Essentials of
Masonry

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Part 1--Selecting and Mixing Mortar Materials

MATERIALS

To construct a concrete-block wall, you will need concrete blocks and the basic ingredients for mortar (cement, lime, sand, and water). Other ingredients that speed up or slow down the curing time of mortar are called admixtures.

1-1. Blocks. When storing concrete blocks, use good storage techniques. Never allow the blocks to get wet before you lay them. Excess moisture in concrete blocks will cause shrinkage when drying. Stockpile the blocks on a raised platform to prevent the absorption of ground moisture. At the end of the workday, cover the stockpile of blocks with a waterproof tarpaulin to protect them from the rain.

1-2. Mortar (cement). Specifications for the types of portland cement are covered by the American Society for Testing and Materials (ASTM) and include types with the following characteristics:

  • Type I is a general purpose cement and the most commonly used. It is generally used in mortar.
  • Types II is used in concrete.
  • Type III is a high early-strength cement. Although Type III takes as long as Type I to set, it will achieve its full strength much sooner. It is sometimes specified for cold weather because it requires shorter protection time. Type III is generally used in mortar.
  • Types IV and V are used in concrete.

1-3. Lime. Cement provides the strength of mortar while lime acts as a plasticizing agent providing smoothness and workability. Lime also increases the water-holding capacity of mortar without it, the mixture would be stiff and unworkable.

1-4. Sand. Sand used in mortar must be clean and free of organic materials; individual particles should be well-graded as to size. In a good mortar, all sand particles are completely coated with cemetitious material (paste). This permits the separate ingredients, or aggregates (sand and stone or gravel), to roll over each other and produce a plastic, workable mortar.

1-5. Water. Water used in mortar should be clean and free of acids, alkalies, salts, and organic matter. As a general rule, drinking water is suitable for making mortar.

1-6. Admixtures. Besides the four basic ingredients (Figure 1-1), any other materials added to the mortar are called admixtures. The most common admixtures are accelerants and retardants.

Figure 1-1.  Ingredients of mortar
Figure 1-1. Ingredients of mortar

1-7. Accelerants. An accelerant is an agent that speeds up the curing time of mortar. One such accelerant is calcium chloride. This is sometimes used when working conditions are very cold (40 degrees Fahrenheit or below) and there is a danger of the mortar freezing before it is completely set. However, since calcium chloride corrodes metal, its use is discouraged by most building codes. Instead, heating devices and Type III cement are recommended.

1-8. Retardants. Retardants are admixtures that slow down the curing time of mortar. These may be specified when conditions are very hot and dry, causing the mortar to set too rapidly and never attain its strength. However, most building codes discourage the use of retardants when other means of retarding, such insulation, may be used.

 

David L. Heiserman, Editor

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Revised: June 06, 2015