When mixing separate bags of cement and lime, hydrated lime must be completely wetted out in the mixing process or it will continue to absorb water after mixing. By following the proper mixing procedures, excellent board life, workability and sand carrying capacities are achieved with lime mortars.
The materials which could be used in a cement-lime mortar are defined in ASTM C270 - Mortar for Unit Masonry.
- Portland Cement - Types I, IA, II, IIA, III or IIIA of ASTM Specification C150.
- Blended Hydraulic Cement - Types IS, IS-A, IP, IP-A, I(PM), I(PM)-A of ASTM Specification C595/595M
- Quicklime - See ASTM Specification C5.
- Hydrated Lime - Type S or SA of ASTM Specification C207. Note: If a Type SA Hydrated Lime product is used, air entrained portland cement products should not be used.
- Aggregates - See ASTM specification C144.
- Water - Water shall be clean and free of amounts of oils, acids, alkalies, salts, organic materials or other substances that are deleterious to mortar or any metal in the wall.
Cement and lime products should be stored in a dry location at the jobsite to prevent deterioration of performance. Sand should be covered to minimize the potential contact with salts in rainwater.
How to read mix designs
The proportions used in cement-lime mortars are determined by units of volume in the proportion specification in ASTM C270. The following nomenclature is normally given to a cement-lime mortar:
- Where X1 = Units of volume of cement (portland or blended)
- X2 = Units of volume of lime (lime putty or hydrated lime)
- X3 = Units of volume of sand
The proper level of sand required can be obtained by multiplying the sum of X1 and X2 times 3.
For example, a Type O mortar is 1:2:9 or
- 1 cubic foot portland cement
- 2 cubic feet hydrated lime
- 9 cubic feet sand
Determining sand content
The most common mistake made in mixing cement-lime (CL) mortar is not adding enough sand. CL mortar will carry more sand due to the presence of dolomitic autoclaved hydrated lime. Lack of the proper amount of sand creates not only poor board life and workability problems but also the possibility of shrinkage cracks in the hardened mortar.
A shovel full of sand is a subjective measurement tool on the job site. Moisture content, temperature, and size of the shovel all have an effect on the actual volume of sand that gets into the mixer. The best method for measuring sand at the jobsite is the use of a cubic foot box. If a cubic foot box is not available, the number of shovels per cubic foot could be determined by shoveling the sand into five gallon pails. The number of shovels of sand required to fill up 1½ 5 gallon buckets is equivalent to one cubic foot. Once this number is obtained, the total number of shovels of sand per batch can be determined as follows:
- Assume that 7 shovels of sand were found to equal one cubic foot
- Assume a Type O mortar mix design of 1:2:9
- The shovels of sand per bag of cement can be calculated by multiplying the nine parts of sand needed times seven. This indicates that 63 shovels of sand are required per bag of cement for a Type O mortar at this jobsite.
Note: Sand shovels per cubic foot can range from 6-9 depending on the variables listed. Seven shovels per cubic foot is used only as an example.
When mixing a pre-blended cement-lime mortar, a full three cubic feet of sand is required for maximum performance of each bag of Mason's Pre-Blend mixed.
The following steps should be taken to maximize the performance of cement-lime mortars mixed at the jobsite:
- Add ¾ of the water required
- Add half of the sand
- Add all of the Type S hydrated lime
- Mix for two (2) minutes
- Add all of the cement
- Add the remainder of the sand
- Add remaining water needed to obtain a workable consistency
- Mix for three additional minutes or a total mix time of five minutes
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