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Guide for Mortar Type Mix
The purpose of this guide is to provide general guidance on appropriate materials and building methods and it is intended to benefit all DIY-ers. The guide will also serve as a brief to enable you to prepare and develop specifications if required.

Mortars for re-pointing projects, especially those involving older type buildings, typically should be custom mixed in order to ensure the proper physical and visual qualities. These materials can be combined in varying proportions to create a mortar with the desired performance and durability.

The strength of a mortar can vary. If mixed with higher amounts of Portland cement, a harder mortar is obtained. The more lime that is added, the softer and more plastic the mortar becomes, increasing its workability.

The actual specification of a particular mortar type should take into consideration all of the factors affecting the life of the building including: current site conditions, present condition of the masonry, function of the new mortar, degree of weather exposure, and damage of the structure.

A mortar strong in compressive strength might be desirable for a hard stone such as granite, whereas a softer, more permeable lime mortar would be preferable for a older type wall of soft brick. Masonry deterioration caused by salt deposition results when the mortar is less permeable than the masonry unit.

A strong mortar is still more permeable than hard, dense stone. However, in a wall constructed of soft bricks where the masonry unit itself has a relatively high permeability or vapor transmission rate, a soft, high lime mortar is necessary to retain sufficient permeability.

Mortar Mix

Mortar components should be measured and mixed carefully to assure the uniformity of visual and physical characteristics. Dry ingredients are measured by volume and thoroughly mixed before the addition of any water. Sand must be added in a damp, loose condition to avoid over sanding.

Mortar mixed for re-pointing is typically pre-hydrated by adding water so it will just hold together, thus allowing it to stand for a period of time before the final water is added. Half the water should be added, followed by mixing for approximately 5 minutes. The remaining water should then be added in small portions until a mortar of the desired consistency is reached. The total volume of water necessary may vary from batch to batch, depending on weather conditions.

It is important to keep the water to a minimum for two reasons: first, a drier mortar is cleaner to work with, and it can be compacted tightly into the joints; second, with no excess water to evaporate, the mortar cures without shrinkage cracks. Mortar should be used within approximately 30 minutes of final mixing.
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