to create efflorescence.

Mortar Specifications

 

Mortar
Type

  

Proportion
(Parts by Volume)

 

Property
(Avg. 28 day
compressive
strength, PSI)

 
  

Portland Cement

  

Hyadrated Lime

  

Aggregate,
Measured in a Damp, Loose Condition

M

1

1/4

Not less than
2 1/4 and not
more than three
times the sum
of the separate
volumes of
cementitious materials.

2,500

           

S

1

Over 1/4
to 1/2

1,800

           

N

1

Over 1/2
to 1 1/4

750

           

O

1

Over 1 1/4
to 2 1/2

350


Mortar Properties
Workability which refers to the plastic properties can be increased by:

  • Using a well graded sand of which approximately 10% passes a NO> sieve
  • Using highly plastic lime, type S, and
  • Increase the air content, but not to exceed 12%.

Bond strength which relates to the hardened properties can be increased by:

  • Mixing maximum water consistent with good workability and permit retempering.
  • Wetting the brick when necessary.
  • Use type S mortar.
  • Keep air content of the mortar to a minimum, and
  • Use mortars with high lime contents.

Field Guidelines for Handling Mortar

  • Water should be clean and mostly free of acids, alkalies or organic material
  • Bags of Portland cement and lime should be stored on pallets, and covered with plastic.
  • Measurement of sand should be done with volume or weight measures, not shovels. Boxes or five gallon paint containers can be used.
  • Mixing of cement, lime, sand and water should be done for at least 3 minutes, and not more than 5 minutes in a mechanical batch mixer, with the maximum amount of water, to produce a workable consistency.

Mortar Uses
There are a variety of mortars, brick, exposure conditions and building uses. The following are many of the factors which influence how to choose the proper mortar for various conditions. The following recommendations are based on Portland cement-lime (PCL) mortars.

High strength mortars, high Portland cement content, are most effective when freeze/thaw action may be considerable. High lime content mortars have lower strengths, but high resistance to moisture penetration. The following table suggests possible mortar choices.

Mortar Type Application

Mortar Type

Application

S (Low Suction)

Exterior paving systems and brickwork below grade

S (Low Suction)

Structural brickwork; i. e., high wind pressure, heavy load bearing seismic designed walls

N (Lime Content)

Exterior brickwork, such as veneer, chimneys and parapets


As a general rule, use the weakest (in compression) mortar that is consistent with the performance requirements of the projects.

Initial rate of absorption of the brick affects the type of mortar used. A brick with a high suction rate will absorb water from the mortar very quickly, potentially leaving the mortar too dry to hydrate properly and bond to the brick. Wetting the brick prior to laying or using a mortar with high water retentivity, a high time content mortar, type N, will compensate for the high suction brick. For low suction brick, 3 to 4 grams, a
higher cement content mortar, type S, may be more effective.

The air temperature at time of installation also affects the mortar choice. For example, in hot weather a high lime content mortar will help to retain the moisture in the mortar for a longer time. In cold weather, a lower lime content, a type S mortar with lower water retentivity, may improve bond strength. Also, a type III Portland cement, a high early strength, will provide extra heat from hydration to enable the mortar to hydrate sufficiently to bond with the brick.

If there is any doubt about which mortar to use, independent testing should be conducted to verify bond strength and Moisture resistance for the brickwork.

All these variable needs to be considered in choosing a mortar. In addition, workmanship is a critical factor. Preconstruction meetings with the mason should be arranged to impress upon him the need for quality workmanship.

Masonry Cements
Masonry cement mortars are widely used, because of their convenience and generally good workability. However, there has been considerable debate concerning whether or not masonry cement mortars are as effective as PCL, mortars with respect to bond strength and moisture
penetration resistance.

The following are some of the concerns:

  • First, the manufacturers of masonry cements, which are proprietary products do not disclose their product formulae.
  • Producers use different amounts and types of ingredients, which leads to inconsistent properties. In a bag of masonry cement, about half the bag is Portland cement, while the other half is ground inert limestone (a fine non-cementitious aggregate. As a filler, Additives are added to provide workability, water retentivity and air entrainment.
  • The second reason is that masonry cement mortars have air contents around 20%, which makes the mortar very workable. However, the high air content means that there will be less cementitious material in contact with the brick, causing less surface adhesion and reduced bond strength. Also, the voids created by the air will allow moisture to more easily penetrate through the mortar.
  • A third reason is that masonry cements contain little or no hydrated lime, although some may contain an inert limestone. Without hydrated lime, autogeneous healing can not occur to seal voids or cracks caused by poor workmanship or shrinking cracks. The lime also provides some additional strength.
  • frequent comment from masons is that they prefer masonry cement mortars over PCL mortars, because PCL mortars are hard to get off the brick - the PCL mortar bonds to the brock too well. Masonry cements have performed reasonably well for many years, however, they should be used with caution.

References

  1. Technical Notes 8 Revised, “Portland Cement-Lime Mortars for Brick Masonry”. Brick Institute of America, Feb. 1989.
  2. Technical Notes 8A Revised, “Standard Specifications for Portland Cement-Lime Mortar For Brick Masonry”, Brick Institute of America , Sept. 1988.

       

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