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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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